Oiii Living Ri'.\tniivt'.\ — BInl.s 



63 



10,000 and 100.000 individuals for most popu- 

 lations, but number from as few as 25 birds for 

 the endangered Eskimo eurlew to about 

 500,000 for the Pacific race of the dunlin 

 (Calidris alpina pacifica). A few other species 

 for which some data are available, such as west- 

 em sandpiper and Wilson's phalarope, have 

 populations that exceed a million (Page and Gill 

 1994). 



Population Trends 



For most species, reliable quantitative data 

 on population trends are either not available or 

 too recent to assess trends. Assessment of long- 

 term population trends is based largely on his- 

 torical accounts of relative abundance and dis- 

 tribution and knowledge of habitat alteration 

 within breeding and wintering ranges. 

 Nonetheless, populations of several species of 

 western North American shorebirds have 

 declined significantly over the past 150 years 

 (Page and Gill 1994). One Arctic breeder, the 

 Eskimo curlew, is on the verge of extinction 

 (Gollop et al. 1986: Alexander et al. 1991). 

 Conversion of native grasslands for agriculture, 

 loss of wetlands, and market hunting before the 

 turn of the century have been attributed as fac- 

 tors primaiily responsible for these declines. No 

 species is known to have increased in overall 

 population size over this period. 



Information on more recent population 

 trends comes primarily from the North 

 American Breeding Bird Survey (BBS), a sys- 

 tem of roadside surveys designed primarily to 

 monitor populations of breeding landbirds. The 

 BBS does not sample most western shorebird 

 breeding populations very well because of its 

 sporadic coverage, poor sampling of wetland 

 habitats, and lack of coverage of the most 

 important shorebird breeding grounds in the 

 Arctic, which are roadless. Despite these limita- 

 tions, BBS does provide valuable trend infor- 

 mation, particularly for grassland species in the 

 temperate zone. Additional information on pop- 

 ulation trends can also be obtained from surveys 

 that target species of concern, such as the snowy 

 plover (Page et al. 1991), or particular habitats 

 of concern, such as the Arctic Coastal Plain of 

 Alaska (D. Troy Ecological Assoc, and British 

 Petroleum Exploration, unpublished report; 

 Andres 1994). 



Recent survey data show a mixture of 

 declining, increasing, and apparently stable 

 population trends (Table). Over the past 25 

 years, western populations of willet and upland 

 sandpiper appear to have been rebounding (J. 

 M. Sauer and S. Droege, unpublished data). 

 Numbers of several other species, such as the 

 black-necked stilt, marbled godwit, and spotted 

 sandpiper, appear to have stabilized (J.M. Sauer 

 and S. Droege, unpublished data). Western pop- 



'J.<S^ 



• 1.000-10,000 



• 10,000-100,000 

 # 100,000-1,000,000 



ulations of several other species, however, have 

 significantly declined over the past 25 years, 

 including the snowy plover, killdeer, mountain 

 plover, American avocet, long-billed curlew, 

 common snipe, and Wilson's phalarope (Table). 

 Such relatively short-term trends among wet- 

 land species are difficult to interpret, however, 

 as they may reflect changes in distribution in 

 response to drought conditions rather than 

 absolute declines in population size (Page and 

 Gill 1994). 



Most changes in populations appear linked 

 to habitat alteration. For example, since 1970 

 the snowy plover, heavily dependent on coastal 

 habitats, has disappeared as a breeding species 

 from over 60% of its historic California nesting 

 sites (Page and Stenzel 1981). Introducing 

 plants to stabilize sand dunes, increasing recre- 

 ational use of beaches, and heavy nest predation 

 by feral foxes threaten to reduce coastal popula- 

 tions further (Page and Gill 1994). Fluctuating 

 water levels in interior wetlands result in unpre- 

 dictable changes in availability of nesting habi- 

 tat away from the coast (Page et al. 1991). The 

 breeding range of the mountain plover has con- 

 tracted markedly in several western states and 

 the continental population has declined signifi- 

 cantly during the past 25 years, probably 

 because of habitat degradation on wintering 

 grounds in central and southern California 

 (Knopf 1994; RL. Knopf, NBS, unpublished 



Fig. 2. Location of important 

 staging areas in we.stem North 

 America used by shiorebirds dur- 

 ing spring and fall migration. Size 

 of dot indicates the estimated peak 

 number of shorebirds at each site. 



