Photoperiod modulates courtship behavior and plasma testosterone in a short‐day passerine

Seasonal reproduction in birds is commonly regulated by photoperiod, and passerines are generally considered long‐day breeders. The Java sparrow Padda oryzivora has been proposed as a possible exception, but experimental evidence for short‐day photoperiodicity has remained limited. Here, we tested the effects of day‐length manipulation on courtship behavior and plasma testosterone levels in male Java sparrows. Birds were exposed sequentially to long‐day, short‐day, and long‐day conditions, and behavioral and hormonal responses were quantified at defined stages across the experiment. Courtship increased after the transition from long to short days and decreased after the return to long days. Plasma testosterone showed a related but more heterogeneous pattern: it did not increase significantly at the group level after transfer to short days, but declined markedly after re‐exposure to long days. In addition, variation in plasma testosterone did not consistently predict variation in courtship across treatment stages. Together, these results provide experimental support for a short‐day reproductive response in Java sparrows, particularly at the behavioral level, and identify increasing day length as a potent inhibitory cue for both courtship and circulating testosterone. More broadly, our findings refine current views of photoperiodic regulation in passerines and highlight the complexity of the relationship between environmental cues, endocrine responses, and reproductive behavior in birds.