Nightjars use warm asphalt roads to cut energy costs before migration
An eight-year study of Red-necked Nightjars in southern Spain found birds preparing for autumn migration increasingly roosted on heat-retaining asphalt during moonlit nights, potentially helping them build fuel reserves more efficiently.
Migrating Red-necked Nightjars appear to use the lingering warmth of asphalt roads as part of an energy-saving strategy while building the fat reserves needed for their autumn journey, according to new research from southern Spain.
The study, published in the Journal of Avian Biology, analysed eight years of observations from a long-term project in Doñana Natural Park. Researchers examined road use by 505 Red-necked Nightjars Caprimulgus ruficollis during the late-summer and autumn fuelling period between 2011 and 2018.
Red-necked Nightjars breed across the Iberian Peninsula and North Africa before migrating to wintering grounds south of the Sahel. Like other nightjars they are aerial insectivores, feeding mainly on moths, and their ability to forage efficiently at night is strongly influenced by moonlight.
The birds regularly use open roads both for feeding and for sitting between bouts of activity. When resting they adopt a flattened posture, bringing much of the underside of the body into direct contact with the road surface. That makes the thermal properties of the substrate potentially important.
Asphalt retains heat absorbed during the day for longer than gravel. In an experiment designed to mimic heat loss from road-sitting nightjars, the researchers found that conductive heat loss was around 5-7 per cent lower on asphalt than on gravel. In principle, a bird sitting on the warmer surface therefore needs to spend less energy maintaining its body temperature.
The clearest pattern emerged with the lunar cycle. During the pre-migration fuelling period, the probability of a nightjar using asphalt rose from about 35 per cent on new-moon nights to around 65 per cent under a full moon. Moon illumination was the only factor to receive strong statistical support; age, sex, body mass, stomach fullness, ambient temperature and time since sunset did not show comparable effects.
That timing may be important because moonlit nights are also when nightjars can forage most effectively. Brighter conditions improve their ability to detect flying insects and extend the time available for feeding. The researchers suggest that combining greater food intake with lower thermoregulatory costs could leave more surplus energy available to be stored as fat, helping birds prepare for migration more quickly.
A supplementary comparison of birds recorded during both breeding and fuelling periods strengthened that interpretation. The tendency to favour asphalt as moonlight increased was detected during fuelling but not during the breeding season, suggesting that the behaviour may be particularly associated with preparation for migration.
However, the study does not show that sitting on asphalt directly increases the amount of fat a wild nightjar gains. The authors caution that road choice is probably an integrated response to several factors. Warm surfaces might also aid digestion, while predator detection, movement patterns and other ecological pressures could influence where birds choose to sit.
Predation may help explain why nightjars do not simply choose the warmer surface every night. Red Foxes are the principal nocturnal predator of nightjars in the study area and are more active on dark nights. The researchers recorded a disproportionate number of fox sightings on asphalt stretches, raising the possibility that nightjars may sometimes trade energy savings for the relative safety of gravel roads when moonlight is poor.
The work was carried out on roads with exceptionally low traffic levels, generally negligible and no more than about 10 vehicles a day, so the findings should not be taken as evidence that busy roads are beneficial habitat. Rather, they reveal how finely nightjars can adjust their behaviour to combine feeding opportunities, temperature and other risks during one of the most energetically demanding stages of their annual cycle.
The authors say direct physiological measurements, such as body-temperature monitoring, will be needed to confirm exactly how much energy wild nightjars save by choosing asphalt and whether that translates into faster fuel accumulation. The results nevertheless highlight a largely overlooked part of migration ecology: birds may speed up fuelling not only by eating more, but also by finding ways to spend less energy while they do it.
The research paper, Migratory nightjars roost on asphalt to reduce thermoregulatory costs and increase fuel accumulation, is available open access in the Journal of Avian Biology.
14 Aug 2026
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