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

• Published in: The Auk 1991-07, Vol.108 (3), p.619-637
• Main Authors: Nora A. Mays, Vleck, Carol Masters, Dawson, James
• Format: Article
• Language: English
• Subjects: Aviculture; Biological and medical sciences; Bird nesting; Birds of prey; Breeding; Female animals; Fundamental and applied biological sciences. Psychology; Hawks; Hormones; Male animals; Mannans; Mating behavior; Parabuteo unicinctus; Thermoregulation. Hibernation. Estivation. Ecophysiology and environmental effects; Vertebrates: anatomy and physiology, studies on body, several organs or systems
• ISSN: 0004-8038; 1938-4254
• DOI: 10.2307/4088103

We measured plasma levels of testosterone (T), estradiol 17β (E), corticosterone, and luteinizing hormone (LH) in adult breeders, adult-plumaged helpers, and juvenal-plumaged 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) did 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.