Assessing the impact of artificial night lighting regulations designed to protect astronomical observatories on seabirds and bats

Artificial light at night is a rapidly increasing driver of global change, affecting both astronomical observations and biodiversity. Regulations such as the Canary Islands "Sky Law" were designed to protect astronomical observations by controlling light intensity and spectral composition, yet their ecological effectiveness remains largely untested. Here, we experimentally assessed whether lighting conditions permitted under this law influence the behaviour of two sensitive nocturnal taxa: seabirds and bats. Field experiments were conducted in Tenerife, Canary Islands, using controlled lighting treatments that varied in intensity (low vs. high) and spectrum (PC amber ~1800K vs. white ~2700K), including a no-light control. We monitored the behaviour of breeding adult Corys shearwaters (Calonectris borealis) using GPS tracking and passive acoustic recording, and quantified bat activity through ultrasonic detectors. Behavioural responses included flight characteristics, colony attendance, vocal activity in shearwaters, and species-specific movement and feeding activity in bats. Generalised linear mixed models were used to evaluate treatment effects while accounting for environmental covariates. Across 211 shearwater flights and extensive acoustic datasets, we found no consistent or significant effects of light treatments on seabird flight behaviour, vocal activity, or bat movement and feeding activity. Instead, environmental variables such as moonlight, seasonality, and interannual variation were stronger predictors of behavioural responses. These results suggest that lighting conditions currently permitted under the Sky Law may have limited ecological impact on the studied taxa under the conditions tested. Further research in less disturbed environments and with broader spectral contrasts is needed to better assess the ecological implications of astronomically motivated lighting regulations.