Changes in Plasma Levels of Luteinizing Hormone and Sex Steroid Hormones in Relation to Multiple-Broodedness and Nest-Site Density in Male Starlings

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Abstract
Changes in plasma levels of testosterone (T), progesterone (P), and luteinizing hormone (LH) were studied over the reproductive cycle in two double-brooded populations of European starlings (Sturnus vulgaris). In one population, nest sites (nest boxes) averaged 6 m apart (dense site), and in the other, nest boxes average 60 m apart (dispersed site). In both populations, plasma levels of T were high during the two egglaying stages when males were mate guarding and defending nest holes. A higher circulating level of T was observed during the first nesting attempt, when all pairs in possession of a box initiated clutches. Furthermore, plasma levels of T were higher in males nesting at the dense site compared with those in males at the dispersed site. Only about 30% of these pairs initiated second clutches after successfully rearing the first brood. Although plasma levels of T rose in males during the second clutch initiation, they were significantly lower than T levels measured during production of the first clutch. Circulating levels of LH became maximal during each egg-laying stage in a manner similar to those of T but did not vary as a function of nest-site density. Changes in circulating T were also examined in relation to a T-dependent secondary sexual characteristic, male bill color. We found that the seasonal change in bill color is sensitive to circulating levels of T only slightly above basal and much lower than those observed during times of male-male aggression. Circulating levels of P were constant from January to July, with no significant changes during the prebreeding and breeding periods. This is consistent with results of studies with captive males.