Roost selection and roosting behavior of male common nighthawks

Many studies to date have documented clear energetic costs and benefits of avian roost selection. Male Common Nighthawks (Chordeiles minor) spend at least half of each day during the summer on a day-roost. Therefore selection of roost-sites likely has implications for survival and long-term fitness. Our objective was to identify characteristics of day-roosts used by male Common Nighthawks in Cypress Hills Provincial Park, Saskatchewan, Canada. We measured features of roost trees and monitored behavior of roosting birds. Nighthawks preferred trees situated on north facing slopes surrounded by trees with significantly lower canopy height compared to randomly measured trees. Roost trees were taller than random trees and occurred in less dense patches of forest. Birds always roosted parallel to branches and adopted a motionless posture. There was a non-significant trend for birds to roost on branches facing east. Birds typically roosted in a direction pointing away from the sun and away from the roost tree trunk. Roost trees emerging from the canopy may provide landmarks for birds as they search for suitable day-roosts, whereas a low tree density surrounding roost trees likely reduces flight costs associated with maneuvering. Our results suggest that roosts chosen by male nighthawks may provide selective benefits in terms of microclimate, energetics and predator avoidance, but further studies are needed to determine which is the most important.