Experimental manipulation of air temperature in captivity appears unsuitable for evaluating fecal glucocorticoid metabolite responses of wild-caught birds to heat exposure

Abstract

Noninvasive measurement of stress-related alterations in fecal glucocorticoid metabolite (fGCM) concentrations has considerable potential for quantifying physiological responses to very hot weather in free-ranging birds, but practical considerations related to sampling will often make this method feasible only for habituated study populations. Here we evaluate an alternate approach, the use of experimentally manipulated thermal environments for evaluating stress responses to high environmental temperatures in wild-caught birds housed in captivity. Using an enzyme immunoassay utilizing antibodies against 5ß-pregnane-3a,11ß,21- triol-20-one-CMO∶BSA (tetrahydrocorticosterone), we quantified fGCMs in captive individuals of three southern African arid-zone species (southern pied babblers [Turdoides bicolor], white-browed sparrow-weavers [Plocepasser mahali], and southern yellow-billed hornbills [Tockus leucomelas]) experiencing daily air temperature maxima (Tmax) ranging from 307–327C to 427–447C. For none of the three species did Tmax emerge as a significant predictor of elevated fGCM concentrations, and no stress response to simulated hot weather was evident. The apparent lack of a stress response to Tmaxp427C in captive southern pied babblers contrasts with linear increases in fGCMs at Tmax 1 387C in free-ranging conspecifics. The lack of an effect of Tmax on fGCM levels may potentially be explained by several factors, including differences in operative temperatures and the availability of water and food between free-ranging and captive settings or the stress effect of captivity itself. Our results suggest that experimental manipulations of thermal environments experienced by wild-caught captive birds have limited usefulness for testing hypotheses concerning the effects of hot weather events on fGCM (and, by extension, glucocorticoid) concentrations.