Peri-urban forest margins are important stopover sites for moult-migrating songbirds

Cities are significant barriers for migrating birds, but providing suitable greenspaces for stopovers can mitigate urban impacts. City planning for greenspaces often focusses on forests as parks, yet brushy edges and other successional habitats may be equally important especially for songbirds who spend weeks at stopovers in cities to moult their feathers ('moult migrants'). To investigate the role of peri-urban landscapes on moult migrants, we captured 48 moulting and 41 post-moult migrating Swainson's thrushes (Catharus ustulatus) within a large (3000 ha) urban park in Montreal, Canada. Each bird was randomly translocated to one of eight sites of varying habitat type and tracked using radiotelemetry. We compared the time spent at each translocation site with habitat characteristics, expecting birds to spend more time in areas with high food availability and concealment from predators. Migrating thrushes spent similar to 4 times as much time in dense forest margins than in fields and mature forests. Berry-producing shrubs that we identified as food for thrushes (by DNA-barcoding of their feces) best characterized those margins. Stopover home ranges were much smaller than the size of the park (50% core range: 10.6 +/- 17.2 ha). This highlights the importance of urban greenspaces with dense forest margins during a vulnerable time for Swainson's Thrushes, and the conservation implications of maintaining these lower-valued greenspaces. Given that city parks are often lacking understory growth, we recommend keeping shrubby patches for migrating birds.