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Background and Justification

Grasslands are recognized by many as the most imperiled ecosystem worldwide. The avian assemblages associated with grasslands also are at risk - grassland bird populations have shown steeper, more consistent, and more geographically widespread declines than any other guild of North American species (Department of the Interior 1996). Breeding Bird Survey data from 1966-1993 indicate that almost 70 percent of 29 grassland bird species adequately surveyed by BBS data had negative population trends; more than half of these were statistically significant.

In addition to range-wide population declines, the distribution and abundance of many grassland species are highly variable in space and time, which complicates conservation plans for grassland bird species. Within a particular geographic location, numbers of grassland birds may fluctuate dramatically from year to year. For example, densities of Le Conte's Sparrow (Ammodramus leconteii) breeding in Conservation Reserve Program fields in Eddy County, ND, increased from 0.03 to 21.47 pairs per 100 ha from one year to the next (Igl and Johnson 1995). At both local and range-wide scales, variation in numbers from year to year may be driven by 1) climate patterns, which may significantly alter vegetation characteristics of the site and hence habitat cues used by birds in selecting breeding territories (Price 1995, Igl and Johnson 1995); 2) changes in the habitat caused by management actions or natural disturbances; or 3) success in raising young at that location in previous years, which may influence return rates and hence population stability at a site. The relative importance of each of these factors has not been well established for grassland species, yet such knowledge is crucial to understand patterns of range-wide population declines and local-scale fluctuations in grassland bird species.

In an attempt to reverse population declines of grassland birds, the management concept of Bird Conservation Areas (BCA's) was suggested as a means to conserve grassland songbird populations (Pashley and Fitzgerald 1996). The notion behind BCA's is that core areas of quality habitat (such as native prairies) that are isolated from hostile habitats (such as woody vegetation) will result in reproductive rates sufficient to maintain population levels of breeding birds (Henderson and Sample 1995). An example of a BCA is given below. The BCA concept implies that the value of high-quality core areas depends on the habitat composition of the landscape matrix in which the core areas are embedded. This concept is being promoted despite the absence of data that validate its usefulness in maintaining viable populations of grassland songbirds.

Figure 1. An example of a Bird Conservation Area, consisting of a core area with high-quality habitat surrounded mostly by neutral habitat and largely isolated from hostile habitat.

Moreover, the U.S. Department of the Interior (DOI) Conservation Strategy for declining birds in grassland ecosystems (DOI 1996) calls for information on effects of habitat and landscape features on population viability of grassland birds. High-priority information needs identified by the DOI include effects of habitat structure and composition on avian communities, and effects of landscape context (e.g., block distribution, surrounding land use, and proximity to hostile environments) on avian numbers and productivity. Furthermore, factors associated with highly variable population numbers (climate, habitat changes, nesting success) are needed to determine causes of population stability or instability over time. This information is critical for developing long-term conservation objectives that will benefit grassland birds, but is lacking for many grassland bird species.

The BCA concept was proposed by the Midwest Working Group of Partners In Flight and supported also by the Prairie Pothole Joint Venture. It was included in the draft of the Landbird Conservation Plan for Physiographic Area 40: the Northern Tallgrass Prairie. This evaluation of the BCA concept is intended to determine whether BCA's do, in fact, meet their intended objectives. The effort addresses needs identified in that Landbird Conservation Plan by evaluating its assumptions.

For this evaluation, we are considering native prairie (parts of which may have been restored) to be high-quality habitat; heavily wooded vegetation, which can harbor large numbers of predators and brood parasites, to constitute hostile habitat; and small-grain and hay fields as examples of neutral habitats.

Objectives

1. To estimate distribution, abundance, and reproductive success of grassland bird species in large and small core habitats embedded within neutral and hostile landscape matrices.
2. To estimate site fidelity (both within-year and between-year) of grassland songbirds and factors that influence them.

Study Areas

In 1998 the study was conducted in two regions in the northern tallgrass prairie, one east of Moorhead, MN, in Becker, Mahnomen, and Clay counties, and the other east of Crookston, MN, in Polk County. Study areas included tracts owned by the U.S. Fish and Wildlife Service, Minnesota Department of Natural Resources, and The Nature Conservancy (Table 1). In 1999 we will add a third study area in southeastern North Dakota, in the environs of the Sheyenne National Grassland.

Methods

Study Design

We are using a two-way factorial experimental design that will address three major questions: 1) Does size of core habitat influence density and nesting success of birds? 2) Does landscape matrix (extent of woody vegetation surrounding the core habitat) influence density and nesting success of birds? and 3) Do landscape matrix and size show interactive effects? Main effects in the design are habitat size and landscape matrix, with several replicate plots within each size*landscape combination. In 1998, bird abundance was assessed on 34 study plots. All study plots were within native or restored prairie of similar vegetation structure and composition, and varied between 1 and 16 ha in size. Nesting success was investigated in 21 of the 34 study plots used for censuses (Table 1). With the addition of the southeastern North Dakota area in 1999, the number of study plots will increase by at least 12.

In total, 18 plots have been established within core areas that are "small" in size (<50 ha), and 16 plots have been established within core areas that are "large" in size (>250 ha; see Fig. 2 below). We searched for nests in 12 study plots within small core areas, and in eight study plots within large core areas. The number of study plots in small core areas was higher than in large core areas to compensate for the generally smaller study plots in small core areas. Differences in abundance and reproductive measures between large and small core areas will be referred to as "main effects of size." Fifteen plots (eight small and seven large) have been established within hostile landscapes (see Fig. 2 below). Hostile landscapes include landscapes that contain large areas of woodland habitat within 5 km of the core habitat. Nineteen plots (10 small and nine large) have been established within neutral landscapes. Neutral landscapes include landscapes that consist of habitats that are thought to have little or no impact on bird populations within the core areas, such as small-grain fields, hay-meadows, or Conservation Reserve Program fields. Differences in abundance and reproductive measures between hostile and neutral landscapes will be referred to as "main effects of landscape composition." Interactive effects between core habitat size and landscape matrix will also be examined.

Figure 2. Study design, and number of study plots in each category.

Field methods

On each study plot, we measured vegetation and bird abundance. Nesting success was investigated on a subset of the study plots (Table 1). Study plots were marked with wooden lath or flags at 50-m intervals along transects that were 100 m apart. Vegetation was assessed at 10-34 measuring points within each study plot, systematically located throughout each plot. The number of measuring points taken within a plot varied with the size of the study plot. Vegetation was measured once, between mid and late July. Measurements included vegetation height, percentage cover by growth form (grass, forb, woody, bare ground, litter, and standing residual) based on a 20×50 cm Daubenmire frame, height-density (Robel readings), numbers of small (<30 cm) and large (>30 cm) woody stems, and litter depth.

Abundance of breeding birds of all species was determined on each study plot by strip-transect censuses (Stewart and Kantrud 1972). Censuses were conducted between June 1 and July 3 by MW, and were repeated once during each breeding season. The maximum number of birds was used to determine density (number of males/100 ha).

Reproductive success of birds was assessed by nest searching and monitoring eggs and young until fledging. Nests were located by observers walking through fields and looking for nests after flushing or observing birds. Stick-sweeps and rope drags were also employed. Nests were marked with a flag 5 m to the north, and were revisited every 3 days to ascertain its status and the incidence of brood parasitism. Nest success was determined by using the Mayfield method (Mayfield 1961). A nest was considered successful if it fledged at least one young of the parental species, and it was considered parasitized, if it contained at least one Brown-headed Cowbird egg or chick.

Nest vegetation was characterized within one week after activity at a nest had ceased. Vegetation was measured at five areas near each nest: directly at the nest and at a distance of 0.5 m from the nest in each cardinal direction. At each of the five points we measured vegetation in the same manner as described above for plot vegetation. Vegetation characteristics at the nest were evaluated to determine the associations between microhabitat (vegetation), local (patch size), and landscape features and reproductive success by species.

Four of the 34 study plots (two plots in large core areas surrounded by neutral landscape, and two plots in small core areas surrounded by hostile landscape) were designated as intensive sampling plots. On these we captured and marked birds to assess factors associated with population stability at a local site over time. Three species—Bobolink (Dolichonyx oryzivorus), Clay-colored Sparrow (Spizella pallida), and Savannah Sparrow (Passerculus sandwichensis)—were captured in mistnets, leg-banded, color-banded, and monitored on each of the four plots. The goal of the intensive-sampling plots is to evaluate the number of young fledged per year and site fidelity for each adult of the three focal species, as well as factors associated with population stability (e.g., climate, reproductive success, change in habitat). Site fidelity will be measured in terms of returning to a site and in terms of returning to the same territory. These will be measured both between years and within years (for birds that move territories within a season).

Results

We recorded 41 species of birds on our study plots (Table 2). The four most common species were Savannah Sparrow, Bobolink, Le Conte's Sparrow, and Clay-colored Sparrow. Greater Prairie-Chicken, Upland Sandpiper, Marbled Godwit, and Northern Harrier were the least common grassland-nesting species, except for theHenslow's Sparrow, which was detected only once on a single plot. Three grassland-nesting species, Henslow's Sparrow, Greater Prairie-Chicken, and Northern Harrier were found only on large neutral plots. Species abundance differed slightly between southern and northern regions. Clay-colored Sparrows were more abundant in the southern region, Savannah Sparrows were more abundant in the northern region, and Grasshopper Sparrows were found in the southern region only.

We found 293 nests of 19 species (Table 3). Ninety-six nests were located by using the stick-sweep method, 72 were found by observing adults, 62 nests were located by haphazardly walking throughout the prairie and flushing birds off their nests, 13 nests were found by rope-dragging, and 50 nests were found incidental to other activities such as walking to study plots, measuring vegetation, or censusing. Although we generally restricted nest searching to the study plots, we did include 27 nests found outside the plot that were in comparable habitat.

Most of the nests found were of three grassland-nesting species, Savannah Sparrow, Clay-colored Sparrow, and Bobolink (Table 3). We did not find any nests of either Bobolink or Le Conte's Sparrow in small hostile plots. This pattern might reflect the lower density of these species in small hostile plots, or it might be caused by lack of females in these areas. More intense behavioral observations in the next years of the study will provide a better understanding on the reasons for this observed pattern. Mayfield estimates of nest success varied greatly among species and among plots. It appeared that ground-nesting species such as Savannah Sparrow, Le Conte's Sparrow, and Bobolink were less affected by patch size and landscape configuration than was the above-ground nester (Clay-colored Sparrow). This pattern is consistent with findings of Winter (1998) in southwestern Missouri. However, a single year of study is inadequate to allow any conclusions or management recommendations.

Cowbird parasitism was relatively low, with 18.2% of all passerine nests (40/220) being parasitized. Parasitized nests had on average one less host egg than unparasitized nests. Further, 30% of all parasitized nests (12/40) failed completely due to removal of all host eggs or host young by adult or young cowbirds.

Between May 28 and July 24 we color-banded 237 birds: 169 Savannah Sparrows, 52 Clay-colored Sparrows, and 16 Bobolinks. However, we found only 13 nests of banded birds: 11 of Savannah Sparrows and one each of Bobolink and Clay-colored Sparrow.

Discussion and Future Plans

The first year of data collection demonstrated the feasibility of our study, but also identified several adjustments that will improve the study.

Density estimates varied greatly among study plots. Additional census plots will give a more reliable understanding of the distribution of species across areas with different patch sizes and landscape configurations. The addition of a third study area in southeastern North Dakota will increase the generality of our results.

Nesting success estimates also were highly variable since the sample size of nests for each species was relatively low. Most nests found were of Savannah Sparrows, Clay-colored Sparrows, and Bobolinks. Therefore, we will focus nest searching on these three species with the goal of increasing sample size by developing a more specific search image. During incubation, nest searching will focus on stick sweeping; rope-dragging will be discontinued. Behavioral observations will be mainly used for nest finding during times when nestlings are being fed. Further, we will conduct more intensive behavioral observations of Bobolinks and Le Conte's Sparrows in small hostile study plots to determine if females are present in these areas.

During the first field season our main goal on the intensive sampling plots was to learn the most efficient technique for mistnetting grassland-nesting birds. Finding nests of color-banded birds, and estimating within-year site fidelity were of lower priority. In the following years, a separate crew will focus on the four intensive sampling plots. This will give them more time for mapping territories, searching for nests, and observing behavior in each plot. In addition to the three focal species, we will also color-band Le Conte's Sparrows. This species was frequently caught but released during the last field season.

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Acknowledgments

Primary funding for the first year of the evaluation was provided by the U.S. Fish and Wildlife Service, Region 3, and the U.S. Geological Survey, Biological Resources Division, through Northern Prairie Wildlife Research Center. Additional funding was provided by the U.S. Fish and Wildlife Service, Region 6, and by the Minnesota Nongame Wildlife Tax Checkoff, the Reinvest In Minnesota Program, and the Minnesota Chapter of The Nature Conservancy through the Minnesota Department of Natural Resources.

Access to study areas was kindly granted by The Nature Conservancy, U.S. Fish and Wildlife Service, and Minnesota Department of Natural Resources (Division of Fish and Wildlife and Division of Parks and Recreation).

We appreciate efforts made by the field assistants: Aaron Brees, Mary Frances Cullen, David Lehmann, Liz Meyers, Les Murray, Monte Sanford, Dan Scheiman, and Laurie Walter. Judd Brink very generously served in a volunteer capacity throughout the field season. Molly Henke and Rolf Koford helped to get the mistnetting started, and Peter Jones was a tremendous help in mistnetting later in the season. Andruss Northrup and Elizabeth Northrup volunteered for part of the field season. Mia Sondreal provided valuable temporary assistance. Jeremy Engelstad helped establish study plots and take GPS-readings.

We are indebted for generous assistance with housing and other logistics to Mary Soler, Paul Soler, and Rick Julian, of the U.S. Fish and Wildlife Service, and Brian Winter, Sonia Winter, and Gordon Yalch of The Nature Conservancy.

We are grateful to Stephen J. Lewis of the U.S. Fish and Wildlife Service for his encouragement and support of this evaluation.

Jane Austin, David Fellows, and Pam Pietz of the Northern Prairie Wildlife Research Center graciously loaned equipment and supplies. Betty Euliss assisted the project in numerous ways.

Cover photo of Le Conte's Sparrow was taken by Ron Batie, Northern Prairie Wildlife Research Center.

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Literature Cited

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