Effects of corticosterone on territorial behavior of free-living male song sparrows Melospiza melodia.

A group of 10 territorial male song sparrows, Melospiza melodia, were given subcutaneous implants of corticosterone in Silastic tubing. A second group of 10 territorial males were given empty implants as controls. After 18-24 hr all males were then subjected to a simulated territorial intrusion (STI) by placing a caged male song sparrow in the center of the subject's territory, and playing tape recorded songs through a speaker placed alongside. Significantly fewer males with corticosterone implants responded to STI than to controls, and the latency to respond was longer. Of the 3 experimental males that did respond to STI, all had a lower frequency of songs and did not approach the simulated intruder as closely as controls. Many males were captured 2-7 days after implantation and blood samples collected for measurement of circulating hormone levels. As expected, plasma levels of corticosterone were high in the group given corticosterone implants. However, plasma levels of luteinizing hormone (LH) were not affected by treatment with corticosterone, and although circulating levels of testosterone were depressed slightly compared with controls, they were within the normal range for territorial and breeding males. There were no differences in body mass despite greatly increased fat depots in males treated with corticosterone. These data suggest that high levels of corticosterone, similar to those measured during stressful episodes both in the laboratory and field, may suppress territorial behavior independently of the adenohypophysial-gonad axis. Since plasma levels of LH and testosterone are not depressed markedly, thus maintaining the gonads in a near functional state, renesting can begin as soon as environmental conditions ameliorate. Such mechanisms could potentially increase the probability of raising viable young after unpredictable, severe weather resulted in failure of the previous breeding attempt.