The reproductive cycle of monogamous passerines is characterized by a sequence of events leading from courtship and pair formation through nest building, egg laying and incubation, and ending with the care of young. Each of these stages is accompanied by equally predictable changes in the pattern of hormone secretion. This report describes endocrine changes in the plasma of free-living Brown-headed cowbirds, Molothrus uter, during a breeding season. Changes in some morphological parameters are also presented.

Cowbirds are brood parasites that show no parental behaviour and lay eggs almost daily for approximately eight weeks. Males are not territorial, but they mate guard extensively. Hcncc, their breeding biology is quite different from that of the nesting species described above. They are hormonally distinct, as well. Luteinizing hormone rises in the spring and remains elevated throughout May and most of June. By July, when breeding has ceased, plasma luteinizing hormone levels are at pre-breeding values. Plasma androgens are also maintained at peak levels for an extended period of time, although they decline briefly between the establishment of dominance hierarchies and the onset of breeding. Circulating levels of corticosterone are elevated until the middle of the breeding season, when they begin to fall, although here, too, there is a transient depression just prior to breeding. In the light of the studies on nesting species, these data suggest that hormone profiles are closely correlated with the breeding and social activities that characterize the species under investigation.