Are house sparrow populations limited by the lack of cavities in urbanized landscapes? an experimental test.

Current urban policies are associated with deep changes in urban structures, which may impoverish urban biodiversity. A major concern is the disappearance of nesting sites for wild vertebrate species living in urban areas. New urban structures without any cracks or cavities may especially preclude cavity nesters from breeding in cities and they may cause population declines. In that context, we experimentally investigated this question in an urban exploiter bird species (the house sparrow, Passer domesticus), which is dramatically declining in most European cities. To test if the lack of cavities is limiting house sparrow populations in urban areas, we equipped 11 sites along an urbanization gradient with nest boxes and we then evaluated the rate of occupancy of these nest-boxes. This urbanization gradient was characterized by very rural places (isolated farms) and moderately urbanized areas (town of medium size, i.e. 60 000 inhabitants). Surprisingly, rural nest boxes were more occupied than urban ones, suggesting that cavity availability is probably more constraining in rural areas relative to urban ones. Therefore, our study suggests that urban house sparrow populations are probably not constrained by a lack of nesting sites in medium size cities with urban designs similar to our city of interest (Niort, Western France). This hypothesis definitely needs now to be tested in further urban landscapes (e.g. large cities and urban landscapes with other architecture and management policies).

当前城市政策与城市结构的深刻变革息息相关，而这类变革可能会削弱城市生物多样性。其中一个核心关切点是，栖息于城市区域的野生脊椎动物的筑巢位点正在消失。无任何裂缝或孔洞的新型城市建筑，尤其会阻碍洞巢鸟类在城市中繁殖，进而可能导致其种群数量下降。在此背景下，我们针对一种在多数欧洲城市中种群数量急剧下降的城市广布鸟类——家雀（Passer domesticus），通过实验探究了这一问题。为验证洞穴缺失是否会限制城市区域家雀的种群规模，我们沿城市化梯度选取了11个样点，并为其安装了人工巢箱，随后对这些巢箱的占用率进行了评估。该城市化梯度涵盖了极度乡村化区域（孤立农场）与中等城市化区域（人口约6万的中型城镇）。令人意外的是，乡村样点的巢箱占用率高于城市样点，这表明相较于城市区域，乡村地区的洞穴可获得性对筑巢的限制作用可能更强。因此，本研究表明，在建筑设计与本次研究的参考城市（法国西部尼奥尔市）相似的中型城市中，家雀的城市种群可能并未受到筑巢位点不足的限制。不过，该假说仍需在更多城市景观（例如大型城市，以及采用其他建筑与管理政策的城市区域）中进一步验证。