Size in the city: morphological differences between city and forest great tits have a genetic basis.

Animals living in cities are smaller than their conspecifics from rural areas but whether such differences are caused by genetic differences or food constraints remains untested. We performed a multi-generation common garden study where we raised great tits (Parus major), originating from eggs collected from multiple Dutch cities and forests under the same conditions for two generations. Offspring from city birds had a smaller tarsus than forest birds in both generations, demonstrating that these morphological differences are genetic. Next, we tested whether size differences are an adaptation to the low food abundance when offspring are raised in the city. Third-generation birds of both origins were given food amounts mimicking being raised in forests or cities during the second part of their nestling development. While the treatment resulted in birds in the lower feeding frequency treatment to be smaller, city and forest birds responded the same way, suggesting that city birds do not cope better with reduced food availability. Our study shows that the smaller size of urban birds has a genetic basis and is not only caused by a plastic response to restricted resources in the urban environment. Our experiment does not provide evidence that these genetic differences have evolved as an adaptive response to a reduced food availability in cities.