Forest degradation, not loss, drives widespread avian population declines

Matthew G. Betts, Forest Ecosystems and Society, Oregon State University, Corvallis, OR, Chris Thomas, Department of Biology, University of York, Zhiqiang Yang, Oregon State University, Corvallis, OR and Javier Gutierrez Illan, Biology, Washington State University, Pulman, WA

Background/Question/Methods
In many regions of the world, forest management has reduced old forest and simplified forest structure and composition via reliance on monoculture tree plantations. We hypothesized that such forest degradation has resulted in long-term habitat loss for forest-associated bird species of eastern Canada (130,017 km2) which, in turn, has affected bird population declines. Back-cast species distribution models revealed that despite little change in overall forest cover, breeding habitat loss occurred for 66% of the 54 most common species from 1985-2020. This habitat loss was strongly associated with population declines for 72% of species, as quantified in an independent, long-term dataset. Since 1985, net forest bird abundance has declined in this region by an estimated 33-104 million birds due to habitat loss alone. The effects of forest degradation may therefore be a primary cause of biodiversity decline in managed forest landscapes.