Feather growth rate and hormone deposition vary with elevation but not reproductive costs in resident Mountain Chickadees

Many organisms engage in metabolic trade-offs to manage costs associated with reproductive output which often leads to these costs carrying over into the future. Compensatory mechanisms vary across life-history strategies and are expected to result in near optimal fitness gains for the investor. Here we investigated whether environmental differences associated with increasing montane elevation and variation in reproductive output of a resident passerine songbird, the Mountain Chickadee (Poecile gambeli), were related to physiological condition during annual molt. Higher elevations are associated with harsher environmental conditions during the winter, which results in later and shorter breeding seasons than at lower elevations. We sampled the outermost tail feathers from adult birds in the fall after their prebasic molt, which initiates closely after reproduction (e.g., after parental care has ceased, ~1–3 weeks). We measured the hormone corticosterone deposited in feathers (fCORT) and feather growth rates for evidence of physiological effort predicted to be driven by several units of reproductive output (e.g., breeding timing, clutch and brood size, offspring mass). There were no relationships between any measure of reproductive output and feather characteristics between elevations or across years, despite substantial variation in reproductive output in the wider population across this same time. However, birds at the high elevation site grew their tail feathers significantly faster and had higher fCORT deposition compared to low elevation birds. These results suggest that although differences in reproductive output and any related signals of associated physiological effort (e.g., fCORT and feather growth rate) may not extend into individual condition during annual molt, shorter breeding seasons associated with harsher environmental conditions may favor faster feather growth as required by earlier onset of winter.