Night-migratory songbird density is highest at stopover sites with intermediate forest cover and low proportion of forest in conifers in the surrounding landscape

Abstract

Some nocturnal migrant forest-breeding songbirds have suffered large population declines in recent decades. Declining availability of high-quality habitat where birds refuel during migration may be contributing to these declines. Our objective was to identify landscape attributes, including the relevant scales of effect, that make sites likely to be used as stopover sites during fall migration. We used autonomous recording units (ARUs) to sample birds between August and October 2018 at 37 fall migration potential stopover sites in southeastern Ontario, Canada. We placed ARUs in forest patches that varied in the amount and type of forest cover within the surrounding landscape. We interpreted recordings at intervals throughout the season to estimate the average numbers of calling birds per minute at each site. We found that bird density was highest at sites with an intermediate amount of forest within 2 km, while density decreased as the proportion of coniferous forest within 6 km increased. We infer that migrating birds avoid forest sites in landscapes with low amounts of forest cover and high proportions of conifer. The lower densities at high forest amounts may result from a dilution effect (birds spread across more forest), avoidance of conifers, which tended to be more abundant at the highest forest amounts, or reduced densities of edges at high forest amounts, if birds use forest edges for foraging. Our study highlights the importance of retaining landscapes with at least 50% forest cover, particularly deciduous forest, as stopover habitat for migrating songbirds.

Keywords

• autonomous recording units
• habitat amount
• landscape composition
• nocturnal migrant birds
• scale of effect
• stopover habitat