Black capped and mountain chickadee nestling fecal samples

Urban and suburban ecosystems are growing in size and number as the human population continues to increase. Not all cities are built the same, however, and it is still unclear how resident organisms are faring in increasingly urban habitats and whether cities are ecological sinks. To investigate how urbanization is affecting breeding success in insectivorous birds, we studied two species of chickadee, the black-capped and mountain chickadee[KG1] , along an urban-rural gradient in Boulder County, CO, USA. We used a citizen-science based experimental framework of nest boxes installed on private land throughout Boulder, CO, USA to local monitor breeding populations of chickadees relative to urbanization. Black-capped and mountain chickadees are two species of closely-related songbirds that co-occur broadly along an elevational gradient. Black-capped chickadees are considered to prefer nesting in low-elevation deciduous trees typically found in cities, while mountain chickadee prefer higher elevation, coniferous forests often considered more rural. However, both species do nest in relatively urbanized habitats, but little is known about how they are navigating these urban habitats. Using arthropod surveys & eDNA metabarcoding paired with field observations, we sought to determine if there are species differences between 1) how chickadee nestling condition varies with the level of urbanization of a nesting site and how urbanization of a nest site infleunces 2) insect availability, 3) nestling diet, and 4) adult provisioning effort. We found that there was no significant effect of urbanization on nestling condition in either species, and that urbanization did not significantly predict insect availability, nestling diet, or adult provisioning effort. We did detect however, differences in diet between the species, highlighting clear ecological niches between these species, despite overlapping breeding habitat. This is, to our knowledge, the first study documenting discrete ecological niches in prey preference for developing chicks and adds to a growing body of literature detailing ecological differentiation between these two species. How this divergence in diet contributes to these species ability to navigate increasingly urban habitats is less clear. It is likely that Boulder, Colorado is made of vegetation patches, effectively urban forest fragments, that allow chickadees living in the city to reproduce successful despite decreased prey quality and quantity associated with urban spaces.随着人口持续增长，城市与城郊生态系统的规模与数量不断扩张。然而并非所有城市的建设模式都完全一致，目前仍不清楚栖息生物在日益城市化的栖息地中生存状况如何，也无法明确城市是否属于生态汇（ecological sink）。为探究城市化如何影响食虫鸟类的繁殖成功率，我们在美国科罗拉多州博尔德县沿城乡梯度开展研究，对象为两种山雀：黑顶山雀（black-capped chickadee）与山地山雀（mountain chickadee）[KG1]。我们依托以公众科学（citizen science）为基础的实验框架，在博尔德市全境的私人土地上安装繁殖巢箱，以此监测城市化梯度下山雀的繁殖种群情况。黑顶山雀与山地山雀是两种亲缘关系相近的鸣禽，二者沿海拔梯度广泛共存。普遍认为黑顶山雀偏好筑巢于城市常见的低海拔落叶林中，而山地山雀则偏爱海拔更高、通常被视为更具乡村特质的针叶林。不过，两种山雀均会在城市化程度较高的栖息地筑巢，但目前对它们如何适应这类城市栖息地的了解仍十分有限。我们结合节肢动物调查、环境DNA宏条形码（eDNA metabarcoding）与野外观测，旨在明确以下两方面的物种差异：1）山雀雏鸟体况如何随筑巢地的城市化水平变化；以及筑巢地的城市化如何影响2）昆虫可获得性、3）雏鸟食谱以及4）成鸟育雏投入。研究结果显示，城市化对两种山雀的雏鸟体况均无显著影响，且无法显著预测昆虫可获得性、雏鸟食谱或成鸟育雏投入。不过我们确实发现两种山雀的食谱存在差异，这凸显了二者尽管繁殖栖息地存在重叠，但仍拥有明确的生态位（ecological niche）。据我们所知，本研究首次针对发育中的雏鸟的猎物偏好，明确了二者的离散生态位，同时为日益增多的关于两种山雀生态分化的研究文献补充了新的证据。目前仍不清楚这种食谱分化如何帮助两种山雀适应日益城市化的栖息地。美国科罗拉多州博尔德市的植被呈斑块状分布，本质上属于城市森林片段，这或许能够解释为何栖息在城市中的山雀尽管面临城市环境中猎物质量与数量下降的问题，仍能成功繁殖。