Plasma Luteinizing Hormone, Steroid Hormones, Behavioral Role, and Nest Stage in Cooperatively Breeding Harris' Hawks (Parabuteo unicinctus)

We measured plasma levels of testosterone (T), estradiol 17fl (E), corticosterone, and luteinizing hormone (LH) in adult breeders, adult-plumaged helpers, and juvenal-plum- aged helpers of the cooperatively breeding Harris' Hawk (Parabuteo unicinctus) in southeastern Arizona. In the males, both adult breeders and most adult-plumaged helpers had elevated T and LH during February and March, when nest building occurs, whereas most juvenal- plumaged helpers showed no change in LH, T, or E throughout the year. Thus adult-plumaged helpers, usually not related to the breeders, are hormonally ready to breed in the spring. We suggest that these helpers attain breeding readiness because potential benefits they can derive from either occasional successful copulations or possible attainment of the breeding position make reproductive readiness advantageous. In contrast, most juvenal-plumaged helpers are aiding their own parents on their natal territory and are not hormonally ready to breed, and thus they physiologically avoid the potential for inbreeding. An exception to this was a juvenal-plumaged male helper with elevated testosterone levels who was not helping his parents. The female breeders we examined had elevated T, LH, and E during the nest-building period, whereas the helpers (both adult- and juvenal-plumaged) not show elevations at any time of the year. During the early part of the nesting cycle, the adult-plumaged female helpers weighed significantly less (ca. 10%) than the breeders. It is possible that poor body condition, subordinate status to the breeding females, or both were related to their lack of breeding readiness. In all birds, corticosterone levels increased with handling time, and were higher during the nesting periods than during non-nesting periods, but showed no consistent relation with helper or breeder status. As in other species that are relatively social, maintain long-term pair bonds, or are tropical in distribution, plasma concentrations of LH and sex steroids in Harris' Hawks, even when elevated, were relatively low. Received 10 May 1990, accepted 13 January 1991.