SPRING PASSAGE AND ARRIVAL PATTERNS OF AMERICAN REDSTARTS IN MICHIGAN'S EASTERN UPPER PENINSULA

We examined the pattern of American Redstarts (Setophaga ruticilla) migrating through and arriving in breeding areas in northern Michigan to evaluate factors that may influence arrival of redstarts. Variation in arrival schedules coincided with variation in endogenous and exogenous factors. Redstarts arrived 3 to 7 days later during a year characterized by cold temperatures and low resource abundance as compared to years in which environmental conditions during the arrival period were more benign. Further, males verified as breeding at our site arrived 2 to 4 days before breeding females while males classified as migrants preceded migrant females by 4 days. Finally, older birds preceded younger for both verified breeders (7 days) and migrants (6 days). These findings demonstrate behavioral plasticity within the constraints of optimal migration theory which places high value on early arrival in breeding areas. Our results suggest that some species of long-distance migrants may adjust spring migration rates in response to environmental conditions. Received 20 March 2008. Accepted 17 September 2008.