Behavioral variation changes across an urbanization gradient in a population of great tits

Abstract

Urbanization is occurring globally at an unprecedented rate and, despite the eco-evolutionary importance of individual variation in adaptive traits, we still have very limited insight on how phenotypic variation is modified by anthropogenic environmental change. Urbanization can increase individual differences in some contexts, but whether this is generalizable to behavioral traits, which directly affect how organisms interact with and respond to environmental variation, is not known. Here we examine variation across three behavioral traits (breath rate, handling aggression and exploration behaviour) in great tits Parus major along an urbanization gradient (n > 1000 phenotyped individuals accross nine years) to determine whether among-individual variance in behavior increases with the degree of urbanization and spatial heterogeneity. Urban birds were more aggressive and faster explorers than forest birds. They also displayed higher among-individual variation for breath rate and aggression (1.5 and 1.8 times increase, respectively), but lower among-individual variation for exploration (3.3 times decrease). Only individual variation in exploration clearly changed along the continuous urbanization gradient; individual differences in exploration declined with increasing impervious surface area. Collectively our results suggest that individuals in the city may have more diverse behavioral stress responses, yet display stronger similarity in their behavioral responses to novelty. Our results suggest that generalizations about urbanization’s impacts on behavioral variation are not appropriate. Instead our results suggest that urbanization can shape individual variation differently across behavioral functions and we may expect decreased individual diversity in urban birds for traits related to behavioral response to novelty.