Feather characteristics as indicators of condition and immunity during the migration season
Feather ornaments and structure play a significant role in avian mate choice, territoriality and molt performance. These characteristics can accurately indicate bird nutritional status and the ability to survive and reproduce. Therefore, given the importance of these feather characteristics during the breeding season, it is central to our understanding of avian ecology to determine whether the same roles as indicators can be applied during the migration season. We investigated whether the size of the white patch and growth bars on the tail feathers of western palm warblers (Dendroica palmarum) and gray catbirds (Dumetella carolinensis), respectively, could be indicators of body and immune condition during the migration season. We expected that white patch would positively relate to both body and immune condition. Further, we expected that growth bar size would trade-off with condition scores, indicating unequal investment to both functions. Western palm warblers (n=45) and gray catbirds (n=28) were captured at the Jekyll Island Banding Station (Jekyll Island, Georgia) during the month of October, 2012. We collected basic morphometric data, a blood sample and the outermost tail feather from each bird in order to test our questions.
We found that the white patch on the tail feathers of western palm warblers was important for explaining body condition during migration, but not overall immune condition (total white blood cell count). Warblers with larger white patches were found to have a significantly higher body condition. In contrast, mean growth bar size of gray catbirds was related to immune condition, but not body condition during migration. Catbirds with slower feather growth (smaller growth bar size) had decreased overall immune condition and individual white blood cell levels, indicating a trade-off between investment between functions. These findings suggest that common feather characteristics can be honest indicators of a bird’s body and immune condition, even after the feather is molted. Our results suggest that feathers can be used to provide insight into which birds might be in the worse condition, and therefore most vulnerable to mortality, during the migration season.