Does an integrated measure of corticosterone fom feathers improve our understanding of avian ecophysiology?

Abstract

The hormone corticosterone (CORT) helps mediate the relationship between birds and their environment. I studied nestling and adult birds in several diverse contexts to understand how sources of environmental variability contribute to variation in an integrated measure of CORT from feathers. In nestling tree swallows (Tachycineta bicolor), feather CORT was positively related to maximum nest box temperatures. Temperatures may not have been challenging for nestlings. Instead, CORT physiology was likely matched to nest box conditions. Experimentally-reduced provisioning resulted in significantly lower feather CORT in nestling Cory’s shearwaters (Calonectris diomedea), suggesting nestlings invoked an adaptive strategy of CORT suppression to cope with extended nutritional challenges. In adults, both enrichment and its removal from the cages of captive Clark’s nutcrackers (Nucifraga columbiana) resulted in significant increases in feather CORT, but this effect was a function of exposure time. In adult tree swallows, increased productivity was positively associated with CORT from feathers grown post-breeding. Although this effect likely reflected the increased energetic expense of raising more young, it could not be separated from individual quality. Traditional measures of habitat and spatial structure related to Dupont’s lark (Chersophilus duponti) population dynamics were not related to feather CORT. However, when ratios of stable isotopes of carbon (δ13C) were used as a proxy for environmental conditions, a significant negative relationship resulted between δ13C and feather CORT. This result suggests that combining feather-based measurements is a particularly strong approach to studying habitat-physiology relationships. Feather CORT quantified hormonal responses to ecological variability in general, rather than in response to any specific type(s) of challenges. A unifying theme of my work was that, when interpreted in proper context, feather CORT was apparently related to energetic demand or exertion. Feather CORT may therefore be a proxy for individual energetics. Building on this, I developed a conceptual model that helps explains how environmental and physiological variation delineates an “ecophysiological niche”, the boundaries of which define a range of CORT values that should contribute positively to fitness. My results suggest that feather CORT will likely be useful in moving both theoretical and applied research towards a more holistic perspective of avian ecophysiology.