Facultative Extension of Fall Migration by Yellow-Rumped Warblers (Dendroica coronata)

Abstract

Abstract Many nocturnally migrating birds show at least some degree of winter site fidelity. Laboratory tests of nocturnal activity, or Zugunruhe patterns, and displacement experiments indicate that migratory distances may be under endogenous control. Yellow-rumped Warblers (Dendroica coronata auduboni) displayed a southerly nocturnal orientation well into winter months, past the conventional migratory period. Additionally, population shifts between highly isolated, limited, desert riparian habitats reflected a facultative migration from north to south. The magnitude of movement was correlated with food-resource abundance, which was dependent on climatic conditions. This suggests that the birds maintain a physiological state that enables them to winter as far north as possible but allows for a correctly oriented movement if conditions become unfavorable. Our results indicate considerable plasticity in what is generally considered a more rigidly fixed process.