Area requirements of grassland birds: A regional perspective

Auk, The, Jan 2001 by Johnson, Douglas H, Igl, Lawrence D

ABSTRACT.-Area requirements of grassland birds have not been studied except in tallgrass prairie. We studied the relation between both species-occurrence and density and patch size by conducting 699 fixed-radius point counts of 15 bird species on 303 restored grassland areas in nine counties in four northern Great Plains states. Northern Harrier (Circus cyaneus), Sedge Wren (Cistothorus platensis), Clay-colored Sparrow (Spizella pallida), Grasshopper Sparrow (Ammodramus savannarum), Baird's Sparrow (Ammodramus bairdii), Le Conte's Sparrow (Ammodramus leconteii), and Bobolink (Dolichonyx oryzivorus) were shown to favor larger grassland patches in one or more counties, Evidence of area sensitivity was weak or ambivalent for Eastern Kingbird (Tyrannus tyrannus), Common Yellowthroat (Geothlypis trichas), Savannah Sparrow (Passerculus sandwichensis), and Western Meadowlark (Sturnella neglecta). Red-winged Blackbirds (Agelaius phoeniceus) preferred larger patches in some counties, and smaller patches in others. Mourning Doves (Zenaida macroura) and Brown- headed Cowbirds (Molothrus ater) tended to favor smaller grassland patches. Three species showed greater area sensitivity in counties where each species was more common. Five species demonstrated some spatial pattern of area sensitivity, either north to south or east to west. This study demonstrates the importance of replication in space; results from one area may not apply to others because of differences in study design, analytical methods, location relative to range of the species, and surrounding landscapes. Received October 13, 1999, accepted August 8, 2000.

THE CENTRAL GRASSLANDS were once North America's most extensive ecosystem, although they have now been brought under submission by settlement. The eastern tallgrass prairie has been nearly completely replaced by intensive agriculture; less than 1% of the original tallgrass prairie remains in most states and provinces (Samson and Knopf 1994). Although the western shortgrass prairie has experienced fewer losses, native grazers such as bison (Bison bison) and prairie dogs (Cynomys spp.) have been largely replaced by domestic cattle, which exhibit very different grazing behaviors and have different effects on vegetation (Peden et al. 1974, Schwartz and Ellis 1981). The mixedgrass prairie, lying between the tallgrass and the shortgrass prairies, has suffered from similar influences.

Associated with that massive scale conversion of prairie has been a concomitant change in communities of birds and other animals that rely on grassland habitats. Historical accounts tell of rich abundances of prairie wildlife that now can be only imagined (e.g. Dinsmore 1994). Widespread and systematic surveys of most bird species did not begin until the mid 1960s, with the advent of the North American Breeding Bird Survey (BBS; Robbins et al. 1986). Thus, quantitative evidence of changes in grassland bird populations exists for only the past 30 years or so, well after most grassland losses occurred. Nonetheless, the BBS indicates that many grassland birds have fared poorly, even during that period. BBS results indicate that, during 1966-1996, grassland-nesting birds had the lowest proportion of species with increasing population trends than of all avian guilds in North America (Peterjohn and Sauer 1999). In North Dakota between 1967 and 1992-1993, numbers of Baird's Sparrows (Ammodramus bairdii), Savannah Sparrows (Passerculus sandwichensis), Chestnut-collared Longspurs (Calcarius ornatus), and Western Meadowlarks (Sturnella neglects) declined by 39% or more; Clay-colored Sparrows (Spizella pallida) and Bobolinks (Dolichonyx oryzivorus) declined at lesser rates (Igl and Johnson 1997).

Although it is convenient to attribute declines of grassland bird populations to losses of native grassland, habitat loss is but the first of three main concerns involving the breeding grounds. A second issue is degradation of remaining tracts of grassland because of improper or inadequate management, disruption of natural disturbance regimes, encroachment of woody and exotic vegetation, and other factors.

In addition to the loss of grassland and the inadequate management applied to extant grassland, a third issue is fragmentation. Habitat fragmentation involves separation of large, contiguous areas of habitat into smaller patches that are isolated from one another. Three types of fragmentation effects can be distinguished: patch-size, edge, and isolation (e.g. Wiens 1995, Johnson and Winter 1999). Patch-size effects are those that cause use or reproductive success to differ among habitat patches of different sizes. Some patch-size effects may be induced by edge effects-phenomena such as avoidance, predation, competition, or brood parasitismthat operate at different levels near a habitat edge than in the interior of a habitat patch. In addition, isolation from similar habitat can affect use of a particular patch by influencing dispersal or habitat use. Here we deal only with effects of fragmentation on occurrence or density of birds using a habitat patch; see Johnson and Winter (1999) for a more general review.