Urban tree canopy cover over 30 % and native trees enhance bird insectivory and tree biosecurity

Urban trees support biodiversity and the provision of ecosystem services but are increasingly threatened by native and non-native insect pests. Biosecurity of urban trees (i.e., tree protection from biological threats such as pests and pathogens) is enhanced by several intrinsic factors, such as tree defences, and extrinsic factors, such as the occurrence of predators of insect pests. Among the predators of insect pests of trees, birds include many species that might substantially contribute to tree biosecurity. We investigated which levels of urban tree canopy cover and tree species richness support the insectivory function delivered by birds and the overall bird diversity. We measured bird predation and bird diversity in three cities in Switzerland along gradients of urban tree canopy cover, including: industrial/commercial areas with low canopy cover; residential areas with intermediate canopy cover; urban parks and cemeteries with high canopy cover; and peri-urban forests. We used caterpillar mimics and naturally occurring invasive insect larvae of Cameraria ohridella to measure bird predation rates. Bird diversity was assessed using rarefaction curves. We found that bird predation on caterpillar mimics increased with tree canopy cover and decreased with exotic tree species richness, whereas predation on live non-native insect larvae was mostly determined by prey density. We found differences in how urban tree canopy cover influenced bird functional groups. Species richness of insectivorous birds included 75 % of forest species when urban tree canopy cover was at least 30 %. The combined influence of native trees and canopy cover has the potential to increase the insectivory function delivered by birds, with expected benefits for non-native insect control. Our findings match current urban planning targets for sustainable and green cities, that include achieving 30 % tree canopy cover in all city districts.