On the behaviour of the white-throated merganser, Mergus serrator, when foraging

Abstract

On an excursion with K. HEIN, J. MOMSEN and U. RADOMSKI on 2 February 1969 near Karlsminde in Eckernförde Bay, we made the following observation. An adult great cormorant, Phalacrocorax carbo, was continuously diving for food. A red-breasted merganser remained close by, together with an adult herring gull, Larus argentatus. When the cormorant surfaced with a fish, the red-breasted merganser flew towards it and attempted to chase it away. The cormorant dived away. This process was repeated twice. I found no references in the literature to the parasitic feeding behaviour of the red-breasted merganser, which was clearly the case here. CURTH (1954), BAUER and GLUTZ (1969) and RINGLEBEN (1951) also have nothing to report on this subject. 

Large contiguous flocks of the red-breasted merganser seem to occur only rarely in Schleswig-Holstein. RINGLEBEN saw a flock of 80 red-breasted mergansers at Schleimünde on 5 May 1946. KUMERLOEVE (1963), like CURTH, does not mention any large flocks for Amrum or for the ferry island/Hiddensee. However, CURTH reports that on 27 April 1951, 334 individuals were present on the ferry island, but without forming a flock. BAUER and GLUTZ report flocks of several hundred saw-billed ducks.

When food is plentiful, large flocks can also form in Schleswig-Holstein: parts of the Orther Rhede in south-western Fehmarn were cordoned off by fishermen with gillnets on 7 April 1971. Apparently, the food supply in front of the net was very favourable for the red-breasted merganser, as I could see a closed flock of 125 males and 45 females hunting eagerly there. After about half an hour of intensive fishing, this flock broke up into smaller groups.