Marine foraging by Ospreys in southwest Scotland: implications for the species' distribution in western Europe

Most breeding Ospreys Pandion haliaetus in the UK are in Scotland and feed on freshwater fish. Some east-coast pairs use estuarine fish in summer but still depend on freshwater foraging in spring. However, a pair in southwest Scotland fed predominantly on marine fish in both 2004 and 2005. This pair took freshwater fish early in the breeding season, but fed mainly marine fish to their young. The marine prey, grey mullet Chelon/Liza, Garfish Belone belone and flatfish (Pleuronectidae), were taken at different stages of the tidal cycle, so these Ospreys could feed from the sea almost throughout the day. A review of dietary studies of Palearctic Ospreys shows that most birds in the north of the range are dependent on freshwater feeding, whereas at lower latitudes and in warm waters they tend to forage in the sea. With current midsummer sea surface temperatUres (SST) of 12-13°C, the UK lies in the transition zone between freshwater-dependent and marine-dependent Ospreys. We can anticipate that, as the species spreads across southern Britain, many birds will feed on sea fish and there is scope for substantial populations on the western and southern coasts. If current c'limate change trends continue and SST rise further, we can also anticipate a marine-foraging Osprey population around the North Sea.