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Bohrer et al 2011 Eagle & Vulture Ecology Letters Paper

Estimating updraft velocity components over large spatial scales: contrasting migration strategies of golden eagles and turkey vultures.

Soaring birds migrate in massive numbers worldwide. These migrations are complex and dynamic phenomena,
strongly influenced by meteorological conditions that produce thermal and orographic uplift as the birds
traverse the landscape. Herein we report on how methods were developed to estimate the strength of thermal
and orographic uplift using publicly available digital weather and topography datasets at continental scale. We
apply these methods to contrast flight strategies of two morphologically similar but behaviourally different
species: golden eagle, Aquila chrysaetos, and turkey vulture, Cathartes aura, during autumn migration across eastern
North America tracked using GPS tags. We show that turkey vultures nearly exclusively used thermal lift,
whereas golden eagles primarily use orographic lift during migration. It has not been shown previously that
migration tracks are affected by species-specific specialisation to a particular uplift mode. The methods
introduced herein to estimate uplift components and test for differences in weather use can be applied to study
movement of any soaring species.