Abstract

Seasonal reproduction in birds is commonly regulated by photoperiod, and passerinesare generally considered long-day breeders. The Java sparrow Padda oryzivora has beenproposed as a possible exception, but experimental evidence for short-day photope-riodicity has remained limited. Here, we tested the effects of day-length manipula-tion on courtship behavior and plasma testosterone levels in male Java sparrows. Birdswere exposed sequentially to long-day, short-day, and long-day conditions, and behav-ioral and hormonal responses were quantified at defined stages across the experiment.Courtship increased after the transition from long to short days and decreased afterthe return to long days. Plasma testosterone showed a related but more heterogeneouspattern: 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 plasmatestosterone did not consistently predict variation in courtship across treatment stages.Together, these results provide experimental support for a short-day reproductiveresponse in Java sparrows, particularly at the behavioral level, and identify increasingday length as a potent inhibitory cue for both courtship and circulating testosterone.More broadly, our findings refine current views of photoperiodic regulation in pas-serines and highlight the complexity of the relationship between environmental cues,endocrine responses, and reproductive behavior in birds.