Abstract
•Area drives annual and seasonal avian species richness in urban green spaces.•Tree canopy cover important for migratory species in the spring.•Bird numbers are not affected by green space shape or isolation.•Ecological data from citizen scientists can inform urban planning and management.
Urban green spaces (UGS) often support diverse bird assemblages. Current evidence suggests large UGS that are circular in shape and contain abundant tree canopy cover will support more bird species independent of UGS isolation. Here, we use bird occurrence information from the eBird citizen-science database over an 18-year period (2002–2019) to estimate annual and seasonal patterns of avian species richness in well-surveyed New York City green spaces. Area was the strongest correlate of annual (n = 102 UGS) and seasonal species richness (n = 43 UGS), whereas shape and isolation lacked significant relationships. Increasing area by 50% resulted in an 11.5% increase in annual and an 8.2% increase in seasonal species richness. There was no evidence that these relationships contained an upper limit. Tree canopy cover was weakly correlated with annual species richness. The richness of nocturnally migrating species was strongly correlated with area in the spring and autumn, especially for non-passerine species. The species richness of nocturnally migrating passerines was strongly correlated with tree canopy cover in the spring, where a 50% increase in canopy cover resulted in a 23.3% increase in species richness. Our findings emphasize the broad importance of area, the value of tree canopy cover for spring migrants, and the limited relevance of shape and isolation. Efforts to enhance area and tree canopy cover will increase the number of resident and migratory bird species, which will likely increase the number of other area-sensitive forest taxa that occur in urban environments.