Data from: Post‐independence mortality of juveniles is driven by anthropogenic hazards for two passerines in an urban landscape

Urban environments impose novel selection pressures with varying impacts across species and life history stages. The post‐fledging stage for migratory passerines, defined as the period of time from when hatch‐year birds fledge until their first migration, is a poorly understood component of annual productivity that potentially limits population growth. We studied two migratory passerines with positive and negative population responses to urbanization, respectively: Gray Catbird (Dumetella carolinensis) and Wood Thrush (Hylocichla mustelina). Our goals were to estimate post‐fledging survival rates for urban bird populations and determine which features of the urban landscape impact mortality risk during the post‐fledging stage. From 2012–2014, we tracked 127 fledglings (60 Gray Catbirds and 67 Wood Thrushes). Over 55 days after fledging, cumulative survival of Gray Catbirds (0.32 [95% CI: 0.22–0.47]) was approximately half that of Wood Thrushes (0.63 [95% CI: 0.52–0.75]). Thus, survival rates during the post‐fledging stage, taken in isolation, do not explain differential trajectories of Gray Catbird and Wood Thrush populations in urban environments. Most mortality (86%) for both species was due to predation. However, after reaching independence from parental care, 6 birds (9.4% of mortalities) died of anthropogenic causes (e.g. building, car strikes). Crossing roads significantly increased mortality risk, but increasing daily movement distance decreased mortality risk. Our results raise the question of whether anthropogenic sources of mortality are compensatory or additive to natural mortality; we emphasize the need to monitor fledgling survival beyond the parental‐dependence stage in order to fully understand the impacts of anthropogenic hazards on juvenile birds.城市环境会施加全新的选择压力，这类压力对不同物种及各生活史阶段的影响存在显著差异。迁徙鸣禽的出飞后阶段（post-fledging stage）被定义为当年孵化幼鸟从离巢至首次迁徙的时段，该阶段是年度繁殖生产力中尚未被充分认知的环节，且可能限制种群增长。我们选取了两种对城市化分别呈现种群正向与负向响应的迁徙鸣禽作为研究对象：灰猫嘲鸫（Gray Catbird，*Dumetella carolinensis*）与林鸫（Wood Thrush，*Hylocichla mustelina*）。本研究旨在估算城市鸟类种群的出飞后存活率，并明确城市景观的哪些特征会影响出飞后阶段的死亡风险。2012至2014年间，我们共追踪了127只出飞幼鸟（其中60只为灰猫嘲鸫，67只为林鸫）。在出飞后的55天监测周期内，灰猫嘲鸫的累积存活率为0.32（95%置信区间[Confidence Interval, CI]：0.22–0.47），约为林鸫（0.63，95%置信区间：0.52–0.75）的一半。由此可见，若仅孤立考量出飞后阶段的存活率，无法解释两种鸣禽在城市环境中种群动态的差异轨迹。两种鸟类的绝大多数死亡事件（86%）均源于捕食作用。但在脱离亲代抚育实现独立后，共有6只个体（占总死亡数的9.4%）死于人为因素，例如撞建筑、撞车。穿越道路会显著提升个体的死亡风险，但每日移动距离的增加则会降低死亡风险。本研究结果提出了一个核心问题：人为致死因素对自然死亡而言是补偿性的还是叠加性的；我们强调，若要全面厘清人为危害对幼鸟的影响，需对脱离亲代抚育阶段后的出飞幼鸟存活率开展监测。