Manx shearwater (Puffinus puffinus) rafting behaviour revealed by GPS tracking and behavioural observations

Abstract

Before visiting or leaving their remote island colonies, seabirds often engage in a behaviour termed ‘rafting’, where birds sit, often in groups, on the water close to the colony. Despite rafting being a widespread behaviour across many seabird taxa, the functional significance of rafting remains unknown. Here we combine global positioning system (GPS) tracks, observational and wind condition data to investigate correlates of rafting behaviour in Manx shearwaters (Puffinus puffinus) at a large colony on Skomer Island, Wales. We test (1) the influence of wind direction on rafting location and (2) whether raft size changes with respect to wind speed. Our approach further allows us to describe day-night trends in (3) raft distance from shore through time; (4) the number of birds present in the nearshore waters through time; and (5) spatial patterns of Manx shearwater rafts in marine waters adjacent to the breeding colony. We find no evidence that wind direction, for our study period, influences Manx shearwater rafting location, yet raft size marginally increases on windier days. We further find rafting birds closer to the shore at night than during the day. Thus, before sunset, birds form a “halo” around Skomer Island, but this halo disappears during the night as more individuals return from foraging trips and raft nearer the colony on Skomer Island. The halo pattern reforms before sunrise as rafts move away from land and birds leave for foraging. Our results suggest that wind conditions may not be as ecologically significant for rafting locations as previously suspected, but rafting behaviour may be especially important for avoiding predators and cleaning feathers.

An assay to investigate factors influencing initial orientation in nocturnally fledging seabirds

“…This method, affordable and weighing very little, could be widely applicable for even smaller species from a range of taxa and thus is a useful addition to the growing toolkit for tracking birds, which mostly comprises expensive, larger devices limited to large species. Further studies, however, could consider validation of vanishing bearings as a proxy for initial orientation by comparing bearings recorded by an observer and those recorded by a GPS device or include calculation of the distance between observer and a bird to improve the detail of an observation (Richards et al 2019 permits asking only a limited number of questions compared to a wider array of hypotheses available when using tracking devices, it could still be useful when researching fledgling birds, which are unlikely to be captured again, and thus could improve our understanding of early migratory navigation. Our assay revealed that juvenile shearwaters do not orient towards their wintering ground during the very start of their initial flight.…”

Biological traits of seabirds predict extinction risk and vulnerability to anthropogenic threats

“…Seabirds are an exceptionally well‐studied faunal group, both on land during the breeding season and at sea through use of technologies such as biologging (e.g., Paleczny et al., 2015; Richards et al., 2019). Thus, comprehensive biological detail on seabird life history, behaviour and ecology is available for trait‐based studies.…”

Exploring the mechanisms of coordinated chick provisioning in the Manx shearwater Puffinus puffinus

“…It has been previously suggested (Congdon et al 2005) that parents may reunite while at sea. Manx shearwaters form large rafting aggregations in the hours before dusk, which may present an opportunity for such interaction between partners (Brooke 1990, Richards et al 2019). However, these rafts form for only a few hours and consist of many thousands of shearwaters, and we did not find evidence for individuals rafting in consistent locations (Supporting information), making this hypothesis unlikely.…”