Urbanization mediates the effects of water quality and climate on a model aerial insectivorous bird

Aerial insectivorous birds have experienced alarming population declines
in eastern North America. Meanwhile, urbanization continues to increase
rapidly, with urban land use comprising 69.4 million acres, or 3.6% of
total land area, in the contiguous United States. Multiple environmental
changes are associated with urbanization, including alterations to local
climate, changes in habitat structure, and potential shifts in both
terrestrial and emergent aquatic flying insects on which aerial
insectivorous birds rely. Here, we investigated the linkages between
urbanization, water quality, and Tree Swallow (Tachycineta bicolor)
reproductive success and body condition at seven river-riparian sites
representing urban and protected land use in Columbus, Ohio over five
consecutive years (2014-2018). Tree Swallows at urban and protected sites
relied on emergent aquatic insects for 37.4 and 30.8% (SD = 28.4 and
24.1%) of their nutritional subsidies, respectively. Despite the loss of
environmental quality generally attributed to cities, Tree Swallows
exhibited greater reproductive success in urban settings where climate was
more amenable to egg and nestling survival, and the breeding season was
longer. Urban-nesting Tree Swallows initiated laying 7.9 days earlier and
fledged 35% more young per nest than those at protected sites. Multiple
characteristics of urban sites appeared to drive these patterns, including
differences in mean and extreme air temperatures and measures of water
quality (e.g., water temperature, nutrient concentrations, turbidity).
However, chronic effects of elevated Hg concentrations – which were 482%
greater in adult swallow blood at urban sites than at protected sites
where swallows exhibited a 17.4% lower trophic position – may disadvantage
individuals in other ways. Further, although Tree Swallows are a good
model aerial insectivore bird species, characteristics of urban landscapes
that benefit Tree Swallows may not advantage other aerial insectivorous
birds owing to differences in life-history and foraging strategies. These
findings implicate urbanization, local climate, and water quality as
important considerations in the conservation of aerial insectivorous
birds.

北美东部的食虫性飞禽（aerial insectivorous birds）正经历着令人警醒的种群衰退态势。与此同时，城市化进程仍在快速推进，美国本土的城市用地总面积达6940万英亩，占全域陆地面积的3.6%。城市化伴随诸多环境变化，包括局地气候改变、栖息地结构重塑，以及食虫性飞禽赖以生存的陆生与水生羽化飞行昆虫种群的潜在动态变迁。本研究于2014至2018年间，在俄亥俄州哥伦布市的7处河滨生境样点（涵盖城市用地与保护用地两类生境），针对城市化、水质与树燕（Tree Swallow, *Tachycineta bicolor*）的繁殖成功率及身体状况之间的关联展开了为期5年的系统性调查。城市与保护地样点的树燕，其营养补给分别有37.4%与30.8%来自水生羽化昆虫（标准差SD分别为28.4与24.1）。尽管城市通常被认为会造成环境质量下降，但城市生境中的树燕繁殖成功率反而更高——当地气候更适配卵与雏鸟存活，且繁殖季更长。在城市筑巢的树燕，其产卵起始时间较保护地样点的个体早7.9天，每巢出飞的幼鸟数量也高出35%。城市样点的多项特征似乎促成了这一结果，包括平均与极端气温的差异，以及水质指标（如水温、营养盐浓度、浊度）的不同。然而，成年树燕血液中的汞（Hg）浓度在城市样点较保护地高出482%，且保护地树燕的营养级较城市样点低17.4%——长期暴露于高浓度汞环境可能会从其他方面对个体造成不利影响。此外，尽管树燕是一种理想的食虫性飞禽模式物种，但由于不同鸟类的生活史与觅食策略存在差异，对树燕有利的城市景观特征，未必能惠及其他食虫性飞禽。本研究结果表明，城市化、局地气候与水质是食虫性飞禽保护工作中需重点考量的核心因素。