Diet composition and diversity does not explain fewer, smaller urban nestlings

The reproductive success of animals breeding in cities is often lower compared to counterparts that inhabit rural, suburban, and peri-urban areas. Urban dwelling may be especially costly for offspring development and survival. Diet composition and diversity may underlie factors that lead to lower fitness, particularly if prey abundance and quality decline in modified environments. Moreover, breeding success may change over the course of a season, an effect that may be augmented in urban areas. In this study, we tested the hypothesis that habitat and date affected nestling house wren (Troglodytes aedon) body condition and survival, and examined whether diet explained differences in nestling success. We monitored urban and rural populations of house wrens breeding in nest boxes, and tested whether clutch size, nestling survivorship, and nestling body condition varied by habitat or by date, and then characterized the diet of a subset of nestlings with DNA metabarcoding of fecal samples. Urbanization had clear impacts on house wren nestling fitness: urban broods contained fewer, smaller nestlings. Early nestling survival decreased as the breeding season progressed, and this effect was more pronounced in the urban population. However, the diets of urban and rural nestlings were similar and did not explain differences in body condition. Instead, across populations, diet changed with date, becoming less diverse, with fewer Lepidoptera and more Orthoptera. Regardless of habitat, adult house wrens provide nestlings with similar types of foods, but other factors, such as quantity or quality of prey delivered, may lead to fitness disparities between urban and rural nestlings.