Plasma corticosterone of city and desert Curve-billed Thrashers, Toxostoma curvirostre, in response to stress-related peptide administration

We compared the activity and responsiveness of the hypothalamo–pituitary–adrenal (HPA) axis of an urban (Phoenix, Arizona) and desert population of a male songbird species (Curve-billed Thrasher, Toxostoma curvirostre), by measuring plasma corticosterone in response to acute administration of corticotropin-releasing factor, arginine vasotocin, or adrenocorticotropin hormone. Urban adult male thrashers showed greater responsiveness than desert birds to an injection of arginine vasotocin or adrenocorticotropin hormone, suggesting a population difference in pituitary and adrenal gland sensitivity. Plasma corticosterone in response to corticotropin-releasing factor injection did, however, not differ between populations. The differential corticosterone response to arginine vasotocin and corticotropin-releasing factor may reflect effects of chronic stress or habituation, which are known to favor arginine vasotocin over corticotropin-releasing factor sensitivity. Efficacy of HPA negative feedback by glucocorticoids was determined by measuring plasma corticosterone in response to acute administration of the synthetic glucocorticoid dexamethasone. This administration decreased plasma corticosterone similarly in urban and desert thrashers, suggesting that the negative feedback of glucocorticoids on the HPA axis in the two populations was equally effective. The higher sensitivity of urban than desert thrashers to adrenocorticotropin hormone and arginine vasotocin may result from up-regulation of the HPA axis in urban birds. This up-regulation may in turn make it easier for city birds to cope with urban environment-associated stressors.