Does Age, Residency, or Feeding Guild Coupled with a Drought Index Predict Avian Health during Fall Migration?

SIMPLE SUMMARY: After a large avian mortality event occurred in New Mexico in the fall of 2020, we performed an analysis using 11 years of fall bird banding data at two locations in north-central New Mexico to investigate the influence of drought on avian health. Carcass studies after the event indicated that starvation was the primary cause of death. Using fall bird banding data along with regional drought indices, we used multiple logistic regression to predict the body health conditions of a bird during the fall migration period. We found that fat scores for younger, insectivorous, migratory birds were less likely to be greater than zero as drought severity levels increased. Our results suggest that migratory insectivores in the southwestern United States may be less resilient to drought-related climate change. ABSTRACT: Birds are good indicators of environmental change and are often studied for responses to climate. Many studies focus on breeding birds, while fewer look at the migration period, which is a critical time for many birds. Birds are more susceptible to unusual climatic events during their migration due to the metabolic stress of long-distance movements. In the fall of 2020, an unusual cold weather event coupled with drought and wildfire smoke led to a large avian mortality event in New Mexico. Later analysis pointed to the mortality being largely due to starvation. This was the impetus for our research. We used 11 years of fall bird banding data from two locations, along with local drought indices, to determine what predicts avian health during the migration period. We used fat score data from over 15,000 individual birds to assess whether drought indices, age, diet, or residency influenced avian health using multiple logistic regression. We found that the probability of positive fat scores decreased as drought severity increased for younger, insectivorous, migratory birds. Insectivores had a higher probability of receiving a fat score greater than zero relative to local drought conditions, which is important, since many North American insectivores are in steep decline. Migratory birds showed a greater response than year-round residents, and older birds showed a lower but significant response compared to hatch-year birds. Our results suggest that migratory insectivores in the southwestern United States may be less resilient to drought-related climate change.