CORTICOSTERONE LEVELS AS INDICATORS OF HABITAT QUALITY - EFFECTS OF HABITAT SEGREGATION IN A MIGRATORY BIRD DURING THE NON-BREEDING SEASON

During the non-breeding season, many species of territorial migratory birds exhibit a non-random pattern of habitat distribution, with males and females occupying different habitats. In this study, we examined possible physiological consequences arising from such habitat segregat ion in one migrant passerine species, the American redstart (Serophaga ruticilla), on its nonbreeding grounds in Jamaica, West Indies. For 2 years, we measured concentrations of corticosterone, at the rime of c apture (baseline) and 30 min after capture (profile of acute corticost erone secretion), in redstarts in two distinct habitats, one occupied predominately by males and one mostly by females. All redstarts in bot h habitat types exhibited similar concentrations of baseline corticost erone levels in fall (October), whereas in spring (March-April), redst arts in female-biased habitat exhibited significantly higher baseline levels regardless of age or sex. In fall, all individuals in both habi tats exhibited significant increases in corticosterone concentration w ith capture and handling, but in spring only redstarts (both sexes) in male-biased habitat continued to exhibit acute corticosterone secreti on. Redstarts in female-biased habitat had elevated baseline corticost erone levels and reduced acute corticosterone secretion. In spring, ba seline corticosterone concentration was negatively correlated with bod y mass, suggesting muscle catabolism associated with high corticostero ne concentrations or possibly that birds are leaner as a result of inc reased foraging effort. These results indicate that redstarts (primari ly females) in female-biased habitats suffered a decline in physiologi cal condition, which could in turn influence their departure schedules , migration patterns and even their condition and arrival schedules on the breeding grounds. Thus, segregation of populations into habitats of different quality during the nonbreeding period may have ramificati ons throughout the annual cycle of such migratory species. Furthermore , these results show the usefulness of plasma corticosterone levels as indicators of physiological condition and thus habitat quality for bi rds during the non-breeding period.