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Management of Northern Prairies and Wetlands for the Conservation of Neotropical Migratory Birds

Bird Populations of the Northern Prairies

A large number of bird species breed in the northern plains. The avifauna includes species of boreal, eastern, southern, and western affinities (Stewart 1975, Johnsgard 1979). Most species are more common elsewhere; I emphasize species for which the area is important because it supports a significant proportion of the species' population. I concentrate mostly on neotropical migrant landbirds, although some short-distance migrants are also included for completeness and comparison (table 1).

Stewart (1975) classified the breeding birds of North Dakota according to their biogeographical affinities (table 2). Of the 190 species included, 56 (29 percent) were associated with the north-central, mixed-grass avifauna. Those 56 species made up 80 percent of the total breeding bird population in 1967 (excluding exotic species).

I suggest species deserve special attention in the northern prairies if a significant portion of their population breeds in the area and they meet any of the following other criteria: (1) their breeding range is small, (2) their total (continental) population is small, (3) they have declined in number or contracted in geographic range, (4) they are restricted to a narrow range of habitats, especially if those habitats are threatened, or (5) there is some major potential threat to their population. The most compelling reason for emphasizing any particular species in an area is that the area supports a substantial portion of the continental population of the species. That point may seem obvious, but considerable management attention is directed toward species in insignificant portions of their range (as noted, for example, by Knopf 1992). This effort may be appropriate if such peripheral populations are genetically distinct from central populations and offer greater potential for adaptation to changing environments (Lesica and Allendorf 1995), but that situation is unlikely to hold for widely dispersing migratory birds. With this perspective, the scheme used by Partners In Flight provides a prioritization scheme for landbirds, based on perceived threat of extinction (Hunter et al. 1993). Alternatively, a focus on endemic species (e.g., Knopf 1988) is valuable but may miss some species that are not endemic but in need of attention.

By such criteria, species such as Baird's sparrow and Sprague's pipit, which have small populations and whose breeding ranges are restricted to the northern Great Plains, deserve more attention in that area than do species such as the brown thrasher and yellow warbler. The latter species also breed in the northern Great Plains, but their distributions are far broader, and they are more common elsewhere. Even in the Plains, they can use artificial habitats such as shelterbelts and suburban plantings, which are increasingly common. Conversely, the Baird's sparrow and Sprague's pipit require grassland, the natural habitat on the northern Plains.

Status and Trends

The mid to late 1960's is a convenient reference point for the status of bird populations in this region. The Breeding Bird Survey (BBS) began in 1966 and became operational in the region in 1967. Also, a statewide survey of North Dakota birds was conducted in 1967 (Stewart and Kantrud 1972); a repeat of that survey in 1992 and 1993 provides a useful contrast (Igl and Johnson 1995b). Considering population changes during the last 25 years or so can be misleading, however. Most of the major changes in habitat in the northern prairies occurred after settlement by Europeans but before the 1960's, and associated changes in bird populations were not tracked by BBS or other programs. Our knowledge of bird populations prior to European settlement is weak, based on comments by early explorers and settlers or inferred from current bird use of habitats that have not been altered dramatically.

Early reports mention huge numbers of waterfowl, shorebirds, and other birds (e.g., Dinsmore 1994). The reports lack quantification, and it is questionable whether low numbers or absences would be reported as faithfully as extreme abundances. Nonetheless, many of the accounts describe grassland birds in numbers unheard of today.

BBS results indicate that during 1966-1991, grassland-nesting birds had a higher proportion of declining species than did any other avian guild in North America (Droege and Sauer 1994, Knopf 1994). BBS trends for the 1966-1994 period are given in table 1 for the Central Region, roughly the area between the Rocky Mountains and the Mississippi River.

From 1966 to 1994, significant decreases outnumbered significant increases by four species to two among neotropical migrants, and by five species to two among temperate migrants (table 1). During the more recent period (1980-1994), significant declines outnumbered significant increases by four to one for neotropical migrants and matched them at five to five among temperate (short-distance) migrants. Declines were consistent in both early (1966-1979) and late (1980-1994) periods for only the grasshopper sparrow, bobolink, and Baird's sparrow, among neotropical migrants, and for northern harrier, horned lark, loggerhead shrike, and lark sparrow, among temperate migrants. Increases were consistent in both time periods for the neotropical migrants, Swainson's hawk, upland sandpiper, and eastern kingbird; for the temperate migrants, ferruginous hawk, vesper sparrow, swamp sparrow, and McCown's longspur; and for the permanent resident sharp-tailed grouse.

Populations of temperate migrants in North Dakota in 1992-1993 did not differ consistently from those in 1967 (Igl and Johnson 1995b; table 3). Numbers of long-distance migrants increased, however, and those of permanent residents more than doubled from the early to the recent period. Examining bird populations by primary breeding habitat, Igl and Johnson (1995b) concluded that species that rely on trees (open habitat with trees, woodland and woodland-edge, and residential-generalist) consistently increased from 1967 to 1992-1993 (table 3). Trends for groups of species associated with other habitat types were not evident.

Such groupings, however, can disguise changes occurring to particular species. Among the grassland birds, numbers of chestnut-collared longspurs, western meadowlarks, savannah sparrows, and Baird's sparrows declined by 39 percent or more; clay-colored sparrows and bobolinks declined at lesser rates (table 4). Horned lark and lark bunting numbers varied without a trend, likely due to changes in precipitation during the study years. Counts of vesper sparrows and upland sandpipers increased by more than 50 percent.

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