Fitness components of avian migration: A dynamic model of Western Sandpiper migration

Western Sandpipers, Calidris mauri, one of the world's most abundant shoreb ird species, migrate from winter sites in the southern USA and Central and South America to breeding grounds in Western Alaska and Eastern Siberia. We describe a dynamic state variable optimization model for these migrations, assuming that individual female sandpipers employ migration strategies tha t maximize their expected lifetime reproduction. The principal environmenta l factors assumed to affect migration decisions are variable wind speeds, s ite-specific predation risks, and the timing of food availability on the br eeding grounds and at the two most northerly stopover sites. The model's pr edictions, which agree closely with data collected in the field, are most s ensitive to changes in wind conditions during the flight phase, rather than foraging opportunities at stopover sites en route. We also show how the mo del can be used to assess potential impacts of habitat degradation on the f itness of this species.