Increased lead and glucocorticoid concentrations reduce reproductive success in house sparrows along an urban gradient.

Urbanization is increasing at a rapid pace globally. Understanding the links among environmental characteristics, phenotypes, and fitness enables researchers to predict the impact of changing landscapes on individuals and populations. Although avian reproductive output is typically lower in urban compared with natural areas, the underlying reasons for this discrepancy may lie at the intersection of abiotic and biotic environmental and individual differences.