Tree cover and geographic origin of tree species drive bird functional groups in urban environments

Urban trees provide habitat and resources for wild species in cities and support biodiversity provision of ecosystem services, among which insect control is particularly relevant. Predators of insects are promoted by the presence of trees and, in turn, may enhance biosecurity of the trees. We investigated how tree canopy cover and tree species composition shaped bird assemblages and their functions, with particular emphasis on insectivory, along tree cover gradients in three cities in Switzerland. Sampling was stratified across four categories of tree canopy cover: 1) areas with low canopy cover 0–20%; 2) areas with intermediate canopy cover 20–40%; 3) parks with high canopy cover 40–60%; and 4) peri‐urban forests as reference sites. We used hierarchical distance sampling models to estimate abundance of birds belonging to different diet‐based functional groups recorded along transects located in different city districts. We then explored correlations with species richness of native and non‐native trees. We found opposite responses of different bird functional groups to tree canopy cover. Insectivore and omnivore abundance were positively related to canopy cover, showing the highest abundances in peri‐urban forests, parks and residential areas. Granivores, instead, preferred industrial and commercial areas. Omnivore and granivore abundance was positively related with species richness of native trees, while insectivore abundance was negatively related to species richness of non‐native trees. We showed that the beneficial effects of urban trees for birds depend on how much native tree species contribute to overall canopy cover in cities. The importance of urban trees for many bird species is mostly related to food resources, such as adult insects and their larvae, whose populations are supported mostly by native trees. The insectivory function and the insect control service provided by birds could be optimised by supplying cities with native trees and by minimizing the share of non‐native trees.