Correlation between plasma steroids and chick visits by nonbreeding adult Nazca boobies

Nonbreeding adult Nazca boobies (Sula granti) often approach unattended chicks and perform parental and/or aggressive behaviors toward the chicks. Earlier descriptions of these encounters included sexual behaviors as well. We studied hormonal correlates of chick visits by such nonbreeding boobies during the breeding season. We compared the hormonal status of chick-visiting adults with that of nonvisiting adults in the colony. We determined levels of corticosterone (CORT), testosterone (T), androstendione, dihydrotestosterone, and estradiol as potential candidate hormones controlling parental, aggressive, or sexual behavior. Only T and CORT differed between chick-visiting and nonvisiting adults. Chick-visiting adults had higher CORT levels after the interaction than nonvisiting adults randomly caught in the colony. To the contrary. T levels after interactions were lower in chick-visiting compared to random nonvisiting adults. T levels, however, did not vary with level of aggression among chick-visiting adults. We suggest that chick-visiting behavior may be permitted by low T levels and activated by, or resulting in, high CORT levels.