DENSITY-DEPENDENT MASS GAIN BY WILSON'S WARBLERS DURING STOPOVER

Abstract

Abstract The need restore energetic reserves at stopover sites constrains avian migration ecology. To describe that constraint, we examined relationships among mass gained by Wilson's Warblers (Wilsonia pusilla) during stopover, abundance of Wilson's Warblers (i.e. capture rate), and arthropod abundance during autumn migration. We found that amount of mass gained by Wilson's Warblers during stopover declined as a function of their abundance. We found no relationship between mass gained by Wilson's Warblers and arthropod abundance. Abundance of Wilson's Warblers was positively associated with arthropod abundance. These patterns are consistent with the hypothesis that mass gain during stopover is density dependent. They also suggest that Wilson's Warblers respond numerically to abundance of food at that stopover site. This numerical response may override any functional response and thereby account for lack of a relationship between mass change and arthropod abundance.