Migratory nightjars roost on asphalt to reduce thermoregulatory costs and increase fuel accumulation

Abstract

Migration is an energetically expensive event in the annual cycle of birds, requiring substantial fuel stores for successful completion. Optimizing fuel deposition rates benefits migrants, as faster fueling reduces the time required to store the necessary energy, maximizing overall migration speed. Studies on fueling optimization have largely focused on increasing energy intake to generate a surplus. In contrast, the role of reducing energy expenditure has received comparatively little attention. We analyzed an 8-year (2011–2018) dataset collected as part of a long-term field study on a population of the migratory red-necked nightjar Caprimulgus ruficollis in southern Spain to assess roosting-substrate preferences during the fueling period. Specifically, we investigated the influence of intrinsic factors (body mass, food consumption, age and sex of the individuals) and environmental factors (ambient air temperature, time since sunset and moon illumination) on nightjars' use of gravel versus asphalt roads as a thermoregulatory mechanism to reduce overall energy expenditure during fueling. An experiment simulating heat loss for road-sitting nightjars confirmed that asphalt is a more thermally favorable substrate than gravel, enabling roosting nightjars to reduce energy expenditure for thermoregulation. We hypothesized that nightjars may thus benefit from using asphalt roads to obtain a more positive energy balance and potentially facilitate fueling. Using an information-theoretic approach and capture data from 505 individuals, we evaluated support for a set of models which made a priori predictions based on ecological reasoning and previous research. We found that nightjars most often sit on asphalt roads on moonlit nights, regardless of age, sex, body mass, food consumption, time since sunset and ambient temperature. Given that the foraging efficiency of nightjars peaks during moonlit nights, individuals preparing for the migratory journey may benefit from reduced thermoregulatory costs on asphalt during full-moon periods, thereby facilitating faster fuel deposition. Our results highlight the potential importance of behavioral thermoregulation for efficient fueling in migratory species.