Monogamous male birds typically allocate less e¡ort to courtship and more to parental behaviour than males of polygynous species. The seasonal pattern of testosterone (T) secretion varies accordingly. Monogamous males exhibit a spring peak in plasma T followed by lower levels during the parental phase, while males of polygynous species continue to court females and maintain T at higher levels. To determine whether testosterone underlies the trade-o¡ between mating and parental e¡ort, we treated male darkeyed juncos (Junco hyemalis) with exogenous T and compared the reproductive success (RS) of T-treated males (T-males) to that of controls. T-males had lower apparent annual RS than controls, probably because elevated T reduced parental care. Nevertheless, annual genetic RS of the treatment groups was similar because (i) T-males su¡ered fewer losses in genetic RS due to extra-pair fertilizations (EPFs), and (ii) T-males gained more genetic RS through their own EPFs. This is the ¢rst hormonal manipulation of an avian phenotype shown to in£uence male RS through EPFs. Together with other studies, it suggests that testosterone may have mediated the evolution of inter-and intraspeci¢c di¡erences in allocation of reproductive e¡ort to mate attraction and parental care.

Animals often cope with adverse events by releasing glucocorticoid hormones, which in turn promote increased energy assimilation. In captive animals, crowding also leads to increased glucocorticoid activity, probably because of increased levels of social competition. We investigated how group size and ectoparasite infestations affected endogenous levels of the glucocorticoid hormone, corticosterone, in colonial cliff swallows, Petrochelidon pyrrhonota, in southwestern Nebraska, USA. Parasites were removed from some colonies by fumigating nests. Baseline levels of corticosterone in breeding adults varied significantly with whether parasites were present, colony size (measured by total number of active nests at a site), and nesting stage. Across all analyses, birds from fumigated colonies averaged significantly lower baseline levels of corticosterone. These levels in adults increased with colony size at nonfumigated sites, especially during the period when nestlings were being fed, but no relation or the opposite one was found for birds in fumigated colonies. Baseline corticosterone levels were unrelated to sex, age, body weight, or testosterone levels in adults. Corticosterone concentrations tended to increase during a bad-weather event when food was scarce. Patterns in nestling and recently fledged juveniles were consistent with those in adults. The increased baseline levels of corticosterone in birds of larger colonies appear related to the larger number of parasites there. Higher levels of corticosterone probably facilitate increased allocation of time and

Abstract. The hormone corticosterone is an important part of animals' response to environmental stress, modulating short-term adaptive changes in behavior and physiology. The hormone testosterone is also critical, especially for males, in regulating the expression of sexual behavior and parental care. These hormones can have costly consequences, however, and within populations individuals show variation in endogenous levels of both corticosterone and testosterone. We studied how annual survival varied as a function of natural levels of these hormones in colonially breeding Cliff Swallows (Petrochelidon pyrrhonota) in southwestern Nebraska, USA, in 2000-2003. We sampled hormone levels of birds caught at colonies and using mark-recapture, monitored their survival through subsequent years in the study area. Maximum-likelihood estimation and model fitting (with program MARK) revealed that birds sampled for corticosterone in colonies of all sizes late in the season had curvilinear survival; individuals with very low and very high levels of corticosterone had lower survival than those with intermediate levels. Annual survival of birds sampled earlier in the season, however, generally declined with increasing corticosterone level. More birds than expected, given the survival functions, had the very low corticosterone levels in nonfumigated colonies later in the year, suggesting perhaps a compensatory benefit unrelated to survival for very low corticosterone levels. In a more limited analysis, testosterone appeared to have little effect on annual survival, although some evidence suggested that females with endogenous testosterone levels below the mean for a given date might have survived better.