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，且对不同物种以及不同生活史阶段的影响存在差异。迁徙鸣禽（migratory passerines）的出巢后阶段（post-fledging stage，即当年孵化幼鸟从离巢至首次迁徙的时段）是年度繁殖产出中认知度极低的环节，且该阶段可能限制种群增长。本研究选取了分别对城市化呈现正向、负向种群响应的两种迁徙鸣禽：灰猫嘲鸫（Gray Catbird，学名Dumetella carolinensis）与棕林鸫（Wood Thrush，学名Hylocichla mustelina）。研究目标为估算城市鸟类种群的出巢后存活率，并明确城市景观的哪些特征会影响出巢后阶段的死亡风险。2012至2014年，我们共追踪了127只出巢幼鸟（其中灰猫嘲鸫60只、棕林鸫67只）。在出巢后的55天观测期内，灰猫嘲鸫的累积存活率为0.32（95%置信区间：0.22–0.47），约为棕林鸫的一半（棕林鸫累积存活率为0.63，95%置信区间：0.52–0.75）。因此，仅单独考量出巢后阶段的存活率，无法解释两种鸣禽在城市环境中的种群动态差异。两种鸟类的绝大多数死亡事件（86%）均源于捕食作用。但在脱离亲鸟抚育后，共有6只幼鸟（占总死亡数的9.4%）死于人为因素（如撞击建筑物、被车辆撞击）。穿越道路会显著提升死亡风险，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新问题：人为致死因素对自然死亡而言究竟是补偿性的还是叠加性的；我们强调，需对亲鸟抚育期后的出巢幼鸟存活率开展监测，才能全面了解人为危害对幼鸟的影响。