Abstract
In migratory species, breeding and non-breeding locations are geographically separate, yet the effects of conditions from one stage may carry over to affect a subsequent stage. Ideally, to understand the mechanisms and implications of ‘carry-over effects’, one would need to follow individuals throughout the year, quantify potential environmental causal factors and physiological mediators during multiple life-history stages, and measure downstream fitness. Owing to current limitations of tracking technology, this is impossible for small, long-distance migrants, so indirect methods to characterize carry-over effects are required. Corticosterone (CORT) is a suspected physiological mediator of carry-over effects, but when collected from blood it provides only a physiological snapshot at that point in time. When extracted from feathers, however, feather corticosterone (CORTf) provides a measure of responses to stressors from previous, and longer, time periods. We collected feathers grown during two life-history stages (post-breeding and subsequent wintering) from individuals of two age classes of a rapidly declining migratory songbird, the cerulean warbler (Setophaga cerulea), on their breeding grounds and quantified CORTf concentrations. We then monitored reproduction and survival of individuals and analysed relationships among CORTf and age, body condition and future fitness. Compared with older males, second-year males had higher CORTf concentrations during both stages. When controlling for age and year, body condition at capture was positively related to CORTf concentrations from winter (especially for older birds). However, we found no relationships between CORTf and fitness (as defined by reproduction and survival). Thus, elevated CORT may represent a beneficial physiological response (e.g. hyperphagia prior to migration), particularly for certain life-history stages, and may mediate the condition in which individuals transition between stages. But for those birds that survive migration, subsequent fitness is likely determined by more recent events and local conditions (i.e. on breeding grounds), which have the potential to counteract conditions from the winter.
Highlights
Animals face a variety of environmental challenges during different stages of their annual cycles
Carry-over effects have been directly quantified for some larger resident or short-distance migrants (e.g. Salton et al, 2015), but this is virtually impossible for many species, small migratory birds that make rapid, long-distance migrations and cannot be monitored on a daily basis throughout their entire life cycle (e.g. Stutchbury et al, 2009; DeLuca et al, 2015)
We evaluated potential carry-over effects of CORT on two response variables related to body condition, namely body mass and a body condition index derived from body mass– wing length residuals
Summary
Animals face a variety of environmental challenges during different stages of their annual cycles. As a result of this, few studies have been able to relate physiological correlates of environmental conditions faced by small migratory birds from multiple life-history stages to subsequent components of fitness at the individual level. This is an important shortcoming in our understanding of how populations are limited, because many species of small migratory songbirds increasingly face human-caused threats throughout the annual cycle (Webster and Marra, 2005), yet most research has focused only on the breeding period (Marra et al, 2015). If higher CORT reflects individuals in better condition, males with higher CORTf would be more likely to pair, initiate nesting earlier, breed successfully, produce more young and/or be more likely to return the following year
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