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.

Rapidly increasing urbanisation is one of the most significant anthropogenic environmental changes which can affect demographic traits of animal populations, for example resulting in reduced reproductive success. The food limitation hypothesis suggests that the shortage of high-quality nestling food in cities is a major factor responsible for the reduced reproductive performance in insectivorous birds. To study this explanation, we collected data on the parental provisioning behaviour of urban and forest great tits (Parus major) in three years that varied both in caterpillar availability (the main food of great tit nestlings) and in reproductive success of the birds. In all years, urban parents provisioned caterpillars in a smaller proportion to their nestlings, but the total amount of food per nestling (estimated by the volumes of all prey items) did not differ between habitats. In the two years with much lower reproductive success in urban than forest habitats, urban parents had higher provisioning rates, but provided more non-arthropod food and brought smaller prey items than forest parents. In the year with reduced habitat difference in reproductive success, urban parents were able to compensate for the scarcity of caterpillars by provisioning other arthropods rather than non-arthropod food, and by delivering larger preys than in the other years. Specifically, in this latter year, caterpillars provisioned by urban pairs were cc. twice as large as in the other two years, and were similar in size to caterpillars provisioned in the forest broods. These results show that although urban great tit parents can provide the same quantity of food per nestling as forest parents by reducing their brood size and increasing the per capita feeding rates for nestlings, they cannot compensate fully for the scarcity of high-quality preys (caterpillars) in poor years. In some years, however, favourable conditions for urban caterpillar development can greatly reduce food limitation in cities, allowing urban birds to achieve higher reproductive success. We suggest that urban green areas designed and managed in a way to facilitate conditions for phytophagous arthropods could improve habitat quality for urban birds.

The plumage of birds plays an essential role in thermal insulation and influences the heat tolerance of birds. These plumage functions are mainly determined by the number and the density of feathers, but it is unclear how feather density responds to environmental changes in wild populations. In urban birds, both high temperature and limited food could generate changes in plumage traits. To investigate the effect of urbanization, we compared the number of feathers in nestlings between urban and forest Great Tits Parus major using a novel non-invasive method. We showed that urban nestlings have fewer feathers than forest nestlings at 6-9 days old. Although the density of feathers was slightly higher in urban nestlings, this was the result of the smaller size of their feather tracts. We suggest that the reduced feather number may be the result of either adaptation to higher urban temperatures, constrained feather development due to limited optimal nestling-food sources in urban environments, or both. Concentrating body feathers in a reduced tract area may also help birds to adapt to higher urban temperatures because this can increase the relative size of bare body surfaces which may facilitate heat dissipation. We suggest several possibilities for future studies that would help to disentangle the underlying mechanisms responsible for the observed patterns.

Extreme weather events are rare, but can have high impact on human societies and biological systems. As the frequency of extreme events are increasing with current climate change, it is important to understand its effects on fitness of individuals and on long-term viability of populations. In this study, we investigated the effects of extreme high ambient temperatures on breeding success of great tits in two urban and two natural forest populations during six years. We found that the number of hot days had habitat-specific effects on nestlings during their development period (from hatching to 15-days age). Average body mass of broods decreased with increasing number of hot days in both forest populations and one city population. However, the negative effect of hot days on chick mass was significantly stronger in forest compared to urban populations, suggesting that forest populations are more vulnerable to extreme hot weather conditions. Unlike other study populations, average chick mass increased with the number of hot days in the hottest urban study site, suggesting adaptation in heat tolerance in this population. Tarsus length of chicks and their survival until fledging was not influenced significantly by hot weather. Hot weather conditions may affect the reproductive output of birds at least two ways: indirectly through food availability and directly through nestling physiology. As the tarsus size and survival of chicks were not affected by hot days in our study, we suggest that the negative effect of hot days on body mass may emerge more likely from the direct physiological effects of heat. These results are one of the first demonstrations that the effects of extreme weather events can differ between wild populations living in different environment.

Video recordings are commonly used to study the types, amount, and size of food items provided to nestling birds. However, the accuracy and repeatability of estimates of the size of food items from video recordings has not been examined. We assessed three aspects of the reliability of measuring prey size from video recordings of Great Tits (Parus major) provisioning nestlings. To test the accuracy of measurements of prey size (length and width) used to determine prey volume, we molded artificial plasticine caterpillars and compared their size and volume as determined using measurements of length and width on screenshots of video recordings (using the vertical diameter of nest‐box entrance holes as a size reference) to their actual size and volume. We also examined within‐ and among‐observer repeatability of measurements of the size and volume of actual prey items delivered to nestlings by adult Great Tits. We found that observers were able to accurately measure prey size and determine volume, with high agreement between the actual size and volume of plasticine caterpillars and the size and volume as determined from measurements made on screenshots from video recordings (rICC = 0.99). In addition, within‐ and among‐observer repeatability were also high (rICC = 0.98 and 0.93, respectively). Overall, our results suggest that the size of prey items delivered to nestlings by adults in video recordings can be accurately measured and those measurements, in turn, can be used to accurately determine the volume of those insect prey.