Abstract: San-Jose et al. recently demonstrated that the colouration of barn owls impacts their hunting success under moonlit conditions, and therefore affects their reproductive success1. They found that near full-moon conditions, the youngest nestlings with white fathers were fed more and were likelier to survive than those with redder fathers. While the study is interesting, the percentage of the Moon that is illuminated (lunar illuminated fraction) is unfortunately a poor proxy for moonlight exposure. We suggest that lunar illluminated fraction should, in general, never be used in biological studies, as alternative variables such as horizontal illuminance better represent moonlight exposure, and therefore offer a greater chance of detecting the effects of moonlight. Here, we provide a brief explanation of how moonlight varies with season and time of night, and stress the need for greater collaboration between biologists and astronomers or physicists in such studies in the future.