Influence of moonlight on nightjars' vocal activity: a guideline for nightjar surveys in Europe

Strictly nocturnal or crepuscular habits on birds are restricted to a very limited number of species such as nightjars. Physiological skills of these species combined with ecological and behavioural adaptations provide them the ability to colonise a less competitive niche. Nightjars' nocturnal ecological adaptations have been largely neglected on bird survey methods, which can bias the obtained results. Here, we studied the factors affecting vocal activity of two nightjar species, red-necked nightjar Caprimulgus ruficollis and European nightjar C. europaeus in a Mediterranean area of southern Portugal by modelling observed vocal activity of both species as a function of astronomic variables, particularly moon phase and day of the year. Red-necked nightjar vocal behaviour was positively associated with brighter moon phases and springtime, whereas European nightjar was only influenced by the season, being more active in the summer. This reveals the importance of light conditions on vocal activity of the former species. We suggest that specifically designed field surveys for these species should be preferentially conducted during brighter nights, when the lunar cycle is close to full moon. Furthermore, positive contacts with birds are maximized from late spring to early summer, at least for the red-necked nightjar.