Do feather traits convey information about bird condition during fall migration?

Abstract

Long-distance migration is energetically costly and often has a significant impact on bird health and condition. Therefore, understanding the health status of birds during long-distance migration is of ecological interest; however, obtaining invasive health measures is not always feasible or practical. Bird feathers can provide information about individual bird condition independent of invasive and expensive blood tests and have been used previously as indicators of condition, even long after the season of feather growth. Here, we investigated this question using 2 migratory songbirds in North America, Western Palm Warblers (Setophaga palmarum palmarum), which have conspicuous white patches on their tail feathers, and Gray Catbirds (Dumetella carolinensis), which have prominent daily growth bars on their tail feathers. Feathers were collected from birds captured on Jekyll Island, Georgia, during their fall migration. From the feathers we quantified the size of white patches on warbler rectrices and estimated growth bar length (as an index of feather growth rate) on catbird rectrices using image analysis. We also obtained 2 measures of bird health for each species: estimates of white blood cell abundance from blood smears (immune status) and size-corrected body mass (body condition). Analyses of feather traits and how they relate to bird health indicated that warblers (n = 42) with larger white patches had significantly higher body condition indices, although no association with white blood cell abundance, and catbirds (n = 27) with faster feather growth had significantly lower white blood cell abundance, but no association with body condition. Given the utility of these feather characteristics, our results suggest these metrics can be used to provide some insights into the biology of songbirds during one of the most critical portions of the annual cycle, migration.