Before-and-after evidence that urbanization contributes to the decline of a migratory songbird

Abstract

Although urbanization is widely believed to be an increasing threat to biodiversity, drawing strong inferences about its effects on wildlife has been challenging because the state of a population prior to development is rarely known. The Wood Thrush (<em>Hylocichla mustelina</em>) is a steeply declining migratory songbird that breeds in North American temperate deciduous forests and winters in Central America. From 1987 to 2001, Wood Thrush abundance and daily nest survival were measured in 72 forest fragments across Waterloo Region, Ontario, Canada. Some of these fragments have since been surrounded by urbanization while others have remained rural. In 2020 and 2021, we reassessed Wood Thrush abundance and daily nest survival in these same fragments by replicating point count and nest monitoring protocols. Our results suggest that Wood Thrush have declined most steeply in fragments with development built within 1 km during the intervening 20-year period (-79% vs. only -57% elsewhere), although experiencing no decline in daily nest survival over the same time period, regardless of development. Our findings provide rare before-and-after evidence that urbanization near breeding habitat is contributing to a pronounced localized decline of a migratory songbird, likely by mechanisms unrelated to nest survival.