Artificial light alters spatial and temporal habitat use by a crepuscular aerial insectivore

Artificial light is increasing worldwide, and has biological effects from molecular to ecosystem levels, which may be particularly severe for crepuscular and nocturnal animals. We investigated how artificial light affected spatial and temporal patterns of habitat use by Common Nighthawks Chordeiles minor in the southern Grassland and northern Boreal regions of Alberta, Canada. We measured two types of artificial light: upward radiance (light emitted directly from ground sources towards space) and skyglow (light scattered back towards Earth). Nighthawks potentially benefit from hunting aerial insects that aggregate near light sources and using skyglow to see their prey, but they may suffer increased predation risk at their cryptic ground nests if artificial light increases their visibility. We predicted a negative association with artificial light for territorial habitat use (where nighthawks are probably nesting) and a positive association for extra‐territorial habitat use (where they are probably foraging). We recorded the different sounds made by nighthawks that reveal when they are defending a breeding territory and then measured the intensity of territorial and extra‐territorial habitat use at 514 acoustic recording stations. Where nighthawks were present in more than 5% of recordings, we also determined associations between diel patterns of vocal activity and artificial light, measured via sun angle. We found a negative association with artificial light for both territorial and extra‐territorial spatial patterns of habitat use in the Grassland, but not in the Boreal. Similarly, we found no effect of artificial light on temporal patterns of habitat use in the Boreal, whereas the morning activity peak in the Grassland occurred earlier at sites with skyglow when cloud cover was high. Our other measurement of artificial light – upward radiance – had no effect on temporal patterns of habitat use in either region, indicating that artificial light did not extend the foraging activity period of nighthawks. Our results may generalize to other crepuscular species, suggesting that artificial light is more likely to affect habitat use during the breeding season at lower latitudes where natural illumination is lower during twilight.