Many seasonally breeding animals exhibit a threshold day length (critical photoperiod; CPP) for gonadal growth, and populations breeding at higher latitudes typically have a higher CPP. Much less is known about latitudinal variation in CPP in migratory population that winter away from their breeding range and must time their reproduction to match favorable conditions at their destination. To address the relationship between migration, breeding latitude, and CPP, we held two closely related songbird populations in a common environment. One population is resident (Junco hyemalis carolinensis), the other winters in sympatry with the residents but migrates north to breed (Junco hyemalis hyemalis). We gradually increased photoperiod and measured indices of readiness to migrate (fat score, body mass) and breed (cloacal protuberance volume, baseline testosterone, and gonadotropin releasing hormone challenged testosterone). To estimate breeding latitude, we measured hydrogen isotopes in feathers grown the preceding year. As we predicted, we found a higher CPP in migrants than residents, and a higher CPP among migrants deriving from higher as opposed to lower latitudes. Migrants also terminated breeding earlier than residents, indicating a shorter breeding season. To our knowledge, this is a first demonstration of latitudinal variation in CPP-dependent reproductive timing in bird populations that co-exist in the non-breeding season but breed at different latitudes. We conclude that bird populations appear to exhibit local adaptation in reproductive timing by relying on differential CPP response that is predictive of future conditions on the breeding ground.

许多季节性繁殖的动物表现出性腺生长的阈值日长（临界光周期；CPP），在高纬度地区繁殖的种群通常具有更高的 CPP。对那个冬天远离繁殖范围的迁徙种群 CPP 的纬度变化知之甚少，它们必须安排繁殖时间以匹配目的地的有利条件。为了解决迁徙、繁殖纬度和 CPP 之间的关系，我们将两个密切相关的鸣禽种群放在一个共同的环境中。一个种群是常住居民（Junco hyemalis carolinensis），另一个种群与居民同冬但向北迁移繁殖（Junco hyemalis hyemalis））。我们逐渐增加光周期并测量迁移准备指数（脂肪评分、体重）和繁殖指数（泄殖腔隆起体积、基线睾酮和促性腺激素释放激素挑战睾酮）。为了估计繁殖纬度，我们测量了前一年生长的羽毛中的氢同位素。正如我们预测的那样，我们发现移民的 CPP 高于居民，而移民的 CPP 更高，来自高纬度而不是低纬度。移民也比居民更早终止繁殖，表明繁殖季节较短。据我们所知，这是在非繁殖季节共存但在不同纬度繁殖的鸟类种群中 CPP 依赖性繁殖时间的纬度变化的首次证明。