Investigating the Impacts of Urbanisation on Blue Tit (Cyanistes caeruleus) Diet and Reproductive Success via Faecal DNA Metabarcoding

Abstract

Urbanisation has altered habitats, affecting wildlife by changing environmental conditions and food availability. For birds like blue tits (Cyanistes caeruleus), urban areas can act as ecological traps, where anthropogenic food attracts them, but they experience reduced reproductive success. To examine the impact of urbanisation on diet and its relationship to blue tit condition and breeding success, faecal DNA metabarcoding was used to analyse the diet of over 500 blue tits across a quantified urbanisation gradient, spanning 19 sites from Loch Lomond to Glasgow City centre, UK. Chapter 2 revealed that as urbanisation increased, nestling diets shifted from Lepidoptera to fattier, less nutritious non-Lepidopteran arthropods and anthropogenic foods such as sunflower seeds and grains. This correlated with poorer nestling condition and fledging success. Chapter 3 focused on breeding blue tits, showing that urban birds, though smaller, maintained their condition by consuming a fattier diet dominated by Hemiptera and anthropogenic food including peanuts. The study was the first to identify sex-related dietary differences in blue tits, which shift with urbanisation, and a convergence of adult and nestling diets in highly urbanised environments. Despite maintaining condition, the reproductive output of urban birds may be limited by nutritional deficiencies. Chapter 4 examined the winter diet of blue tits, finding that urban birds had slightly higher body and muscle mass compared to rural birds, despite similar size and condition. Urban birds consumed fewer Hymenoptera and more non-lepidopteran arthropods and anthropogenic food, reflecting a shift to a lower protein, higher fat diet. This dietary shift likely helps urban birds gain weight and survive during winter but may not provide sufficient nutrients to prepare optimally for breeding. Collectively, these findings suggest that urban environments are nutrient-limited which could be a key driver of the reduced reproductive success observed in many urban bird populations.