Behavioral DiverCity: individual differences in behavior change along an urbanization gradient

Urbanization is occurring globally at an unprecedented rate and, despite the eco-evolutionary importance of individual variation, we still have 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 well known. Here we examined variation across three behavioral traits linked to stress reactivity, anti-predator response, and novelty-coping (breath rate, handling aggression, and exploration behavior) in great tits Parus major along an urbanization gradient. We phenotyped > 1000 individuals across 9 yr, to test whether individual differences in behavior increased with urbanization and spatial environmental heterogeneity. We used two different approaches: a city vs. forest comparison (ie a binary descriptor) and an urbanization gradient approach (ie a continuous quantitative score from 0 to 1) to explore the influence of built-up areas at different spatial scales. Our results reveal that urban individuals display more diverse stress-related and anti-predator behaviors (breath rate and handling aggression), yet show more similarity in their exploratory behavior than forest counterparts. However, there was no evidence that individual variation changes along the percentage of built-up areas for any traits. This study suggest that generalizations about how behavioral traits respond to urbanization will differ across behavioral dimensions. In particular, we may expect decreased individual diversity in urban birds for traits related to behavioral response to novelty.