ABSTRACT
Abstract. Climate change is having a dramatic effect on many migratory species. Changes in climate may lead to changes in food availability or other proximate cues that affect migratory behavior. We used 13 years (2000-2012) of data on songbird banding and raptor migration counts and captures during autumn migration in the intermountain West to evaluate whether regional temperature or precipitation or hemispheric climate indices predicted autumn migratory timing and energetic condition. We examined overall trends and evaluated the effects of diet and migratory distance on phenology and conditional responses. For the 13-year study period, no temperature, precipitation, or climate index trends were evident. There was no change in migratory timing for all species combined, but trends were apparent when evaluated by diet and migratory distance. The magnitude of these changes varies by diet and by migratory distance, but not as predicted by previous research of autumn timing in other parts of the globe. Long-distance migrants tended to migrate later in autumn, whereas short-distance
migrants exhibited no change in timing. Annual variation in timing was predicted by regional temperature and precipitation and by hemispheric climate indices, and the predicted effects differed by diet and migratory distance. Granivores responded to the broadest set of climate indices, whereas avivores responded to the least. Frugivores responded with the greatest magnitude to annual variation in climate. We did not meas ure food availability but in most cases the predictive effect of climate on migratory timing of birds was consistent with predicted effects on food. Frugivorous birds migrated earlier in warmer years when fruit quality and quantity were expected to be lower. Energetic condition meas urements supported the food hypotheses in some, but not all cases. The different responses of species to annual variation in climate suggest that different species integrate difference cues in their decision to migrate.
