GPS tracking of the foraging movements of Manx Shearwaters Puffinus puffinus breeding on Skomer Island, Wales

We report the first successful use of miniature Global Positioning System loggers to track the ocean-going behaviour of a c. 400g seabird, the Manx Shearwater Puffinus puffinus. During three field seasons, breeding birds were tracked during the incubation and chick rearing periods on their foraging excursions from the large colony on Skomer Island, Pembrokeshire, UK. Foraging effort was concentrated in the Irish Sea. Likely foraging areas were identified to the north off the Mull of Galloway (some 330km from Skomer), in Dundalk Bay, Cardigan Bay, and more diffusely to the west of the colony both locally and off southern Ireland. No foraging excursions were recorded significantly to the south of the colony, conflicting with the conclusions of earlier studies based on ringing recoveries and off-shore observations. We discuss several explanations including the hypothesis that foraging may have shifted substantially northwards in recent decades (although the possibility that breeding birds forage further south in the very early part of the season remains open). We found no obvious relationship between birds’ positions and water depth, although night-time observations were more likely to be in shallower water than day-time observations. We also found that despite the fact that shearwaters can be observed rafting off-shore from their colonies in the hours prior to making landfall at night, breeding birds are usually located much further from the colony in the last eight hours before arrival, a finding that has significance for the likely effectiveness of marine protection areas if they are only local to the colony. Short sequences of precise second-by-second fixes gathered every two hours enabled us to measure movement speed and direction. Movement speeds were bimodal, corresponding to sitting on the water and flying, with flight behaviour separable into erratic (perhaps indicative of searching for food) and directional (perhaps indicative of travelling). Movement behaviour had a diurnal pattern, with flight predominating in the morning and evening, with more sitting at night and around midday. We also provide a first direct measurement of mean flight speed during directional flight (approximately 40 kmh-1), slower than a shearwater’s predicted Vmr (maximum range velocity) and suggesting that birds are exploiting wave or dynamic soaring during long-distance travel