The altered ecological and environmental conditions in towns and cities strongly affect demographic traits of urban animal populations, for example avian reproductive success is often reduced. Previous work suggests that this is partly driven by low insect availability during the breeding season, but robust experimental evidence that supports this food limitation hypothesis is not yet available. We tested core predictions of the food limitation hypothesis using a controlled experiment that provided supplementary insect food (nutritionally enhanced mealworms supplied daily to meet 40%-50% of each supplemented brood's food requirements) to great tit nestlings in urban and forest habitats. We measured parental provisioning rates and estimated the amount of supplementary food consumed by control and experimental nestlings, and assessed their body size and survival rates. Provisioning rates were similar across habitats and control and supplemented broods, but supplemented (and not control) broods consumed large quantities of supplementary food. As predicted by the food limitation hypothesis we found that nestlings in (a) urban control broods had smaller body size and nestling survival rates than those in forest control broods; (b) forest supplemented and control broods had similar body size and survival rates; (c) urban supplemented nestlings had larger body size and survival rates than those in urban control broods; and crucially (d) urban supplemented broods had similar body size and survival rates to nestlings in forest control broods. Our results provide rare experimental support for the strong negative effects of food limitation during the nestling rearing period on urban birds' breeding success. Furthermore, the fact that supplementary food almost completely eliminated habitat differences in survival rate and nestling body size suggest that urban stressors other than food shortage contributed relatively little to the reduced avian breeding success. Finally, given the impacts of the amount of supplementary food that we provided and taking clutch size differences into account, our results suggest that urban insect populations in our study system would need to be increased by a factor of at least 2.5 for urban and forest great tits to have similar reproductive success.

中文翻译：

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食物供应限制了城市中的鸟类繁殖：一项关于大山雀的实验研究

城镇生态和环境条件的改变极大地影响了城市动物种群的人口特征，例如鸟类繁殖成功率通常会降低。以前的工作表明，这部分是由于繁殖季节昆虫的可用性低，但尚无有力的实验证据支持这种食物限制假设。我们使用受控实验测试了食物限制假说的核心预测，该实验为城市和森林栖息地的大山雀雏鸟提供补充昆虫食物（每天供应营养增强型粉虫，以满足每种补充幼虫食物需求的 40%-50%）。我们测量了父母的供应率并估计了对照和实验雏鸟消耗的补充食物量，并评估它们的体型和存活率。不同栖息地以及对照和补充幼体的供应率相似，但补充（而非对照）幼体消耗了大量的补充食物。正如食物限制假说所预测的那样，我们发现（a）城市控制育雏中的雏鸟比森林控制育雏中的雏鸟体型和雏鸟成活率更小；(b) 森林补充和对照育雏具有相似的体型和存活率；(c) 城市补充雏鸟的体型和成活率高于城市对照雏鸟；至关重要的是 (d) 城市补充雏鸟的体型和存活率与森林控制雏鸟中的雏鸟相似。我们的研究结果为雏鸟饲养期间食物限制对城市鸟类繁殖成功的强烈负面影响提供了罕见的实验支持。此外，补充食物几乎完全消除了栖息地在存活率和雏鸟体型方面的差异这一事实表明，除了食物短缺之外，城市压力因素对鸟类繁殖成功率的影响相对较小。最后，考虑到我们提供的补充食物数量的影响并考虑到离合器大小的差异，我们的结果表明，我们研究系统中的城市昆虫种群需要至少增加 2.5 倍，对于城市和森林大山雀有类似的繁殖成功。补充食物几乎完全消除了栖息地在存活率和雏鸟体型方面的差异这一事实表明，除了食物短缺之外，城市压力因素对鸟类繁殖成功率的影响相对较小。最后，考虑到我们提供的补充食物数量的影响并考虑到离合器大小的差异，我们的结果表明，我们研究系统中的城市昆虫种群需要至少增加 2.5 倍，对于城市和森林大山雀有类似的繁殖成功。补充食物几乎完全消除了栖息地在存活率和雏鸟体型方面的差异这一事实表明，除了食物短缺之外，城市压力因素对鸟类繁殖成功率的影响相对较小。最后，考虑到我们提供的补充食物数量的影响并考虑到离合器大小的差异，我们的结果表明，我们研究系统中的城市昆虫种群需要至少增加 2.5 倍，对于城市和森林大山雀有类似的繁殖成功。