Reproductive endocrinology of a free-living, opportunistically breeding passerine (White-winged Crossbill, Loxia leucoptera)

The aim of the study was to investigate, in a free-living opportunistic breeder, the White-winged Crossbill (Loxia leucoptera), the degree to which annual changes in photoperiod and differences in the timing of breeding in different years correlate with the secretion of reproductive hormones. Seasonal changes in concentrations of luteinizing hormone (LH) and prolactin were compared in White-winged Crossbills in interior Alaska (64°50′N, 147°50′W) in plasma samples taken in May-December 1994 and July 1998-August 1999. The birds were in breeding condition in May-July 1994 and in February-April 1999. Additional observations were made on plasma testosterone (T) in males in 1998/1999. In 1998, in adult males, plasma LH and T were low in September-October (LH: <0.4 ng/ml; T: undetectable) and increased significantly in November (LH: 3.65 ± 2.20 ng/ml) and December (T: 0.69 ± 0.88 ng/ml). Thereafter, plasma T increased to 2.35 ± 2.03 ng/ml in March/April and returned to basal levels by June/July. In 1998/1999, in females, plasma LH did not change seasonally. In males in 1994, plasma LH was higher in June (2.15 ± 1.22 ng/ml) and July (2.86 ± 0.69 ng/ml) than in the same period in 1999 (June: 1.20 ± 1.82 ng/ml; July: 1.37 ± 1.03 ng/ml), which is consistent with the later breeding season in 1994. In 1994 and 1998/1999 in both sexes, concentrations of plasma prolactin were elevated when day lengths exceeded about 12 h, being high during late spring to early fall (males: 40-130 ng/ml; females: 40-140 ng/ml) and low during winter (< 15 ng/ml in both sexes). In 1994 and 1998, molt occurred during the fall, at a time when breeding did not occur, although conifer seeds were sufficiently abundant to support a large bird population. These data are consistent with the view that White-winged Crossbills have the potential to breed at all times of the year except when molting in the fall. It is likely that a photoinduced increase in prolactin secretion is a factor that induces molt. Crossbills may not breed when they are molting because of increased metabolic demands for feather formation and for the maintenance of basal metabolism at a time of year when ambient temperatures fall well below 0°. Alternatively or in addition, the birds may become reproductively photorefractory.