Does the urban gradient influence the composition and ectoparasite load of nests of an urban bird species?

Urbanisation has profound impacts on birds via, for example, changes in activity budgets, distributions and movements influenced by resource availability, and the connectedness of preferred habitats. We live in an urbanising world and yet understand little about how urbanisation affects the basic biology of organisms that share urban spaces with us. A case in point is nest construction and nest maintenance behaviours that require significant investments of time and energy by birds early in the breeding attempt. Here, we studied how position on an urban gradient in the city of Birmingham, UK, influenced the composition and ectoparasite load of nests of Blue Tits (Cyanistes caeruleus). In total, we deconstructed 131 nests removed from nestboxes at the end of the breeding season in 2014 at 30 different locations along an urban gradient. Nest composition varied significantly along this gradient with significant relationships between feather content and built cover (negative), and connected tree cover (positive). Notably, anthropogenic materials were found in 73 % of nests but their inclusion was unrelated to position on the urban gradient. The only identifiable ectoparasites in nests were Siphonapterans (fleas) and although ectoparasite load was unrelated to position on the urban gradient, it was positively related to nest mass. Taken together, we show that even for a common species that is often referred to as an 'urban adapter', the urban gradient influences nest composition and ectoparasite load, and thus potentially reproductive outcomes of small passerines. The challenge is to roll out this approach over multiple years to test the applicability of our findings over longer timeframes and their broader implications for a wide range of bird species that are routinely found breeding in increasingly urbanised landscapes globally.