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A Fifteen-year Study of Fall Stopover Patterns of Catharus Thrushes at an Inland, Urban Site

Abstract Little is known about how landbirds use urban habitats as migratory stopovers despite increasing urbanization in North America and the importance of the migratory period to annual survival of birds. I examined 15 years of autumn stopover data for three species of migratory Catharus thrushes from an urban natural area in metropolitan Detroit, Michigan, USA. The majority of birds were in good condition on first capture with 59% having more than a just a trace of fat, a higher proportion than reported for passerines at other fall stopover sites. Condition was similar among species, and there was little difference in fat class or mass between age groups. A linear regression of the condition of each bird at first capture and time of capture indicated positive diurnal mass gains in two species and mass loss in the third. Fourteen percent of thrushes banded were recaptured. Lean birds were not more likely to be recaptured than fatter birds and, among recaptured birds, there was no difference in stopover period or mass or fat increases between young and adults. Seventy-nine percent of recaptured birds gained mass, and mass and fat class increases were significant for all species. These results are discussed in view of the high prevalence of non-native fruit resources and rates of human disturbance at the study site.