The Effects of Urban Living upon Diet Variation and Overall Condition of Blue Tit Chicks in Manipulated Broods

Abstract

Urbanisation is expanding rapidly and brings with it substantial threats to biodiversity. The milder climates of towns and cities, as well as the presence of nesting sites near to accessible food sources, can make them beneficial environments for many bird species. Unfortunately, much of the food found in urban environments may be of insufficient nutritional value, especially for the development of nestlings, and so urban populations of many species exhibit reduced reproductive success compared to forest populations. Whether this negative effect is consistent throughout urban areas is unclear; microhabitats within cities could host sufficient food, while some urban parents could be able to provide a better quality diet to their young. Here, the nestling diet variation of the blue tit, Cyanistes caeruleus, is assessed using diet metabarcoding of blue tit chick faecal samples from an urban and a forest population. This is combined with a brood manipulation experiment to explore the impact of increased parental stress upon nestling diet and condition. The impacts of enlarging broods - increased nestling mortality and worse quality fledglings - were consistent across the city and forest, which may be attributable to phenotypic plasticity in clutch size. Urban clutches averaged 2.5 fewer eggs than forest clutches, although its likely urban adults were physiologically constrained in egg production. Overall, city nestlings appeared to be in worse condition than their forest counterparts, and caterpillars, their preferred food, comprised just 39.36% of the insect food items found in their diet samples, compared to 69.24% in the forest. City diets were more diverse and exhibited greater variation both between and within nest boxes, with some urban chicks seemingly receiving as many caterpillars as their forest counterparts. This suggests that not only is it possible for blue tits to raise their chicks in urban habitats, but also that some of their offspring may be no worse off than forest nestlings. A greater understanding of the factors that could allow such success is needed, with improved and targeted management potentially helping towns and cities provide optimal breeding environments for blue tits and other bird species.