Data from: Impact of urbanization on abundance and phenology of caterpillars and consequences for breeding in an insectivorous bird

Urbanization can have marked effects on plant and animal populations' phenology, population size, predator-prey interactions and reproductive success. These aspects are rarely studied simultaneously in a single system, and some are rarely investigated, e.g. how insect phenology responds to urban development. Here, we study a tri-trophic system of trees – phytophagous insects (caterpillars) – insectivorous birds (great tits) to assess how urbanization influences i) the phenology of each component of this system, ii) insect abundance and iii) avian reproductive success. We use data from two urban and two forest sites in Hungary, central Europe, collected over four consecutive years. Despite a trend of earlier leaf emergence in urban sites there is no evidence for an earlier peak in caterpillar abundance. Thus, contrary to the frequently stated prediction in the literature, the earlier breeding of urban bird populations is not associated with an earlier peak in caterpillar availability. Despite this the seasonal dynamics of caterpillar biomass exhibited striking differences between habitat types with a single clear peak in forests, and several much smaller peaks in urban sites. Caterpillar biomass was higher in forests than urban areas across the entire sampling period, and between 8.5 and 24 times higher during the first brood's chick-rearing period. This higher biomass was not associated with taller trees in forest sites, or with tree species identity, and occurred despite most of our focal trees being native to the study area. Urban great tits laid smaller clutches, experienced more frequent nestling mortality from starvation, reared fewer offspring to fledging age, and their fledglings had lower body mass. Our study strongly indicates that food limitation is responsible for lower avian reproductive success in cities, which is driven by reduced availability of the preferred nestling diet, i.e. caterpillars, rather than phenological shifts in the timing of peak food availability.

城市化会对动植物种群的物候（phenology）、种群规模、捕食-被捕食相互作用以及繁殖成功率产生显著影响。上述研究维度极少在同一生态系统中被同时探讨，其中部分议题更是鲜有涉及——例如昆虫物候如何响应城市化进程。本研究以树木-植食性昆虫（phytophagous insects，即毛虫（caterpillars））-食虫鸟类（大山雀（Great Tits））构成的三营养级系统（tri-trophic system）为研究对象，旨在评估城市化对以下三方面的影响：①该系统各组分的物候，②昆虫种群丰度，以及③鸟类繁殖成功率。本研究使用采自中欧匈牙利2处城市样地与2处森林样地、历时4个连续年度的调查数据。尽管城市样地的树木展叶物候呈现提前趋势，但并未观测到毛虫种群丰度峰值提前的现象。因此，与学界广为流传的理论预测相悖，城市鸟类种群的提前繁殖，并未与毛虫可利用量的峰值提前存在关联。尽管如此，毛虫生物量的季节动态在不同生境类型间呈现出显著差异：森林样地仅存在一个清晰的单一峰值，而城市样地则存在多个峰值更低的次级峰值。在整个采样周期内，森林样地的毛虫生物量均高于城市区域；在第一窝雏鸟的育雏期，森林样地的毛虫生物量更是达到城市样地的8.5至24倍。这种更高的生物量与森林样地中树木的高度并无关联，也不受树木物种组成的影响；尽管本研究的多数目标树种均为研究区域的本土物种，但上述差异依然存在。城市生境中的大山雀窝卵数更小，雏鸟饥饿致死率更高，成功抚育至离巢阶段的幼鸟数量更少，且离巢幼鸟的体质量更低。本研究有力证实，城市生境中鸟类繁殖成功率偏低的根源在于食物资源受限：这一限制由雏鸟偏好的食物（即毛虫）可获得性降低所致，而非食物峰值出现时间的物候偏移。