Urbanization influences breeding abundance of a migratory songbird: a 20-year before-and-after study in 73 forest fragments
Urbanization is a likely threat to biodiversity, but drawing strong inferences about its effects on wildlife is challenging because the state of a population prior to development is rarely known. From 1987 to 2001, the abundance and nest success of a declining migratory songbird, the Wood Thrush (Hylocichla mustelina), were quantified in 73 forest fragments across Waterloo Region, Ontario, Canada, some of which have since been surrounded by urbanization. In 2020 and 2021, we revisited these same fragments and found that Wood Thrush have (1) ceased to breed in many fragments they once occupied, (2) declined most steeply in fragments with new development in the surrounding landscape, but (3) experienced no decline in nest success over the same time period, regardless of development. Our findings provide rare before-and-after evidence that urbanization near breeding habitat may be contributing to a regional decline of a migratory songbird, likely by mechanisms unrelated to nest productivity.