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), and insectivorous birds (Great Tits) to assess how urbanization influences (1) the phenology of each component of this system, (2) insect abundance, and (3) 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.

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.

Animals living in urbanized habitats often show reduced fear responses to humans compared with their rural conspecifics. This is usually assumed to be the result of habituation, but may also be explained by differential colonization or local adaptation. To contrast these hypotheses, we studied the fear responses of urban and rural house sparrows (Passer domesticus) to humans, by measuring flight initiation distances (FID) in free-living flocks and observing the hiding behavior of wild-caught individuals in response to repeated human disturbance in captivity. We found that although sparrows had shorter FID at urban compared with rural sites, sparrows from both habitat types were equally likely to hide when they were disturbed for the first time in the new captive situation. Both urban and rural sparrows decreased their time spent hiding over the course of 8 trials, but the decrease was faster in urban sparrows. This difference was primarily due to a decrease in the sparrows’ immediate response (reactivity) to the disturbance, whereas the speed of recovery after disturbance increased similarly over trials in urban and rural birds. These results demonstrate that urban individuals habituate faster to human disturbance than their rural conspecifics. Our findings suggest that the reduced fear of urban animals is the result of behavioral plasticity, whereas we found no evidence for their higher intrinsic boldness as predicted by differential colonization and local adaptation.