How is artificial light linked to where and when Common Nighthawks use their habitat?

Details

Artificial light is increasing worldwide and has biological effects from molecular to ecosystem levels, potentially with particularly severe effects on crepuscular and nocturnal animals. Researchers investigated spatial and temporal patterns of habitat use by Common Nighthawks (Chordeiles minor) in the southern Grassland and northern Boreal regions of Alberta, Canada. They measured upward radiance, light emitted directly from ground sources toward space, and skyglow, light scattered back toward Earth.

Nighthawks might benefit by hunting airborne insects that gather near lights and using skyglow to see prey, but artificial light could also make their cryptic ground nests more visible and increase predation risk. The researchers predicted a negative association between artificial light and territorial habitat use, where the birds were probably nesting, and a positive association with extra-territorial use, where they were probably foraging. They recorded the different nighthawk sounds that reveal when the birds are defending a breeding territory, then measured the intensity of territorial and extra-territorial habitat use at 514 acoustic recording stations. At sites where nighthawks appeared in more than 5% of recordings, they also assessed associations between artificial light and daily patterns of vocal activity, using sun angle to measure timing.

In the Grassland, artificial light was negatively associated with both territorial and extra-territorial spatial patterns of habitat use; no such association was found in the Boreal. Artificial light had no detected effect on temporal habitat-use patterns in the Boreal, while in the Grassland the morning activity peak occurred earlier at sites with skyglow when cloud cover was high. Upward radiance had no detected effect on temporal habitat-use patterns in either region, indicating that artificial light did not extend the nighthawks’ foraging activity period. The researchers suggest the results may apply to other crepuscular species and that artificial light is more likely to affect breeding-season habitat use at lower latitudes, where natural illumination during twilight is lower.