Corticosterone Secretion in Response to Adult Alarm Calls in American Kestrels

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Many nestlings respond to alarm calls with anti-predator behavior, but their physiological responses to such calls are poorly understood. Alarm calls could stimulate increased secretion of corticosterone to mobilize energy reserves in anticipation of an interaction with a predator. However, elevated corticosterone levels can be costly to nestlings because they induce catabolic reactions at a time when the birds are engaged in anabolic processes. Thus, there may be constraints on the secretion of corticosterone if the catabolic costs outweigh the behavioral benefits. Furthermore, many studies use baseline corticosterone levels as a correlate of body condition, and if nestlings respond hormonally when they hear alarm calls rather than when they are first handled by (or catch sight of) an investigator, then determination of baseline hormone levels would be difficult. To investigate this phenomenon, we broadcast taped calls of either adult conspecific alarm calls or songs of heterospecifics to 15-20 d old American Kestrel (Falco sparverius) nestlings. Playbacks were started when adults were absent, but adults returned and produced alarm calls at two control and three experimental nests. Blood was collected from randomly selected nestlings at each nest 13 min after the onset of a playback and within 2 min of opening the nest box, and the plasma was assayed for corticosterone. There was no difference in corticosterone values between nestlings that heard alarm calls and those that heard control vocalizations. Hearing natural kestrel alarm calls did not affect the results, although our small sample sizes resulted in low power to detect differences. These data indicate that 15-20 d old kestrels do not increase corticosterone secretion in response to adult conspecific alarm calls, and suggest that studies of corticosterone secretion in nestling kestrels will not be affected by the occurrence of adult alarm-calling.

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