Individual variation among and within natural populations can have eco-evolutionary implications by, for example, affecting species interactions or evolutionary potential. Urban systems present a unique opportunity to evaluate how environmental change shapes variation since urban phenotypic differentiation is widely documented on contemporary timescales. We introduce and test three hypotheses to determine how urbanization affects phenotypic variation at different population levels. Combining 21 long-term datasets in a mega-analysis approach, we synthesize how urbanization impacts variation in tarsus length and lay date among and within subpopulations of great and blue tits (Parus major, Cyanistes caeruleus) at a continental scale. Our synthesis reveals that urbanization is associated with increased phenotypic variation within subpopulations by 11% on average, and by as much as 25% across the species and traits examined. We also find some evidence (for tarsus length in great tits) that urbanization increases differentiation between subpopulations. We did not, however, find that urbanization increases differences between subpopulations in their within-subpopulation variation. Our synthesis provides novel insights into how urban contexts impact individual diversity at different spatial scales and we highlight future directions that could establish the genetic and environmental effects that underlie these continental patterns of urban phenotypic variation.

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.

Successful urban colonisation by formerly rural species represents an ideal situation in which to study adaptation to novel environments. We address this issue using candidate genes for behavioural traits that are expected to play a role in such colonisation events. We identified and genotyped 16 polymorphisms in candidate genes for circadian rhythms, harm avoidance, and migratory and exploratory behaviour in 12 paired urban and rural populations of the blackbird Turdus merula across the Western Palearctic. An exonic microsatellite in the SERT gene, a candidate gene for harm avoidance behaviour, exhibited a highly significant association with habitat type in an analysis conducted across all populations. Genetic divergence at this locus was consistent in 10 of the 12 population pairs; this contrasts with previously reported stochastic genetic divergence between these populations at random markers. Our results indicate that behavioural traits related to harm avoidance and associated with the SERT polymorphism experience selection pressures during most blackbird urbanization events. These events thus appear to be influenced by homogeneous adaptive processes in addition to previously reported demographic founder events.