Baseline and acute levels of corticosterone in Florida Scrub-Jays ( Aphelocoma coerulescens): Effects of food supplementation, suburban habitat, and year

Abstract

Recent work has shown that Florida Scrub-Jays ( Aphelocoma coerulescens) living in suburban habitats, as well as birds that reside in a natural habitat (hereafter referred to as wildland birds) provided supplemental food during the pre-breeding period, have lower baseline corticosterone (CORT) levels than nonsupplemented wildland birds, suggesting that predictable and abundant foods can lower stress levels. Here, we investigate whether the acute CORT response to capture stress reflects our earlier findings. During the pre-breeding seasons in 2004 and 2005 we captured over 200 Florida Scrub-Jays and collected four blood samples from each bird over a 30 min time period. We compared hormonal and morphometric data from birds in four treatment groups: wildland birds that were provided with supplemental food during the pre-breeding period, wildland birds that were provided with supplemental food during the winter and pre-breeding period, nonsupplemented wildland birds, and suburban birds that effectively have access to ad libitum human-source food year-round. Suburban birds had lower baseline levels, and a more acute initial CORT response (i.e., rate of increase) than wildland birds, but we did not observe any differences among the treatment groups of wildland birds. Baseline CORT concentrations were unusually low across treatments in 2005, a year that was characterized by increased adult body mass, superior body condition, and a high fledgling success rate, all of which suggest favorable environmental conditions. Thus, abundant or more reliable food sources in 2005 may have overwhelmed any potential benefits of supplemental feeding and explain the absence of any strong experimental effects on CORT for that year.