Interchange of individuals between two Whooper Swan Cygnus cygnus populations, and its effect on population size estimates

This study aims to determine the level of movement of individuals between the Icelandic and Northwest Mainland European (NWME) Whooper Swan Cygnus cygnus populations, and to assess the extent to which this interchange affects total population estimates. Ringing, resighting and recovery data for Whooper Swans ringed across Europe since the early 1900s were compiled from the EURING Data Bank, national ringing schemes and individual ringers. Birds were assigned to the biogeographical population (Icelandic or NWME) in which they were ringed. Of >18 000 Whooper Swans ringed in 17 European countries, 172 individuals (0.94%) were later found outside the nominal range of their assigned biogeographical population. The proportion of ringed swans from the Icelandic population that were subsequently found ‘out of range’ did not differ significantly from the proportion recorded for the NWME population, indicating no directional bias in population interchange. Population switching by Whooper Swans in western Europe occurs consistently, but currently at very low levels. Our results reinforce the view that such levels of population interchange are unlikely to have caused major inaccuracies or biases in the total numbers recorded during the coordinated censuses used to estimate population size.