Shearwaters' nest attendance patterns throughout the lunar cycle: Are oceanographic conditions decisive for timing of nest arrival?

Abstract

The moon cycle has long been recognized as a driving factor of animals' behavioural patterns. However, its influence on seabirds' nocturnal behaviour remains poorly understood. Using a long-term GPS tracking dataset from Cory's and Cape Verde shearwaters (Calonectris borealis and Calonectris edwardsii, respectively) in the Berlengas, Azores, Madeira and Cape Verde archipelagos, North Atlantic Ocean, we investigated nest attendance patterns of breeding birds in relation to oceanographic and natural light conditions during mid-chick rearing. We intended to understand if inter-annual changes in oceanographic conditions around each colony were related with timing of nest arrival and respective light intensity at burrow entrance. For this, oceanographic conditions were evaluated using proxies for marine productivity and metrics of shearwaters' foraging effort. We also wanted to investigate if birds adjusted nest attendance in relation to moonlight throughout the lunar cycle, and whether these patterns changed with contrasting oceanographic conditions. Shearwaters stayed longer at sea after sunset in years of good oceanographic conditions, returning ashore earlier in years of poor conditions, mostly under twilight. Breeding birds also seemed little influenced by moonlight at colonies, although this did not seem to be the case at sea. In particular, they adjusted nest arrivals with the onset of waning moon nights (nights starting without moon), but delayed nest entrances throughout waxing moon nights (nights entailing a growing period of moonlight until moonset). However, this pattern was not extensible to years of poor foraging conditions, when birds returned ashore regardless of moonlight conditions. Combined, these results show that the nest attendance behaviour is mediated by moonlight in Cory's and Cape Verde shearwaters, a pattern that seems to be regulated by prevailing oceanographic and foraging conditions.