Behavioural thermoregulation in man-made habitats: surface choice and mortality risk in Red-necked Nightjars

Capsule Red-necked Nightjars' (Caprimulgus ruficollis) choice of road surface type is significantly affected by air temperature, which may lead to increased traffic-induced mortality under cool weather conditions. Aims To assess how seasonality and weather conditions affect road surface use by Red-necked Nightjars and, if so, to identify possible patterns in human-induced mortality as a result of Nightjars' decisions. Methods In 2009-2011, the number of sitting birds were counted along gravel and paved roads in southwestern Spain to evaluate environmental correlates of surface use by Nightjars. Thermal benefits from using each surface type were determined based on local weather data, while seasonality in road casualty rates was assessed at the regional scale. Results Nightjars' occurrence on roads varied seasonally with the nature of the surface. Nightjars' decisions were significantly affected by the thermal regime, with birds being attracted towards the warmth of paved roads during migration, when cool weather (< 20 degrees C) usually prevailed, or at low temperatures (< 14 degrees C). Paved roads provided birds with a significantly warmer substrate than nearby gravelled or sandy areas. However, traffic-induced mortality significantly increased when surface use shifted from gravel to paved roads. Conclusion Plasticity in surface choice presumably helps Nightjars to minimize thermoregulatory costs during adverse weather conditions, although the adaptive value of thermal-related decisions appears to incur mortality costs for Nightjar populations.