Herkenning van Aalscholver Phalacrocorax carbo en Kuifaalscholver P. Aristotelis in het licht van de recente toename van Aalscholvers in ons land

The recent increase in claims of Shags along the Dutch North Sea coast in August and early September, the appearance of a Swedish paper on identification of Cormorant and Shag with exetnsive descriptions, drawings and photographs (Alström 1985) and the spectacular increase in the breeding populations of NW European Cormorants, especially in the Netherlands (Hansen 1984, Rooth 1985) were considered sufficient justification for a critical examination of aal main specific characteristics and some illustration of how the increase in breeding Cormorants is reflected in post-breeding dispersal of predominantly juvenile birds in Dutch coastal areas. It was found that most of the features usually used for specific identification of Cormorant and Shag may overlap and therefore not be absolutely reliable. Only facial structure, bill and head shape and close examination of plumage characteristics in correctly aged birds (out of breeding plumage) will ascertain specific identification by observers lacking extensive experience with both species. Size, bill length and bill depth, overall colour pattern and behavioural features (e.g. diving with a little jump, wing action in flight, etc.) should only be used as additional arguments. Experienced observers may ascertain more easily, but great care should always be taken. Considering the extensive dispersal of mainly young Cormorants at the end of the breeding season and the fact that neither young nor adult Shags usually have traveled more than 100 km away from their breeding colonies by August (Galbraith et al. 1981), it would seem advisable to consider any cormorant-like bird along the Dutch coast in August/September to be a Cormorant, unless strong arguments would indicate otherwise.