House sparrows (Passer domesticus) adjusted hypothalamic-pituitary-adrenal axis negative feedback and perch hopping activities in response to a single repeated stimulus.

Chronic stress has been extensively studied in both laboratory and field settings; however, a conclusive and consistent phenotype has not been reached. Several studies have reported attenuation of the hypothalamic-pituitary-adrenal axis during experiments intended to cause chronic stress. We sought to determine whether this attenuation could be indicative of habituation. Importantly, we were not investigating habituation to a specific stimulus-as many stress physiology studies do-but rather we assessed how the underlying physiology and behavior changed in response to repeated stressor presentation. We exposed house sparrows (Passer domesticus) to a single stimulus twice per day at random times for 8 consecutive days. We predicted that this period of time would be long enough for the birds to determine that these acute stressors were not, in fact, dangerous and they would, therefore, acclimate. A second control group remained undisturbed for the same period of time. We measured baseline, stress-induced, negative feedback strength, and maximum production of corticosterone as well as neophobic behavior before, during, and after this 8-day experiment. When birds experienced a stimulus for 4 days, their negative feedback strength was significantly diminished, but recovered after the second 4 days. Additionally, perch hopping decreased and recovered in this same time frame. These data suggest that distinct physiological and behavioral responses arise when house sparrows are exposed to the same stressor for several consecutive days as opposed to many stressors layered on top of one another. Furthermore, they indicate that habituation-as with chronic stress-can appear differently depending on the metric being examined.