LEOPOLD MF, BRUIN CJW, CAMPHUYSEN CJ, WINTER C & KOKS B (2003) Why do Ospreys Pandion haliaetus so rarely utilise Dutch marine waters for feeding?. LIMOSA 76 (4): 129-140.

Ospreys migrate by the hundreds through the Netherlands each year, on their way from Fenno-Scandian breeding grounds to African wintering quarters and vice versa. In northern Europe, these birds use mainly freshwater habitats for breeding, but in Africa many birds switch to marine waters and marine prey. On migration, Ospreys may be seen feeding anywhere in Europe. Birds passing through the Netherlands tend to follow major fresh water bodies such as rivers, but some follow the western coastline, particularly in spring. Even these birds only feed rarely in marine waters, but instead mostly seek out dune-ponds and freshwater canals in or just inland from the dunes or dikes. We collected the rare observations of birds that did feed in Dutch marine waters and found that if marine prey were taken, these were almost invariably mullets (Mugilidae). Mullets are large roundfish that often swim very close to the water's surface. Mullets are thus attractive prey fish, that are also commonly taken by Ospreys in southern Europe and in Africa. Most observations on Ospreys catching marine fish in the Netherlands were made on days with belowaverage wind speeds, suggesting that feeding at sea is easier then. By and large, however, passing or staging Ospreys at the coast go inland to feed, taking large cyprinids like Rudd Scardinius erythrophthalmus, Bream Abramis brama or Grass Carp Ctenopharyngodon idella that occur in often high densities in clear and quiet dune ponds. Feeding in such fresh water ponds thus seems more profitable than feeding in the more turbid marine waters and even birds staging on marine sites, habitually fly inland when they want to feed, often to return with their catch to a feeding post at sea. Three cases have been recorded of birds staging for longer periods in marine waters (five days to several weeks), and specialising on taking mullets there. Most other cases of Ospreys taking mullets in the Netherlands should probably be attributed to opportunistic birds, catching such fish en route to their wintering or breeding grounds.

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