We compared the abundance and reproductive success of grassland birds in rotationally- and continuously-grazed cattle pastures in southeast Minnesota to determine if either grazing practice was more beneficial to grassland bird populations. Savannah sparrows (Passerculus sandwichensis) were the most abundant species, and we found no difference in savannah sparrow nest success between grazing systems (P > 0.1). Cattle stocking rate and vegetation density were the best predictors of nest success in all pastures. Nest survival rates trended downward with decreasing vegetation density. Intervals of <30 days between grazing events on rotationally-grazed paddocks contributed to low nest productivity.
Recent declines of grassland bird species in the Midwest (Knopf 1994, Herkert et al. 1996) correlate with a 6.5 million ha decline in the regional acreage of pastures and hay fields between 1966 and 1992 (Herkert et al. 1996). In the Eastern and Central United States, 60% of grassland species declined between 1980 and 1998 (Murphy 2003). The magnitude of these declines suggests that local and regional extinctions are likely if grasslands are not managed to minimize breeding season disturbances (Herkert 1994). Because native prairie exists in less than 5% of its original range in Minnesota (Samson and Knopf 1994), birds that traditionally nested on prairies have become largely dependent on privately held, non-native, agricultural grasslands for breeding habitat (Johnson 1996). Most grasslands in the Midwest are used to produce forage that is harvested either mechanically or by grazing animals, primarily cattle (Herkert et al. 1996).
A growing number of farmers are practicing intensive rotational grazing (IRG) and finding it more cost effective than continuous grazing (U.S. Department of Agriculture 1996, Undersander et al. 1993). To practice IRG, a pasture is divided into small paddocks that support a herd for a half-day to two or thee days, depending on forage availability and quality, and animal condition (Blanchet et al. 2003). Ideally a 10-12.5 cm (4-5 inch) stubble remains after animals have been rotated to the next paddock, and the grass is rested for about a month before animals are allowed to graze it again (Blanchet et al. 2003). IRG is becoming increasingly popular on farms located in the unglaciated region of southeast Minnesota. Although statistics for Minnesota are not available, >10% and >15% of dairy farmers in Wisconsin (Paine et al. 1996) and Pennsylvania (U.S. Department of Agriculture 1996), respectively, are using IRG and its popularity is increasing (Paine et al. 1996). This grazing method is touted as bird-friendly (Undersander et al. 2000, but see Temple et al. 1999), although Johnson and Ward (1997:8) suggest that “if grassland birds are attracted to these fields under an IRG system, the question of their breeding success still needs to be addressed.”
Farmers who manage for increased reproductive success of birds nesting on their pastures should benefit through value-added product labels, and consumers who care about grassland birds should be given the information they need to support these farmers. A growing number of farmers in Minnesota and across the nation are marketing their products directly to consumers, and products with labels touting environmental and human health benefits are becoming popular nationwide (McLaughlin 2004). A recent U.S. Department of Agriculture (Payne 2002) survey found a 63% increase in the number of farmers markets between 1994 and 2002. In addition, bird watching increased by 155% between 1983 and 1995 nationwide (Iverson 2002), and participation in wildlife viewing in Minnesota increased 53% between 1996 and 2001 (U.S. Fish and Wildlife Service 2002b). Ecologically-sound, bird-friendly products such as shade-grown coffee, which represents 5% of sales in the United States gourmet coffee market (Griswold 1999), can help provide habitat for birds who winter in the tropics. However, Murphy (2003) has indicated that recent grassland bird declines in Midwestern and eastern states were linked to changes in agricultural land use regardless of a species’ migratory behavior or nesting habits. Whether a bird uses a cavity or makes a cup nest on the ground, and whether it flies to Argentina or to Alabama, if it is a grassland bird it is more affected by land management practices on North American farms than habitat changes occurring in the southern hemisphere (Murphy 2003). North American farmers have become de facto managers of grassland bird habitat. They exert a great deal of control over the success of grassland bird populations, and could benefit financially through sales of certified ‘bird-friendly’ beef and milk.
Savannah Sparrows were useful in our study because of their abundance, ubiquity, and habitat niche. They were abundant enough for us to find enough nests to make single-species statistical comparisons between treatments and farms. Because Savannah Sparrows prefer mid-range vegetation heights, soil moisture, grazing intensities and vegetation densities (Herkert et al. 1993) they can indicate ecological minimum needs for a number of rarer, tall-grass species such as the Bobolink, Sedge Wren, Eastern and Western Meadowlark, and Dickcissel. Providing good Savannah Sparrow nesting habitat is a starting point for any grassland farmer interested in providing habitat for other grassland nesters.
Our objective was to determine if rotationally- or continuously-grazed pastures better supported stable grassland bird communities. We found and monitored nests to determine if pastures and treatments had reproductive success capable of supporting stable populations of birds. We also measured vegetation and other aspects of nest sites and pastures to explore how these variables were correlated with success. Broadly, our goal was to provide farmers with the information to make land management and marketing decisions that enhance their financial bottom line as well as the reproductive success for grassland birds on their pasture. We also aimed to provide resource managers with tools to design government programs that promote more bird friendly practices on pastures. Finally, we provided bird-watchers and scientists with data to inform a new eco-labeling program (Institute for Agriculture and Trade Policy 1998). In this way, consumers’ grocery purchases could support farmers who provide productive grassland bird habitat.
Objectives for the project were initially both short- and long-term. Short-term objectives included:
Build a more trusting relationship between university researchers, resource managers, and farmers.
Share ideas about grazing and birds with a broad range of interested folks.
Identify how rotational and continuous grazing affect the nest success of grassland birds.
The long-term objectives were:
To provide information to farmers to fuel a powerful direct marketing campaign for bird-friendly farm products.
To improve the quality of life for farmers in the project as they learn more about the needs of birds on their pastures.
We located grassland bird nests on six privately-owned cattle pastures in Winona, Wabasha, and Olmsted Counties of southeast Minnesota, 2002-2003 (Driscoll 2004, Figure 1). We chose upland pastures that were similar in topography, soil type, whole pasture cattle density, and vegetation. Further, we selected farms based on physical similarity and from a pool of willing cooperators, with only the grazing practices different among farms. Throughout, we refer to each pasture with a two or three letter abbreviation of the owner’s last name (i.e. BE, KOR, RU, DI, HO, KOC).
We searched for nests and determined bird densities only in pasture areas that were >100 m from forests and water sources. Three pastures were continuously-grazed and three were rotationally-grazed; feedlots, non-vegetated areas, and cattle loafing areas were excluded. Continuously-grazed pastures were partitioned into three or fewer paddocks and each paddock was grazed at least one month continuously during the nesting season. Rotationally-grazed pastures were partitioned into 10 or more paddocks and the cattle were moved to a new paddock every 1-5 days. Four pastures were grazed by beef cattle and two were grazed by dairy cattle (Driscoll 2004, Appendix A). Pasture vegetation consisted primarily of cool-season, non-native grasses and forbs. We advised farmer cooperators to manage their pastures without considering our research because we wanted to study how grassland bird reproductive success was affected by successful grazing operations that were not experimentally controlled.
We excluded all data from paddocks that were managed inconsistently with our designated management categories. For instance, we excluded data from two paddocks of a designated rotationally-grazed pasture that were either hayed or grazed continuously, and were never rotationally-grazed. We also excluded data from a designated continuously-grazed area on one farm that was not grazed until the last week of the nesting season (Driscoll 2004, Appendix B).
Nest Searches and Reproductive Success
We randomly located on each pasture approximately 5 ha (12.5 acres) of plots to search for grassland bird nests; 62% of plots were > one ha (2.5 acres). We used stick searching and observation to find grassland bird nests from 21 May through 10 August during both study years. The stick search method for finding nests involves sweeping an ~2-m stick through the grass in front of you as you walk. If done with a line of people, birds flush close enough for you to find the nest. Each nest search plot was extensively searched > once a week, and plots where birds were heard were searched more often until we found the nest. Once all established study plots had been searched within the week, we searched outside of study plots. Areas around known, active nests were left undisturbed during searching. We avoided nest searching and bird surveys in rainy and windy conditions to prevent undue human influence at the nest and because birds do not generally sing in the rain.
After finding nests, we quickly noted the nest characteristics, such as species (Baicich and Harrison 1997), height, number of eggs and nestlings, age of nestlings, height of tallest vegetation, and species of vegetation closest to the nest. We marked the nest with carefully-placed flagging and orange-painted tongue depressors pressed into the ground ~ 1 meter on each side. We then noted the compass azimuth and distance to a particular landmark, such as a fencepost, and then walked >25 m from the nest to record data such as nest identification number, species, farm, location, date, and time, as well as nest characteristics. To track nestling success, we visited each nest every three days from 22 May to 10 August until the nest was depredated, abandoned, or the young fledged. Once a nestling left the nest we considered it “fledged” and called it a “fledgling.”
Avian Community Composition
We noted breeding male birds by sight and song along transects in each pasture to determine the presence and absence of nesting pairs. We separately noted non-resident birds that flew over the transect, and birds heard off of the transect. Transects were randomly located and were 100 m wide and 500 m long. Transects were bisected into 300 m and 200 m sections in continuously-grazed pastures because trees and waterways split continuous pastures into smaller grassland sections. Rotationally-grazed transects were straight and unbisected. We surveyed each pasture four times during the 2002 season (within four days of 7 June, 23 June, 12 July, and 28 July), and three times during the 2003 season (concurrent with whole pasture vegetation data collection in the 2003 season, within four days of 1 June, 1 July, and 1 August). We averaged the second and third surveys of 2002 to coincide with vegetation data collected within four days of July 1st. We recorded surveys at a pace of 5 minutes per 50 m beginning at sunrise, and ending before 9:00 a.m. Birds located on the edge of the area (i.e., on the fence or a pole) were counted as in the area. We identified the following as grassland nesting birds (Kaufman 2000): savannah sparrow (Passerculus sandwichensis), Grasshopper Sparrow (Ammodramus savannarum), Bobolink (Dolichonyx oryzivorus), Red-winged Blackbird (Agelaius phoeniceus), Eastern Meadowlark (Sturnella magna), Western Meadowlark (Sturnella neglecta), Brown-headed Cowbird (Molothrus ater), Killdeer (Charadrius vociferous), and American Goldfinch (Carduelis tristis). We combined Eastern and Western Meadowlarks in our analysis to reflect our inability to distinguish between the two while they were in flight.
Nest Site Attributes
We measured vegetation at each nest site within three days of its last occupation to examine the relationship between reproductive success and nest site vegetation. We divided nest site attributes into three categories: percent cover variables, plant structure variables, and distance variables. We used a Daubenmire frame (Daubenmire 1959) to measure percent cover of grass, forb, downed litter, standing litter, woody stems, bare soil, cow pie, and rock at the nest and one meter from the nest in each cardinal direction. The height of the tallest vegetation and the litter depth were measured at each corner of the frame in each of its five placements. We used a Robel pole (Robel et al. 1970) to gauge vegetation density, also called a “visual obstruction reading” or VOR, at the nest and one meter from the nest in each cardinal direction. We estimated the distance from each nest to the first and second closest edge, the nearest shrub, shrub clump, tree, tree clump, and the distance to the nearest cattle-accessible water. Habitat edges included forests, waterways, roads, rowcrops, and hayfields. We counted fences as habitat edges in 2002 but did not count them as habitat edges in 2003. We visually estimated the distance that cattle had to travel to water to determine if cattle proximity to water affected nest success.
We randomly located 20 permanent vegetation measurement sites in each pasture to assess how birds were affected by vegetational changes at the field scale. We marked each point with a 18 cm (7 inch) spike tipped with a brightly colored washer pounded flush to the ground. Points were relocated with the aid of a metal detector. We measured the same vegetation variables at these 120 sites as we did at the nest within four days of 1 June, 1 July, and 1 August, in 2002 and 2003. We excluded 1-4 points on four pastures for various reasons (Driscoll 2004, Appendix B). When the slope of a site was >15 degrees, we measured vegetation density in two directions at the elevation of the site instead of four measurements, one in each cardinal direction.
We spent one day near the end of each field season on each pasture identifying as many plant species as possible. We used native wildflower (Tekiela 1999) and grassland (Johnson and Larson 1999) books, as well as a tree book (Brockman 2001), and two weed books (Royer and Dickenson 1999, University of Illinois at Urbana-Champaign 1995) to identify pasture plants. We consulted Gleason and Cronquist (1963) and the USDA plants database website (U. S. Department of Agriculture 2004a) for taxonomic authority.
We randomly chose six to eight paddocks per rotationally-grazed farm per year to characterize how paddock vegetation density (VOR) changed with grazing. We chose 10 random points per paddock and marked them with a steel stake and colored washer pounded flush with the ground. All 10 points were measured on the same day, the first time within one to three days before, and the second time within one to three days after the paddock was grazed.
We noted the paddock location of cattle on each visit and which paddocks had been grazed since our last visit. The 2003 season was more thoroughly documented in this respect than the 2002 season. We defined the interval between grazing events as the time elapsed from the first day a paddock was grazed during a grazing period until the first day a paddock was grazed during the next grazing period.
We interviewed farmer cooperators during 2002 to gather information on farm characteristics (e.g., acreage, amount of farm used for grazing, number of cattle grazed, size and number of paddocks, etc., Driscoll 2004, Appendix A). We used this information to determine cattle density and the percentage of cattle diet provided by each pasture. We counted any cow, bull, or steer, or a cow and her <3-month-old calf, or a calf over three months old, as one animal (Blanchet et al. 2003). We measured the perimeter of pastures to assist in determining overall cattle density and included all areas within the pasture, and not only areas 100 m from trees and water features.
We excluded from analysis a nest that never had eggs, a nest that was not found a second time, nests where savannah sparrow adults were raising only cowbird young (2), and a nest where the method of finding the nest was inconsistent with our protocol (Driscoll 2004, Appendix C). Six of the eight Bobolink nests we located were on continuously-grazed pastures. The Bobolink nest dataset was reduced to seven nests because one of the Bobolink nests we located on rotationally-grazed pasture KOR failed to meet our inclusion criteria. The area harboring the excluded nest was not grazed at all during the nesting season (Driscoll 2004, Appendix C).
Our measure of apparent nest success was at least one young surviving to fledge from a nest. We used Micromort 1.3 (Heisy and Fuller 1985) to calculate daily survival, interval survival, and variances for nests, eggs, and nestlings (Mayfield 1961, 1975). We used a pair-wise approach with Bonferroni adjusted alpha values (Moore and McCabe 1999) to compare daily survival rates (DSR) among farms (Hensler 1985, equation 6). To achieve a simultaneous error rate of P £ 0.05 for the experiment, we considered pair-wise comparisons £P = 0.005 to be significant.
We used methods in Brawn and Robinson (1996) to estimate population stability for savannah sparrows in each pasture, and all seven Bobolink nests, regardless of pasture:
l = PA + PJb, where l was the population growth rate, PA = adult survival rate, PJ = juvenile survival rate, and b = number of juveniles produced per nest attempt. Populations were stable where l=1.0, declining where l<1.0, and increasing where l>1.0. We used savannah sparrow and Bobolink adult survival data from Wheelwright and Rising (1993) and Martin and Gavin (1995), respectively. We assumed fledgling survival to be half of adult survival (Brawn and Robinson 1996, but see Weatherhead and DuFour 2000), and inserted our observed number of fledglings/nest (b) for each pasture. Because the adult survival rate is unknown on our sites, we used the number of young fledged/nest on each pasture as a constant and determined what adult survival rate was necessary to support a stable population of savannah sparrows on each pasture.
We used the density of male passerines to compare species density, richness, and diversity among continuously- and rotationally-grazed pastures. We compared species richness between treatments by counting the number of species heard in each survey. Prior to analyses, we averaged each species density from the three surveys each year, over two years (n = 6) for each pasture. We calculated Shannon’s Diversity Index for each pasture (Begon et al. 1986). We choose Shannon’s index over Simpson’s index because we found that it provided more consistent results with small sample sizes.
We used logistic regression and employed a forward selection routine to determine which nest area attributes influenced savannah sparrow nest success on all pastures (Hosmer and Lemeshow 1989). At each step we subtracted the deviance of the model with the addition of the variable in question, from the deviance of the established model. We added variables whose difference was greater than the chi-squared critical value at the a = 0.05 significance level, until all such variables were added to the model (Cook and Weisberg 1999). Variables considered for the logistic regression included previously noted percent cover variables and distance variables, vegetation height, vegetation density, litter depth, and percent of nest covered by vegetation. We confirmed model selection using Akaike’s information criterion (AIC, Cook and Weisberg 1999).
During analysis of pasture vegetation, we reduced the multiple subsamples to a single measure for each pasture. To do this, we averaged the 1-4 measures for each point, over two years for each variable, then averaged all of the points on a pasture resulting in a pasture mean for each variable, for each year. We employed stepwise linear regression to determine which pasture-scale variables affected savannah sparrow daily survival rate among pastures. Variables considered for the linear regression model included all previously noted percent cover variables, vegetation height, vegetation density, litter depth, cattle density, savannah sparrow density, and percent of cattle diet provided by the pasture. In order to determine how well simple linear regression explained the relationship between vegetation densities (VOR) and savannah sparrow densities on all pastures, we visually compared lowess smoothes of the mean function (Cook and Weisberg 1999) to linear and second- through fifth-order polynomial functions.
We determined the proportion of times each plant species was identified out of 12 identification periods, one period per year per pasture. Species of plants that were found on all years in all pastures rated a 1.00, whereas plant species that were found only 11 out of 12 times were rated 0.92, and so on. We then ordered plant species by proportion of occurrence to visually determine if treatments had similar plant species assemblages.
We used one-way Analyses of Variance (ANOVA) to determine if change in vegetation density after grazing and number of exposure days differed between rotationally-grazed pastures. All procedures were run with the student version of Statistics 7 (Analytical Software 2000) and/or Arc (Cook and Weisberg 1999).
We located 80 nests, including 68 savannah sparrow nests, eight bobolink nests, two eastern meadowlark nests, one western meadowlark nest, and one mallard nest. We included 60 savannah sparrow nests in the analysis (see Appendix C), 23 in continuously-grazed, and 37 in rotationally-grazed, pastures. We found no differences between grazing systems in the daily survival rate (DSR = 0.92 and 0.92, Z = 0.07, P = 0.94, Driscoll 2004, Table 1), the egg survival rate (Z = 1.12, P = 0.26, Driscoll 2004, Table 2), or the nestling survival rate (Z = 1.18, P = 0.24, Driscoll 2004, Table 2). We compared survival between five of the six pastures regardless of treatment (Driscoll 2004, Table 3) and found no statistically significant differences (all P > 0.09, Driscoll 2004, Table 4). We did not make any comparisons to DI pasture as we were unable to locate any nests on that pasture. Although we did not find statistically significant differences between reproductive rates on the pastures where we found nests, there was a range of Interval Survival Rates within treatments and between all farms regardless of treatment (Driscoll 2004, Figure 2, Table 3).
Wheelwright and Rising (1993) estimated that the savannah sparrow adult survival rate was ~ 50%. At 50% adult survival only pasture HO supported a stable population of savannah sparrows (Driscoll 2004, Figure 3, l = 1.16). All other pastures had lower rates of nest success and thus required higher rates of adult survival to support stable populations. The mean number of young fledged per nest (b) on rotationally-grazed pastures was 1.59 for BE (0.39 SE), 0.50 for KOR (0.25 SE), and 1.62 for RU (0.40), with a combined mean of 1.49 (0.32 SE). The mean number of young fledged per nest (b) on continuously-grazed pastures was 2.08 for HO (0.60 SE) and 0.18 for KOC (0.05) with a combined mean of 1.17 (0.37 SE). We found no nests on DI pasture and have no record of fledglings. At 62% adult survival the group of rotationally-grazed pastures would support a stable savannah sparrow population. The adult survival rate had to rise to 67% for all six pastures and 73% for continuously-grazed pastures to support a stable population.
We analyzed seven bobolink nests and they fledged a mean of 2.71 (1.02 SE) chicks per nest. The DSR for bobolink nests regardless of treatment was 0.93 (0.02 SE). A collection of sources gathered by Martin and Gavin (1995) suggest that male and female bobolinks exhibit different annual survival rates. These rates range from > 34% to 61% for females, and > 57% to 70% for males (Martin and Gavin 1995). If the bobolink nests had all been located in the same pasture, they would represent a stable population at 45% adult survival.
We found no relationship between the density of savannah sparrows on a pasture and their interval survival rate (Pearson’s r = 0.05, n = 5, P = 0.94). Intermediate densities of savannah sparrows experienced higher success than either higher or lower densities (Driscoll 2004, Figure 4).
Cattle were at least partially responsible for most savannah sparrow nest loss (> 64%, Driscoll 2004, Table 5). Trampling, eating vegetation around the nest and thus exposing the nest, and defecating on the nest were all noted causes of nest failure perpetrated by cattle. Other reasons for failure were abandonment, predation, cowbird parasitism, and haying activities. Causes of nest failure appeared to be similar between grazing treatments. Cowbird parasitism was a partial cause for failure in 9% of nest failures in rotationally-grazed pastures (Driscoll 2004, Table 5). Two nests located in continuously-grazed pastures were parasitized by cowbirds, but savannah sparrow nests that had only cowbird young were removed before analysis (Driscoll 2004, Appendix C) and therefore Table 5 attributes no nest failures to cowbird parasitism in continuously-grazed pastures.
Avian Community Composition
We found no differences between rotational and continuous grazing treatments in the density of all passerine species (t = 0.83, 2 df, P = 0.49) or the density of grassland nesting species (t = 0.29, 2 df, P = 0.80). Rotationally-grazed sites had 8.33 +/- 1.33 (SE) species per pasture and 4.33 +/- 0.88 (SE) of those were grassland nesters. Continuously-grazed sites had 9.00 +/- 2.65 (SE) species per pasture, and 4.67 +/- 1.45 (SE) of those were grassland nesters. No single species had a higher density in one treatment than another (all P > 0.10, Driscoll 2004, Table 6). However, savannah sparrow densities among pastures were different (One-way ANOVA, F = 10.29, P < 0.01, Driscoll 2004, Figure 5). DI had a lower density of savannah sparrows than all other pastures except KOR pasture, and KOR pasture had a lower density than RU pasture (Tukey T = 6.64, 30 df, P = 0.05, Driscoll 2004, Table 7). Community diversity and evenness of birds between the treatments were not different for the whole passerine community (t = 3.22, 4 df, P = 0.08, and t = 1.26, 4 df, P > 0.1, respectively), or the subset of grassland nesters (t = 1.27, 4 df, P > 0.1, and t = 1.20, 4 df, P > 0.1, respectively, Driscoll 2004, Figure 6).
Nest Site Attributes
Variables that affected simple nest success at the nest sites (i.e., a nest fledging ³ 1 chick or not) included vegetation density, distance to nearest shrub, percent cover of downed litter, and percent cover of cowpies (logistic regression, all coefficient P-values < 0.05 except percent cover of cowpies, Lack of Fit for model P = 0.95, Driscoll 2004, Table 8). Vegetation density was the best predictor of nest success; nests were 58 times more likely to fledge at least one chick with a one decimeter increase in vegetation density (VOR, Driscoll 2004, Table 8). Percent cover of downed litter, distance to shrubs, and percent cover of cowpies involved much smaller odds ratios. For every 1% increase in cover of downed litter a nest was 1.32 times more likely to survive (Driscoll 2004, Table 8). A lower 95% confidence limit of 1.07 showed that the relationship between fledging success and litter coverage was positive (Driscoll 2004, Table 8). For each additional meter distance from a shrub, a nest was 0.97 as likely to be successful (Driscoll 2004, Table 8). Finally for every 1% increase in cowpie cover a nest was 0.26 as likely to be successful. There was evidence of great variability at the nest sites as all odds ratios had broad confidence limits, except for distance to nearest shrub (Driscoll 2004, Table 8). We also looked at each variable separately and found that successful nests had lower percent cover of bare soil (t = 2.47, 22 df, P = 0.02), lower percent cover of cow pies (t = 3.06, 22 df, P < 0.01), higher vegetation densities (t = 4.92, 22 df, P < 0.01), and higher vegetation heights (t = 3.23, 22 df, P < 0.01), than failed nests (Driscoll 2004, Table 9). Successful nests also had marginally greater percent of nest covered by vegetation (t = 1.81, 22 df, P = 0.09), and litter depth (t = 1.95, 22 df, P = 0.07, Driscoll 2004, Table 9). We found that successful nests located in rotationally-grazed pastures had higher percent cover of forbs (t = 3.30, 7 df, P < 0.01) and lower percent cover of downed litter (t = 5.6, 7 df, P < 0.01) than successful nests in continuously-grazed pastures (Driscoll 2004, Table 10). No other nest-site characteristics were different between treatments. Pasture Vegetation We found no differences between rotationally- and continuously-grazed pasture vegetation attributes when we compared the overall vegetation of pastures where nests were found (all P-values > 0.05, Driscoll 2004, Table 11). However, percent cover of grass (P = 0.07) and percent cover of soil (P = 0.06) may be biologically significant considering the small pasture sample size (Driscoll 2004, Table 11). We also observed temporal changes in VOR values on some pastures (Driscoll 2004, Figure 7). Pasture RU had a notably different pattern than all other pastures. Its average vegetation density continually increased throughout the nesting period, even during the drought of 2003 (Driscoll 2004, Figure 7).
Ninety plant species were identified on the pastures and rotationally- and continuously-grazed pastures had similar sets of species (Driscoll 2004, Table 12). Species present on all pastures during both years included Bull Thistle (Cirsium vulgare), Daisy Fleabane (Erigeron strigosus), Purple Alfalfa (Medicago sativa), Common Dandelion (Taraxacum officianale), Red Clover (Trifolium pratense), and White Clover (Trifolium repens) (Driscoll 2004, Table 12). Species present on all rotationally-grazed pastures both years included the previous list with the additions of: Whorled Milkweed (Asclepias verticillata), Canada Thistle (Circium arvense), Smooth Brome (Bromis inermis), Common Plantain (Plantago major), and Western Ragweed (Ambrosia psilostachya) (Driscoll 2004, Table 12). Species present on all continuously-grazed pastures both years included the first list with the additions of: Kentucky Bluegrass (Poa pratensis) and Musk Thistle (Carduus nutans) (Driscoll 2004, Table 12).
We heard and saw few or no savannah sparrows on pastures with little or no vegetation (Driscoll 2004, Figure 8). The greatest mean increase in savannah sparrow density across all pastures occurred when vegetation density was between 0.4 and 1.0 dm VOR (Driscoll 2004, Figure 8). Interval survival rates trended downward with decreasing vegetation density (Driscoll 2004, Figure 9).
We found that the best predictors of difference in savannah sparrow DSR among pastures were whole pasture cattle density (Student’s t = 5.94, P < 0.01) and whole pasture vegetation density (Student's t = 18.57, P < 0.01). If we had not collected data on either cattle density or vegetation density, vegetation height (Student's t = 5.51, P < 0.01) would have been the best predictor of high savannah sparrow DSR. The difference in litter depth between farms was statistically significant (One-way ANOVA, P < 0.01) although the difference between the farm with the deepest and shallowest litter depth was only 1.65 cm. Each farmer who practiced rotational grazing managed their vegetation differently. We combined both years of the study to examine vegetation removal on paddocks that were rotationally-grazed. The change in vegetation densities in pastures BE and KOR was 1.08 dm (0.12 dm SE) and 0.75 dm (0.14 dm SE), respectively, while RU pasture’s mean change in density was 1.79 dm (0.12 dm SE) and significantly higher than the other two (One-way ANOVA, P < 0.01, Driscoll 2004, Table 13). RU farmers allowed the vegetation in an average paddock to reach a relatively high density of ~2 dm (VOR) and then managed the cattle so that ~0.25 dm (VOR) remained after grazing. In contrast, farmers managing pastures BE and KOR allowed the vegetation to reach between 1.0 and 1.5 dm and only allowed the cattle to graze the vegetation down to ~0.5 dm (VOR, Driscoll 2004, Figure 10). The number of days between before- and after-grazing measurements was not different among farms (one-way ANOVA, P = 0.13, Driscoll 2004, Table 13). Cattle Stocking Rate, Interval Between Grazing Events, and Diet Cattle stocking rates and savannah sparrow daily survival rates (DSR) of nests were inversely related (Linear Regression, F = 5.90, 5 df, P = 0.07, R2 = 0.60) when calculated for each farm. Whole pasture cattle stocking rates ranged from 5.37 cattle/ha at KOR pasture to 1.62 cattle/ha at HO pasture (Driscoll 2004, Table 14 [or 2.15 (KOR), 1.66 (DI), 1.33 (BE), 1.29 (KOC), 1.04 (RU) and 0.65 (HO) cattle per acre]). Average cattle densities per treatment were 3.00 (0.74 SE) cattle/ha (1.20 cattle/acre) for continuously-grazed, and 3.77 (0.83 SE) cattle/ha (1.52 cattle/acre) for rotationally-grazed pastures, and were not different (one-way ANOVA, F = 0.48, P = 0.53). RU farm maintained a numerically higher stocking rate on its paddocks than the other two rotationally-grazed farms (Driscoll 2004, Table 14). The interval between grazing events at paddocks in rotationally-grazed pastures during the 2003 season was 31 days for BE paddocks (4 days SE), 30 days (1 day SE) for RU paddocks, and 22 days (3 days SE) for KOR paddocks (Driscoll 2004, Table 15). Shorter interval times at KOR pasture are a result of higher whole-pasture cattle density and fewer paddocks in the rotational grazing system at that pasture. Pastures provided 50-100% of the diet of cattle, and reflected different animal management strategies. According to farmer cooperators, RU, KOC, and HO pastures provided 100% of the diet, besides salt, to cattle grazing on them, BE pasture provided 80-90% to cattle, whereas KOR and DI pastures provided only 62% and 50%, respectively, to cattle (Driscoll 2004, Table 14 and Appendix A). Cattle stocking rate and percent of cattle diet provided by pasture were inversely related (Linear Regression, F = 7.34, 5 df, P = 0.05, R2 = 0.65). Percent of cattle diet that was pasture was not directly correlated with savannah sparrow daily survival rates (Linear Regression F = 1.34, 4 df, P > 0.1, R2 = 0.31), but was indirectly tied through cattle density. Percent of cattle diet that was pasture and vegetation density (VOR) in pastures were positively correlated (Linear Regression F = 18.69, 5 df, P = 0.01, R2 = 0.82, Driscoll 2004, Figure 11).
Pastures as Grassland Bird Habitat
Nest success of savannah sparrows in both rotationally- and continuously-grazed pastures was 0.92 daily survival rate (DSR) and was similar to research conducted on grazed, ungrazed and mowed grasslands in the upper Midwest. Daily survival rates in Wisconsin were 0.92 for eight savannah sparrow nests monitored on rotationally-grazed pastures and 0.95 for nine savannah sparrow nests monitored on continuously-grazed pastures (personal communication, S. Temple, University of Wisconsin, Madison, Temple et al. 1999). The latter measure is similar to the DSR (0.95) in continuously-grazed HO pasture. Differences between results in Wisconsin and Minnesota may be a result of low sample sizes and variation in research protocol. Temple et al. (1999) assumed successful fledging of all nests active before 1 July, when mowing was allowed to commence. We did not restrict mowing activities and continued to record nest successes and failures through early August each nesting season. Ungrazed grasslands in North Dakota and Minnesota supported a DSR of 0.94 for savannah sparrows (Winter and Faaborg 1999) and Waterfowl Production Areas (WPA), and Conservation Reserve Program (CRP) lands in these two states supported DSRs of 0.86 – 0.95 (Koford 1999). In contrast, 12 nests had a daily survival rate of 0.23 on mowed airports in east central Illinois (Kershner and Bollinger 1996).
Ungrazed prairie fragments and CRP fields produce more fledglings/nest than pastures. The savannah sparrows’ population at one of our study sites teetered on the edge between population stability and decline, but generally both continuously- and rotationally-grazed pastures did not support stable populations. Records of adult savannah sparrow survival have fluctuated wildly among years and areas of North America, ranging from 28%-42% in Nova Scotia, to 68%-70% in Michigan, but were usually below 50% (Wheelwright and Rising 1993). When we assumed ³50% adult survival, only HO pasture had sufficient reproductive success (2.08 chicks per nest) to support a stable population. Other pastures in our study were even less likely to support stable populations. Johnson and Temple (1990) examined nest success in tall grass prairie fragments in western Minnesota and found that unparisitized savannah sparrow nests successfully fledged 3.2 chicks/nest, and could have served as savannah sparrow sources at 39% adult survival. In comparison, other grassland species such as grasshopper sparrows and field sparrows that nested in Missouri CRP fields successfully fledged 2.82 to 3.94 chicks/nest (McCoy et al. 1999).
Singing male savannah sparrow densities of 2.00/ha for rotationally-grazed pastures and 1.36/ha for continuously-grazed pastures were high compared to other research. Illinois densities range from 0.25/ha (Herkert 1994) to 1.91/ha (Kershner and Bollinger 1996). North Dakota idle short grass prairie and alfalfa-wheatgrass fields supported densities of 0.07/ha (Koford 1999), and 0.31/ha (Renken and Dinsmore 1987), respectively. Northern tallgrass prairie remnants in North Dakota and Minnesota supported a range of densities from 0.34/ha to 1.73/ha, uncorrelated to size and hostile/neutral habitat designation (Winter et al. 2000a). RU pasture had particularly high average numbers of savannah sparrows per survey at 2.83/ha, although according to some unpublished data, RU pasture savannah sparrow density may be declining. Amateur birders surveyed the pasture yearly from 1994 to 1996 and found 3.17, 3.94, and 3.66 savannah sparrows/ha, respectively (Arthur ‘Tex’ Hawkins, U.S.F.W.S., unpublished data).
Results of our pasture-scaled research inform farmer decisions today, and point out the need for large scale meta-population research to determine how pasture fragments should be managed to sustain grassland bird populations in the future. If a greater percentage of the overall savannah sparrow population in a region is dependent on pastures than on CRP fields, then savannah sparrow conservation efforts should concentrate more on working agricultural landscapes, in comparison to set-aside programs such as CRP. In our research, pastures supported similar daily survival rates but produced fewer fledglings per nest than ungrazed grasslands. Rotationally-grazed RU pasture was a good example. It attracted high densities of savannah sparrows, but might have served as an ecological trap for grassland nesters when cattle were rotated through the paddock before chicks could fledge. In addition, Best et al. (2001) found grassland species occur most commonly in rowcrops when rowcrops border grassland. Our research did not examine cover classes bordering pastures so we cannot determine how nearby habitats affected success. More research regarding meta-population dynamics of grassland birds in agricultural areas is necessary to understand the interrelated nature of pasture fragments in what farmer Ralph Lentz (personal communication) calls “a sea of corn.” However, we address grassland bird habitat issues at the pasture or field scale, which is the common scale used by farmers managing their land. In this regard our research could be instrumental in bridging the gap between scientists with ideas, and farmers who implement these ideas on the farming landscape.
Farmer Actions Determine the Success and Productivity of Grassland Birds
Cattle stocking rates can affect passerine densities and nest success through effects on vegetation density and directly through trampling (Jenson et al. 1990, Paine et al. 1996). Continuously-grazed HO pasture exhibited the highest overall vegetation density (VOR), and highest nest success, with the lowest stocking density. At the other end of the spectrum, continuously-grazed DI pasture had the highest cattle stocking rate, the lowest density of savannah sparrows, and despite exhaustive searches, no nests were found there. Rotationally-grazed RU pasture was stocked with one more cow/ha (+62%) than HO pasture and RU pasture had almost twice the density of savannah sparrows. Unfortunately savannah sparrows at RU pasture had less than half the daily survival rate of birds that nested on HO pasture, showing that density and success do not necessarily have a positive relationship (Van Horne 1983). Continuously-grazed pasture KOC and rotationally-grazed pasture BE have almost identical cattle densities but young reared on KOC pasture reach only one-third the interval survival rate of young reared on BE pasture, suggesting that some rotationally-grazed pastures can out-perform continuously-grazed pastures when compared at the same cattle density. Bélanger and Picard (1999) found that a “moderately grazed” island in the St. Laurence River, with < 1 cow/ha supported the highest density of savannah sparrows at 2.12 pairs/ha. Both an intensively grazed and an ungrazed island supported lower densities (Bélanger and Picard 1999), confirming that savannah sparrows use early successional habitat (Askins 1993). As well, Kantrud (1981) found much higher densities of savannah sparrows under light grazing conditions, compared to medium and heavy conditions. Cattle trampling, eating vegetation adjacent to the nest, and defecation caused 61% and 67% of the nest failures in rotationally-and continuously-grazed pastures, respectively (Table 5). This is supported by Paine et al. (1996) who measured nest success in Intensive Rotationally Grazed (IRG) pastures in Wisconsin. They used unwashed pheasant eggs and found that trampling damaged 75% of the imitation nests. In Oklahoma, Jensen et al. (1990) used imitation nests that consisted of clay pigeons and documented trampling that increased exponentially over time for all stock densities. They found that stock densities above 2.5 Animal Units/ha could result in significant disturbance of ground nesting birds. Environmental correlates of savannah sparrow success are different in different regions of North America (Swanson 2001, Winter et al. 2000b, Koford 1999, Kershner and Bollinger 1996). We found that increasing vegetation density (VOR) was strongly correlated to savannah sparrow density and nest success on both treatments. Vegetation density at successful nests was twice that at unsuccessful nests, with an average difference of 8.6 cm. Temple found that savannah sparrows had better success in continuously-grazed pastures with vegetation density of 0.59 dm, than in rotationally-grazed pastures with a vegetation density of 4.76 dm (personal communication, S. Temple, University of Wisconsin, Madison, Temple et al. 1999). Bélanger and Picard (1999) found the greatest number of savannah sparrows where vegetation density was 24 cm on a St. Laurence River Island in Canada. Herkert (1991) suggests that savannah sparrows prefer intermediate vegetation densities and that grazing that leaves ³ 40% vegetation cover and vegetation height ³25 cm could be used as a management tool to enhance habitat. Savannah sparrows require some downed plant litter before they will set up territories (Herkert 1994) although successful savannah sparrow nests on our study pastures had a shallower litter layer than unsuccessful nests. Overall litter depths at continuously- and rotationally-grazed pastures ranged from 0.58-0.88 cm, somewhat less than the 1.8 cm Renken and Dinsmore (1987) found in North Dakota pastures. Savannah sparrows were common on native prairie with a deeper litter layer, than in crested wheatgrass fields with shallower litter (Sutter and Bringham 1998). Although many herdsman consider a field to be “rank” when it has “too much” litter, farms should not have a problem providing the <0.5 cm of litter that we found at successful savannah sparrow nests. There appeared to be a positive correlation between the percent of cattle diet provided by a pasture, the density of pasture vegetation, and nest success. Farmers who managed their pastures to provide at least 80% of daily feed for cattle grazing on them supported the highest savannah sparrow nest successes. Generally, such farmers pay closer attention to the long-term health of their pastures in an attempt to lower the risk of depleting pasture resources (Loeffler et al. 2000). Toward this goal they spent more time monitoring vegetation, maintained the pastures at a higher overall vegetation density, and stocked cattle at lower rates (Loeffler et al. 2000). Farmers who allow their forage to remain taller after grazing assist desired forage to better compete with undesired forage, such as thistles (Loeffler et al. 2000, Rook et al. 1994), and we found that higher density forage on cattle pastures can harbor more successful grassland bird nests. In addition, when farmers are feeding more forage they feed less grain, reducing their need to plow up more grassland to plant crops for cattle feed. More land can be kept in permanent cover when farmers pasture their cattle and especially when grassland farmers depend upon forage for 80% to 100% of meat or dairy cow diets on their farm. More working land kept in permanent cover provides more habitat for grassland birds and other wildlife Thistles were a common “undesirable” on our study sites. They thrive in disturbed areas, and reduce the availability of more palatable pasture forage. The Musk Thistle Carduus nutans L., can reduce pasture yields by 23% if it is not controlled (Giles et al. 1996). Thistles were considered noxious weeds in southeast Minnesota counties, and farmers were required by law to control them through clipping, haying or application of herbicides (State of Minnesota 2003). We suggest that if grasses and other palatable forbs are maintained at a higher density, thistles, and the need to mow them, may be reduced. A reduction of mowing activities will save farmers time and fuel, and prevent the destruction of many young birds and nests. Another important variable for birds nesting in rotationally-grazed pastures was the interval between grazing events. Savannah sparrows experienced higher daily survival rates at pastures RU and BE with 30 and 31 day return times, respectively, in comparison to KOR pasture with a 22 day return time. Because intensive rotational grazing involves a high density of cattle in a small paddock, most nests in that paddock are trampled during a grazing event. Therefore, to provide for better grassland bird success in an intensive rotationally-grazed) system, farmers must not return cattle to a paddock for the amount of time it takes a pair of birds to find a suitable nest site, build a nest, lay a clutch of eggs, incubate eggs, hatch, feed, and fledge young. When vegetation density was grazed to an average of 0.25 dm VOR in a paddock before rotation, as was the case at RU pasture, the minimum time for savannah sparrows to progress from building the nest to fledging chicks was approximately 30 days. Producers who allow pasture forage to grow a minimum of 1 dm VOR and produce seeds, and do not allow the same paddock to be grazed again for a minimum of 30 days (4-6 more days for larger species such as bobolinks), are more likely to support a healthy population of grassland birds, as well as enjoy increased long term plant vigor. Grass grows fastest from late May through June in the upper Midwest (Matches and Burns 1995), coinciding with peak nesting season for grassland birds. Grass has a higher protein level in its vegetative state before it goes to seed (Matches and Burns 1995) and to gain the most profit from a rotational grazing system, grazing specialists recommend that grass stands be clipped to prevent them from maturing and producing seeds (personal communication, Howard Moechnig, Natural Resources Conservation Service, Rochester, Minnesota). In an attempt to prevent the grass from going to seed and “getting ahead” of their cattle, and to store forage for winter, some producers in our study clipped and hayed their pastures. KOR pasture was the most intensively clipped pasture and had the lowest nest success of all rotationally-grazed pastures. Countering the popular view of clipping for productivity, researchers have found that when grass plants in a pasture are prevented from ever growing tall and going to seed they also are prevented from forming deep, healthy root systems (Loeffler et al. 2000). Shallow rooted grasses are more susceptible to drought, winter die-off, and grazing pressure (Blanchet et al. 2003, Andrae 2004). Producers who graze and clip their pastures to prevent forage plants from going to seed often must interseed into pastures to retain good quality forage for their cattle. Although cutting hay and clipping pasture are actions as detrimental to grassland bird nests as they are necessary for grassland farmers, a few avenues for compromise are available. We saw five of the six pastures either hayed or clipped at least once during our two year study, and some paddocks were clipped after each grazing beginning in mid-June each year. In addition to our nest failure data (Driscoll 2004, Table 5), savannah sparrow productivity declined by 80% following mowing for hay in Saskatchewan (Dale et al. 1997) and mowing caused 44% of all grassland nest failures on an Illinois airport (Kershner and Bollinger 1996). Fortunately, some old farming patterns are changing. DeVore (2003) noted that a southeast Minnesota dairy farmer found no decrease in pasture or herd performance after eliminating clipping from his rotationally-grazed pastures. In addition, some farms have changed their mowing equipment and methods to protect birds. Onandaga Farms in Ontario uses a flushing bar attached to the mower to flush adult birds from the nest (MacKenzie and Kemp 1999). The Oklahoma Cooperative Extension Service recommends harvesting fields from the inside out, or by starting on one end and traversing back and forth until the other end is reached (Green 1997). These methods are intended to flush birds to previously cut areas. It is imperative to protect and enhance the largest contiguous tracts of grassland and pasture. Although still common (U.S. Fish and Wildlife Service 2002a), savannah sparrows are declining (Sauer et al. 1995) and recent experiences in Europe and the United States have shown that common grassland species have experienced dramatic declines when agriculture practices intensify (Peterjohn and Sauer 1999, Murphy 2003, Stoate 2001). Declines are due in large part to the transformation of pasture acreage and strip cover to row crop fields (Peterjohn and Sauer 1999, Koford and Best 1996), and the fragmentation of grassland (Warner 1994). Midwestern private pastures provide the most abundant grassland habitat (U. S. Department of Commerce 1994, Cunningham 2000) but farmers who are managing for grassland birds should protect the largest tracts possible because birds, such as the savannah sparrow, are area sensitive (Herkert 1994). Savannah sparrows rarely occurred on tracts <10 ha in Illinois although their territories are usually <1 ha (Herkert 1994). Opportunities for Cooperation Between Farmers and Citizens Fortunately, both producers and birds can benefit from protecting birds in pastures. The Conservation Security Program of the 2002 U. S. Farm Bill and a private “bird-friendly” labeling initiative are two ways citizens and farmers can act to help prevent further grassland bird population declines. Our research provides information on how grazing practices affect the nests and young of declining grassland bird populations, and informs the debate over Conservation Security Program rules regarding pasture management. The Conservation Security Program (CSP) of the 2002 U. S. Farm Bill was passed by congress as an entitlement program to “reward the best” farmers who practice sound conservation measures on their farm and “motivate the rest” of the farmers to do the same (U.S. Department of Agriculture 2004b). The CSP will be implemented soon, and has great potential to benefit grassland birds. Unlike past farm programs designed to protect natural resources, such as the Conservation Reserve Program, CSP targets “working” farmlands, including pastures. Each farmer entering the program must draw up a conservation security plan with a Natural Resources Conservation Service agent (U.S. Department of Agriculture 2004b). In response to declining populations of grassland birds, we recommend that the Natural Resource Conservation Service also work with local, regional, and national birding groups to assist farmers in monitoring grassland bird populations on their farms. Program money should be allocated to buy simple monitoring equipment, and provide some basic bird identification training. A project in the Netherlands encouraged farmers to mow around nests by paying them for clutches of birds bred on their land, and grassland bird breeding success increased significantly (Musters et al. 2000). The CSP could help prevent further declines by implementing a similar initiative in the United States. We encourage consumers to demand bird-friendly farm products as this is the best way for non-farmers to influence farming practices (Jim Ellis, Food Alliance Midwest, personal communication). A carefully monitored “bird-friendly” label could provide farmers a financial incentive for bird-friendly pasture stewardship. Murphy (2003) presents compelling evidence that grassland birds have declined regardless of where they over-wintered, and he argues that greater attention must be paid to grassland nesting habitat to prevent future population declines. The shade-grown coffee campaign is an excellent model of bird-friendly marketing. We suggest that northern birders and consumers work toward bird-friendly labels modeled on the shade-grown coffee campaign but that addresses issues of pasture management on grassland bird breeding grounds. The label could serve as a link between conservation-conscious birders and sustainable farmers in the region, or throughout the country. We hope farmers will utilize our research and the work of other grassland bird specialists to protect grassland bird habitat, and profit from it.
Educational & Outreach Activities
Our research team, lead by the graduate student PI, spent the time to build strong relationships between farmer cooperators, natural resource personnel and university researchers. The PI lead four, informal, multi-farm tours that included local residents, professors from the University of Minnesota, and DNR employees. Many morning chats instigated to ask where the cows would be “rotated to” next, ended with philosophical discussions about the future of farming, or an explanation of why a specific farming practice was chosen. One farmer cooperator put the team to work identifying the species of birds utilizing his small manure lagoon, and another assisted the team with nest searching. In August 2003 the research team held a picnic at Whitewater State Park for all cooperators. We all grilled local vegetables and meats and had a turnout of about 40 people. Some farmers leaving the picnic said that they would like to be informed about further opportunities to participate in on-farm research.
We collaborated with the Beginning Farmer and Agricultural Lender programs of the Land Stewardship Project to put on a Birds and Grazing Field Day on June 16th, 2004, at a cooperator’s farm. Approximately 35 people attended and evaluations were collected from 17 people. Evaluation results found that on average attendees traveled 80 miles to attend our event and they heard about it through mailed flyers (4), newspapers (3), email (8), word of mouth (5), and posted flyers (2). Five people reported learning “a lot” about grass-based dairying, nine learned “some,” and one learned “a little.” Nine learned “a lot” about grassland bird habitat, seven learned “some,” and 1 learned “nothing.” Fourteen said that it was “very likely” that they will use what they learned at the field day and five said that “maybe” they would use what they learned. Of all evaluators, two were dairy farmers, two were sheep farmers, three were beef cattle farmers, and one farmed with other livestock. An on-line article about the event can be found at: http://www.landstewardshipproject.org/pr/04/newsr_040618.html
In addition, our PI has made a number of presentations to diverse groups. These include:
Driscoll, Melissa A., John P. Loegering, and Vernon B. Cardwell. Reproductive Success of Grassland Passerines in an Agricultural Landscape: The Contribution of Alternate Grazing Systems to Bird Population Viability. Conservation Biology Seminar, University of Minnesota, St. Paul Campus. April 14, 2003. 60 Minutes.
Driscoll, Melissa A., John P. Loegering, and Vernon B. Cardwell. Reproductive Success of Grassland Passerines in an Agricultural Landscape: The Contribution of Alternate Grazing Systems to Bird Population Viability. Wildlife Extension Agents Annual Event, St. Paul Campus. April 24, 2003. 2 – 15 Minute presentations.
Driscoll, Melissa A., John P. Loegering, and Vernon B. Cardwell. Reproductive Success of Grassland Passerines in an Agricultural Landscape: The Contribution of Alternate Grazing Systems to Bird Population Viability. Society for Conservation Biology, Duluth, MN 28 June – 2 July 2003, 15 minutes.
Driscoll, Melissa A., John P. Loegering, and Vernon B. Cardwell. Reproductive Success of Grassland Passerines in an Agricultural Landscape: The Contribution of Alternate Grazing Systems to Bird Population Viability. Presentation to FW 5603W Habitats and Regulation of Wildlife. Instructor: Peter Jordan, University of Minnesota. November 4, 2003. 30 minutes.
Driscoll, Melissa A. Creating and Enhancing Natural Habitats on the Farm. Presentation to the tri-state Women in Sustainable Agriculture Annual Fall Gathering, Spring Valley, MN. November 7 – 9, 2003. 75 minutes.
Driscoll, Melissa, John, P. Loegering, Vernon B. Cardwell. Reproductive Success of Birds Nesting in Rotationally- and Continuously-Grazed Cattle Pastures in Southeast Minnesota. Given to CB Seminar March 22, 2004. 60 minutes.
Driscoll, Melissa. Reproductive Success of Birds Nesting in Rotationally- and Continuously-Grazed Cattle Pastures in Southeast Minnesota. Thesis defense given at the University of Minnesota, May 11, 2004. 60 minutes.
Driscoll, Melissa, John, P. Loegering, Vernon B. Cardwell. Reproductive Success of Birds Nesting in Rotationally- and Continuously-Grazed Cattle Pastures in Southeast Minnesota. Given at the International Society for Conservation Biology Conference, Columbia University, New York, New York, July 31, 2004. 15 minutes.
Big Woods Dairy Field Day, June 9, 2004, 10 minute discussion about bird requirements in a rotationally grazed system, at Nerstrand Big Woods state Park, Nerstrand, Minnesota.
Pastureland Field Day, August 18, 2004, five minute presentation about bird requirements in a rotationally grazed system, north of Mantorville, Minnesota.
In addition this work resulted in a Masters Degree thesis for our PI, Melissa Driscoll, titled: Reproductive success of savannah sparrows (Passerculus sandwichensis) and other grassland birds nesting in rotationally- and continuously-grazed cattle pastures in southeast Minnesota. This thesis is available by emailing Melissa Driscoll at firstname.lastname@example.org or email@example.com.
The labels “continuously-grazed” and “rotationally-grazed” cattle pastures describe cattle movement on a pasture, but do not indicate if the pasture management system is likely to provide habitat to support a stable savannah sparrow population. High daily survival rates for savannah sparrows are correlated with lower cattle density, higher average density of vegetation in a pasture, and the percent of cattle diet provided by the pasture. Because evidence points to an intensification of agriculture as the main reason for grassland bird population declines in the Midwest (Herkert 1994, Murphy 2003), a halt or reversal of that trend is likely to benefit grassland birds (and see Ormerod and Watkinson 2000). Intensive rotational grazing may make it feasible for producers to switch from intensified confinement dairying and beef-cattle feedlots, with their high demand for row-crops, to a system that better supports farmers and birds.
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Our research directly involved five farms (six pastures). In addition, two other farmers helped design the research. About 35 farmers, agency officials, and students attended our Birds and Grazing field day. The PI shared our research results with about 20 agency officials including Minnesota Department of Natural Resources Commissioner Gene Merriam during a field day at Big Woods Dairy in Nerstrand, Minnesota in June 2004. The PI also interviewed the farmers at Big Woods dairy and discussed their grassland management practices as it related to her grassland bird research. The PI spoke to 45 farmers at a Pastureland Dairy Cooperative field day near Rochester, Minnesota in August 2004.
We recommend that farmers implement the following practices to provide more source habitat for grassland birds in the Upper Midwest:
Maintain pastures at an average vegetation density (VOR) > 1 dm to leave enough vegetation for nesting birds.
Rest each rotationally-grazed paddock for a minimum of 30 days during the period from mid-May through mid-July, which coincides with the passerine nesting season.
Increase cattle use of pasture forage and decrease their use of hay and mixed grain rations during the growing season. This generally will encourage a greater understanding of pasture dynamics including nutrient management, water cycles, and cattle behavior on pastures.
Stock cattle at densities that promote strong forage health. Birds are dependent upon forage to hide their nest and provide habitat for insects that they feed to their chicks. Generally stock cattle at a lower density. Think of the tall grass as your insurance against drought.
Reduce or eliminate clipping from your pasture maintenance routine.
Time cutting and clipping to avoid the nesting season. If the pasture is cut for hay, cutting should be early, say at the end of May, so that the pasture can be left uncut in June to protect bird nests during the main nesting period.
Carefully monitor grass height and density to ensure adequate nesting habitat.
Monitor bird populations on your farm to determine how changes to your management system affect the number of grassland species and the quantity of birds on your farm
Additional recommendations are common in the literature regarding grassland birds and grazing or are common sense from a birder’s perspective.
Use a flushing bar. This can be attached to your mower and it flushes adult birds before the mower runs over the nest (Green 1997).
Fence off some part of the pasture for most or all of the nesting season (Undersander et al. 2000). Wait to graze, clip, or hay that area until July 20th (Herkert et al. 1996).
Clip or hay only a part of the pasture at one time so that there is a refuge for nesting birds (Undersander et al. 2000). Try haying from the inside of a pasture out to help birds escape from the mower blades (Green 1997).
Hay or graze more intensively near trees, roads, and shrubby areas first and let the center of your grassland (the better nesting habitat) go longer between mowing or grazing.
Walk out to the pasture instead of driving a vehicle.
Resist letting your dog come out to the pasture with you, and keep your house cat in the house.
Plant some pastures to warm season forage plants. This will even out the forage available throughout the summer (Blanchet et al. 2003), and if left ungrazed will provide nesting habitat in the spring and early summer.
Keep grazing, and encourage others to graze. Grassland bird habitat is decreasing across the country and each additional pasture can provide more habitat.
Areas needing additional study
More research is needed to determine how the landscape mosaic of plowed fields, pastures, CRP, woodland and wetlands in farm country support or endanger dwindling grassland bird populations. It would be useful to have a larger study that includes more pastures and categorizes pastures by the landscape that surrounds them. Then researchers could determine at what habitat scale reproducing birds are affected by pasture management.
In addition, it would be helpful to know how grazing for birds and forage health affect the economic bottom line for farmers in this region. If grass farmers can capitalize on a bird-friendly label there is a possibility of recouping funds lost by allowing forage densities to remain high throughout the grassland bird nesting season (May and June). Perhaps the agronomic benefits of taller forage (i.e. less winter forage die-off, less need for interseeding, higher resistance to drought conditions) pay for bird-friendly farming without a bird-friendly marketing label.
It would be extremely helpful to more carefully define the adult and juvenile survival rate for savannah sparrows and other grassland bird species to assist in determining source/sink population dynamics in a region.