Seasonal changes in adrenocortical responses to acute stress in polygynous male bush warblers (Cettia diphone)

The adrenocortical response to acute stress as measured by increase in plasma levels of corticosterone following capture, handling, and restraint over an hour was estimated in free-living male bush warblers at different times of the breeding season on their breeding ground in Chichibu, Honshu, Japan. Initial plasma levels of corticosterone showed a seasonal change, being moderate in April, highest in May to July and declined in late August. In contrast, the stress response was suppressed at the early stage of the breeding season, April and May, and just after the breeding season, late August. In June, July, and early August, the response was obvious although the initial levels of corticosterone were high. These results are different from those of monogamous passerine birds so far studied in which adrenocortical response is obvious at the early stage of breeding season and the response is suppressed later stage when the male is involved in parental care. The results suggest that during a course of the breeding season, male bush warblers that take a polygynous breeding strategy may decrease the amplitude of the adrenocortical response to acute stress at the early stage of the breeding season so as to allow establishment and defense of a territory. The birds became more sensitive to acute stress later the breeding season when they already had females incubating, feeding hatchlings or even fledgling. This change of stress response is likely to be a reflection of polygynous breeding strategy of the species.