Ecological causes and consequences of intratropical migration in temperate-breeding migratory birds

New discoveries from direct tracking of temperate-breeding passerines show that intratropical migration (ITM) occurs in a growing number of species, which has important implications for understanding their evolution of migration, population dynamics, and conservation needs. Our large sample size ([Formula: see text]) for purple martins (Progne subis subis) tracked with geolocators to winter sites in Brazil, combined with geolocator deployments at breeding colonies across North America, allowed us to test hypotheses for ITM, something which has not yet been possible to do for other species. ITM in purple martins was not obligate; only 44% of individuals exhibited ITM, and movements were not coordinated in time or space. We found no evidence to support the resource hypothesis; rainfall and temperature experienced by individual birds during their last 2 weeks at their first roost site were similar to conditions at their second roost site after ITM. Birds generally migrated away from the he...

通过对温带繁殖雀形目鸟类的直接追踪所获得的新发现表明，热带内迁徙（intratropical migration）在越来越多的物种中存在，这对理解其迁徙进化、种群动态及保护需求具有重要意义。我们对通过地理定位器追踪至巴西越冬地的紫燕（Progne subis subis）样本量较大（[Formula: see text]），结合在北美各地繁殖群体中部署的地理定位器数据，使我们得以检验关于ITM的假设——这一点对于其他物种而言尚无法实现。紫燕的ITM并非强制性行为；仅44%的个体表现出ITM，且其迁徙活动在时间或空间上并无协同性。我们未发现支持资源假说的证据；个体鸟类在首个越冬栖息地最后两周经历的降雨量与温度，与ITM后第二个越冬栖息地的条件相似。鸟类通常会从...