Flying the gantlet: Population characteristics, sampling bias, and migration routes of Eared Grebes downed in the Utah desert

Abstract

In January 1997, an estimated 35,000 Eared Grebes (Podiceps nigricollis), about 3% of the population that stages at Great Salt Lake, Utah, were downed by snowstorms while migrating between Great Salt Lake and Mexico. Another 920 grebes crashed on the northbound migration in late March 1997. We examined 1,500 carcasses to determine the characteristics of the population during fall versus spring migration with regard to age and sex ratios, body mass, plumage abnormalities, presence of parasites, and morphological defects. In many cases, large samples derived from mass downings provide a reasonable index of the composition of birds aloft. However, we found important differences in sex ratios between two samples derived from the same flight, which indicates that assumptions regarding unbiased sampling require testing. The possibility that regular catastrophic events in central and southern Utah have influenced the evolution of migration routes in Eared Grebes deserves further consideration.