Abstract
Urban environments are expanding rapidly, and with urbanization come both challenges and opportunities for wildlife. Challenges include combating the anthropogenic disturbances such as light, noise and air pollution and lower availability of natural food sources. The benefits are many, including the availability of anthropogenic food sources, breeding boxes and warmer temperatures. Thus, depending on the context, urbanization can have both positive and negative effects on fitness related traits. It is well known that early-life conditions can have lifelong implications on fitness; little is however known about development in urban environments. We reciprocally cross-fostered urban and rural nestling great tits (Parus major L.) to study how growing up in an urban versus rural habitat affected telomere length (TL)-a suggested biomarker of longevity. We show, for the first time, that growing up in an urban environment significantly shortens TL, independently of natal origin (i.e. urban or rural). This implies that the urban environment imposes a challenge to developing birds, with potentially irreversible effects on lifespan.