Urban environmental predictors of group size in cliff swallows (Petrochelidon pyrrhonota): A test using community-science eBird data

Studies on the impacts of urbanization on birds have explored a wide variety of behaviors (e.g. foraging, breeding, migratory), but there is little research on the impacts of cities on avian coloniality. Various urban-environmental factors may impact colonial birds, including the predominance of impervious surfaces in cities, as well as the urban heat island effect and shifts in habitat and resource availability (e.g. food, water). Here we used five years of community-science data available in eBird to investigate urban impacts on group size in cliff swallow (Petrochelidon pyrrhonota), an abundant colonial bird species that now breeds readily under bridges and other built structures over or near water in Phoenix, Arizona, USA. We hypothesized that, based on the unique breeding habits of these neotropical migratory birds in this desert environment, swallows in Phoenix would form larger groups in areas with more food and water sources and with more built structures. In fact, we found that proximity to water sources and cropland, but not impervious surface density, was positively and significantly related to group size. These results suggest that, in this desert ecosystem, an abundance of often-human-provided food/water resources permits cliff swallows to form larger social groups during breeding. Although many studies show harmful impacts of cities on local wildlife, our findings highlight how urban and/or agricultural ‘oases’ may relieve some native species from natural resource stressors and permit them to thrive and increase in group size in human-impacted environments.

有关城市化对鸟类影响的研究已涵盖了多样的鸟类行为（如觅食、繁殖、迁徙），但针对城市对鸟类集群营巢习性（avian coloniality）影响的研究仍较为匮乏。诸多城市环境因子可能对集群营巢鸟类产生影响，包括城市中不透水地表的占比、城市热岛效应，以及栖息地与资源（如食物、水源）可获得性的变化。本研究依托eBird平台的五年公民科学观测数据，针对城市化对崖燕（Petrochelidon pyrrhonota）种群集群规模的影响展开调查。崖燕是一种常见的集群营巢鸟类，目前在美国亚利桑那州凤凰城的水域上方或周边的桥梁及其他人工建筑下已广泛繁殖。基于这类新热带迁徙鸟类在该沙漠环境中的独特繁殖习性，我们提出如下假设：凤凰城的崖燕会在食物、水源更充足，且人工建筑更多的区域形成更大的种群集群。研究结果显示，种群集群规模与水源及农田的距离呈显著正相关，但与不透水地表密度无显著关联。上述结果表明，在该沙漠生态系统中，人类活动提供的充足食物与水源，使得崖燕在繁殖期能够形成更大的社会集群。尽管诸多研究证实了城市对本土野生动物的负面影响，但本研究结果凸显了城市及/或农业‘绿洲’如何帮助部分本土物种摆脱自然资源胁迫，使其在人类活动影响的环境中得以繁盛并扩大种群集群规模。