Behavioral and hormonal basis of polygynous breeding in male bush warblers (Cettia diphone).

Plasma levels of testosterone and corticosterone were measured in free-living male bush warblers captured on their breeding ground at different times of the breeding season. Their territoriality was also estimated from their singing response to song playbacks. The pattern of change detected in the levels of plasma testosterone was different from that of "typical" monogamous species but similar to that of polygynous species. In "typical" monogamous species, plasma testosterone levels elevated during territory settlement and courtship behavior and then declined to low, stable levels during incubation. In bush warblers, plasma levels of testosterone were already high (1-2 ng/ml) upon arrival in late March and peaked (2. 5-4 ng/ml) in early June. They then decreased but relatively high levels were maintained until early August. In late August the testosterone concentration was 0.03 ng/ml or less. Plasma levels of corticosterone also showed a seasonal change, being highest in May to July and declining in late August. Territoriality showed clear seasonality, reflecting the levels of circulating testosterone. Upon arrival, latency periods for responses to song playback were long and singing activity was rather low but this behavior was soon stabilized and a high degree of territoriality was maintained to late August. These results suggest that high levels of circulating testosterone and corticosterone allow males to pursue a polygynous breeding strategy, to hold a territory, and to maintain breeding activity for a prolonged period, characteristics which are likely to be adaptations to dense bushes with high rates of predation and brood parasitism of this species.