Developmental photoperiod alters the onset and capacity for reproduction in male dark-eyed juncos ( Junco hyemalis )

Natural selection favours individuals able to physiologically adjust to seasonal environmental cues, such as photoperiod, and time their reproduction accordingly. Although it is known that differences along the hypothalamic-pituitary-gonadal (HPG) axis can explain differences in reproductive timing across the dark-eyed junco (Junco hyemalis) range, it remains unknown whether these differences arise from local adaptation or early-life environmental effects. We exposed nestlings from a single population to photoperiods mimicking development at the northern and southern extents of the species' breeding range. In autumn, we adjusted the photoperiod in the northern treatment to mimic southward migration, after which all birds experienced the same daylengths through the winter to simulate mixed overwintering flocks in the wild. Beginning early March, birds were injected with gonadotropin-releasing hormone (GnRH) to measure induced testosterone as a proxy for HPG axis maturation; brood patches, cloacal protuberances and fat were also measured. Males raised under northern developmental photoperiods delayed the onset of reproductive readiness compared with control males. Sample date affected females' GnRH-induced testosterone and fat deposition but not in clear patterns. These results indicate the daylength under which male songbirds develop can influence their reproductive schedules. Further research should discern the interplay of plasticity and genotypes and potential sex-specific differences.