Survey trends of North American shorebirds: population declines or shifting distributions?

ABSTRACT

We analyzed data from two surveys of fall migrating shorebirds in central and eastern North America to estimate annual trends in means per survey and to determine whether trends indicate a change in population size or might have been caused by other factors. The analysis showed a broad decline in means per survey in Atlantic Canada and the northeastern United States (North Atlantic region). For example, 9 of 9 significant trends in this region were <1 (P=0.004), and the mean, annual rate of change among 30 species was 0.9783, a decline of −2.17% per year (P<0.001). Trends in the midwestern United States (Midwest region) showed no clear pattern. The mean among 29 species was 1.0090 (P=0.35). Only 4 of the trends were significant. Several hypotheses were evaluated to identify causes of the declining means per survey in the North Atlantic region. The most likely hypothesis appears to be a decline in the breeding populations that supply migrants to the North Atlantic region, but a change in movements, for example passing through the region more quickly in recent years, cannot be excluded as an explanation. Further surveys of arctic breeding areas coupled with analysis of long-term survey data from western North America would be helpful in determining whether the declines found in this analysis are also occurring in other areas.