Date of Award


Document Type

Honors Thesis (Open Access)


Colby College. Biology Dept.


Judy Stone

Second Advisor

Catherine Bevier

Third Advisor

Allison Barner


Breeding, molt, and migration place considerable yearly demands on migrating passerines in North America. Due to conflicts in energy allocation, many species have evolved migration strategies that prevent temporal overlap in these processes. However, there is recent evidence that some passerines which breed in western North America migrate to intermediate stopover sites to carry out molt, conflating molt and migration. Factors influencing the evolution of such a strategy are believed to include both aridity on breeding grounds and dependable flushes of late-summer productivity in the southwestern United States and northwestern Mexico. This productivity occurs due to the brisk rainfall of the North American monsoon, and it results in a dramatic increase in many types of food resources. In this study, I use precipitation records and pre-existing surveys of breeding birds in the Great Basin region to investigate the relationship between migration strategy and influence of monsoon precipitation. Specifically, I model abundance changes using various precipitation and population explanatory variables in five molt migrants and five non-molt migrant controls. This study did not provide strong evidence that monsoon rains have distinct effects on passerines of divergent migration strategies. However, I found that molt migrants may be subject to weaker density-dependent effects, possibly due to relaxed constraints on the molting ground.


passerine, molt migration, monsoon, precipitation, phenology, climate change