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

By Kane Brides l, Sverrir Thorstensen, Ólafur Einarsson, Dmitrijs Boiko, Ævar Petersen, Svenja N.V. Auhage, Graham McElwaine, Axel Degen, Bjarke Laubek, Pelle Andersen-Harild, Morten Helberg, Didier Vangeluwe, Jeroen Nienhuis, Maria Wieloch, Leho Luigujõe

Population delineation based on sound knowledge of movements, distribution and numbers involved of where two populations of the same species may occur is an important prerequisite in population monitoring.

In Europe, two populations of Whooper Swan Cygnus cygnus occur, namely the Iceland population which breeds in Iceland and winters chiefly in Britain and Ireland, whilst the Northwest Mainland European (NWME) population winters in western continental Europe and breeds primarily in Fennoscandia, European Russia and the Baltic States (Rees et al. 2019).

Over 18,000 Whooper Swans have been marked, either with colour-marks or with metal rings in Europe, of these 12,324 (67.4%) were marked in the range of the Icelandic population, whilst 5,954 (32.6%) have been marked on mainland Europe (Brides et al. 2023). Our recent study used ringing, re-sighting and recovery information to determine the level of movement of individuals between the two populations and to assess the extent to which this interchange affects total population estimates.

Birds were assigned to the biogeographical population (Iceland 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.

Ringing locations (blue) and re-encounter locations (red) of Whooper Swans ringed in Iceland, Britain or Ireland and subsequently recorded outside the range of the Icelandic population.

The study updates previous estimates of the number of Icelandic Whooper Swans from the population that winter in mainland Europe (given as up to 600 birds in the 1990s; Gardarsson 1991, Cranswick et al 1996) and the estimates of Finnish-breeding birds wintering in southern Britain each year (at least 200 birds; Laubek et al 1998). The estimate of 600 birds amounted to 3.3% of the numbers reported for the Icelandic population in 1991 (18,035 individuals; Kirby et al 1992), whilst 200 Continental birds migrating to Britain represented 0.03% of the total NWME population size at that time (59,000 individuals in the mid-1990s; Laubek et al 1999).

Assuming that the probabilities of appearing out of range reported here for ringed individuals are indicative of movements for the whole population, the estimates of the numbers of individuals wintering out of range are now 340 (95% CI 272–408) and 434 (347–520) for the Icelandic population in 2015 and 2020 respectively. In contrast, 1,106 (831–1384) out-of-range individuals were estimated for the NWME population in 2015, whilst more recent census data have yet to be reported. Hence our revised estimates are lower for the Icelandic population (3.3% down to 1.0%) and higher for the NWME population (0.03% up to 0.80%).

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.

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Brides, K., S. Thorstensen, O. Einarsson, D. Boiko, Æ. Petersen, S.N.V, Augahe, G. McElwaine, A. Degen, B. Laubek, P. Andersen-Harild, M. Helgberg, D. Vangeluwe, J. Nienhuis, M. Wieloch, L. Luigujõe, J. Morkūnas, Y. Bogomolova, I. Bogdanovich, S.W. Petrek, K.A. Wood & E.C. Rees. (2023) Interchange of individuals between two Whooper Swan Cygnus cygnus populations, and its effects on population size estimates. Ringing & Migration 37, 1-12.

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