BONIER, F.; MARTIN, P.R.; RAMENOFSKY, M.; WINGFIELD, J.C.; University of Washington; University of Washington; University of Washington; University of Washington: Departure Biology of an Arctic-breeding Passerine

In the fall, migratory birds prepare to depart the Arctic for southerly clines to overwinter. Whereas a great deal is known of the preparatory stages for spring (northward) migration, relatively little is known about fall migration, particularly for populations breeding in the most northern range of their distribution. We explored fall migratory life history stage in Gambel�s White-crowned Sparrows at Toolik Lake, Alaska (longitude W149.6, latitude N68.6). The spring migratory life history stage is initiated by the prealternate molt and fattening both of which are induced by spring photoperiods. The molt is restricted to replacing feathers on the head and body. In fall, the daylength is decreasing and molt is a much more extensive involving replacement of feathers as well a components of skin, blood, and bones. These differences provide a unique comparison of the two migratory life history stages. We document changes in fat scores, mass, molt, and plasma levels of corticosterone from the conclusion of breeding through the prebasic molt and migratory fattening in preparation for departure. In the field, birds did not fatten until molt was nearing completion. Our results provide new insight into the preparation for fall migration in a population located at the extreme northern edge of its geographic range.