Avian Conservation and Ecology (Jun 2022)
Understanding widespread declines for Common Terns across inland North America: productivity estimates, causes of reproductive failure, and movement of Common Terns breeding in the large lakes of Manitoba
Abstract
Common Tern (Sterna hirundo) breeding populations in inland North America have declined significantly since the 1970s. A 2012 survey of the large Manitoba lakes, previously the largest known inland population stronghold, reported a 57–67% decline in 20 years. A further 38% decline by 2017 highlights the urgent need for research and management. We use ground-based estimates of productivity and analysis of microsatellite markers to provide the first detailed insight into breeding status and movements of Common Terns in this region. At six breeding colonies in 2012, we recorded breeding success in fenced plots, counted fledglings, documented predators and floods, and collected blood samples for microsatellite analysis of movement. Productivity ranged from 0.0 to 2.0 chicks fledged per nest, being highest at large colonies (> 1000 nests) located far away from human settlements (20–30 km). Large-scale breeding failure from predation occurred at smaller colonies close to human settlement. The most common predators were Black-crowned Night Herons (Nycticorax nycticorax) and Great Horned Owls (Bubo virginianus), but we also report three novel predators: Bald Eagles (Haliaeetus leucocephalus), gray wolves (Canis lupus), and river otters (Lontra canadensis). Microsatellite analysis suggested little eastward emigration, but instead a 100-fold increase in immigration from the Great Lakes between the 1990s and 2010s. Substantial population declines in the Manitoba Lakes despite this influx imply that net losses are occurring within inland-breeding populations. Terns now appear to switch frequently between breeding colonies in the region, possibly in response to predation and/or flooding. Although some colonies achieved productivity during the one-year study, continued population decline indicates that monitoring and studies of adult survival and movement are needed, especially given the on-going environmental changes within the region. Only by coupling these data with further efforts in unsurveyed boreal regions can the status of inland-breeding Common Terns be determined and strategies developed to curb apparent, large-scale population declines.
