Year-class differences in the reproductive system, plasma prolactin and corticosterone concentrations, and onset of prebasic molt in male dark-eyed juncos (Junco hyemalis) during the breeding period

Year-class differences in reproductive function were investigated in a free -living population of adult male Dark-eyed Juncos, Junco hyemalis, breeding in interior Alaska. Second-year males (SY, entering their first breeding s eason) were compared with after-second-year males (ASY, entering at least t heir second breeding season). We measured body mass, size of the cloacal pr otuberance (CP), testis mass, onset of prebasic molt, and concentrations of plasma luteinizing hormone (LH), testosterone (T), corticosterone (CORT), and prolactin (PRL) throughout the reproductive season (April to mid-July). There were no differences in SY and ASY body weights but SY males had smal ler CPs and testis masses than ASY males during gonadal recrudescence and a t the end of the breeding season. Plasma LH was elevated from April until m id-June and then decreased in the same way in both year classes. In contras t, plasma T was high from April until mid-May and was lower in SY than in A SY juncos shortly after they arrived on their breeding grounds at the end o f April, but not at other times. In July, SY males started to molt earlier, suggesting that they became photorefractory earlier than ASY males. Plasma PRL increased progressively in both year classes between April and early J une and decreased in early July. At this time, plasma PRL decreased earlier in SY than in ASY males. Plasma CORT changed seasonally, but did not diffe r between SY and ASY juncos. Thus, year-class differences in CP sizes and t estis mass apparently did not result from SY males secreting less LH or mor e PRL or CORT than ASY males. It is suggested that differences in reproduct ive condition in SY and ASY juncos are mediated by interactions with conspe cific birds and do not result from an intrinsic effect of age. (C) 2000 Aca demic Press.