Knobbelzwanen in de stad Groningen in de strenge winter 1978/79

During the severe winter of 1978/1979, up to 120 Mute Swans Cygnus olor spend the winter in the city of Groningen. Several ringed birds were observed and could be identified by reading the number of the metal ring. Almost all had been ringed in Denmark. Counts of the number of swans are presented in table 1. Small numbers were recorded in ponds in city parks ('vijvers'). Most swans were recorded in canals ('koelwatercircuit'). The local power plant used the water in these canals for cooling purposes. As a consequence, they remained free of ice during the whole winter. These canals were also used by high numbers of other species of waterbirds during the severe winter of 1978/79.