Diet and habitat use of scoters Melanitta in the Western Palearctic-a brief overview
If patterns of scoter distribution and abundance are to be understood, there is a need to know upon which prey items these birds feed, how they obtain these prey items and the habitats from which these food items are most easily harvested. Dietary studies and descriptions of habitats exploited by Com m on and Velvet Scoter in the non-breeding season are reviewed. The existing literature strongly suggests that, outside of the breeding season, these species forage mainly upon marine bivalve m ol
... arine bivalve m ol luscs (especially those less than 4cm long) that live on the surface or within the upper 3cm of clean, coarse, sandy substrates in waters less than 20m deep. Although there is a large energetic cost to diving, han dling and crushing such prey prior to digestion, such sedentary prey items often occur in very high densities, offering a locally abundant and predictable feeding resource. Since single species often dominate the diet, but dominant food items differ between feeding areas, it seem s likely that scoters simply take whatever prey is locally available in suffi cient abundance to fulfil nutritional needs. Large differences in docu mented prey size frequency distributions suggest that scoters may not select for specific prey size classes below an upper digestive limit. However, in the absence of any precise understanding of how scoters obtain their prey, nor any sim ultaneous studies of available benthic food abundance and size class distributions in scoter diets, it is not possible to confirm if differences sim ply reflect differences in profitability between different prey at different sites at different times of the year. There rem ains a considerable need to study the basic feeding ecology and the behaviour of scoters and their prey if their patterns of distribu tion and abundance are to be better understood.