Understanding changing demographic rates in a declining population of Bewick’s Swans
By Kevin Wood
Numbers of Bewick’s Swans wintering in northwest Europe declined by almost 40% between 1995 and 2010, but the causes are unclear. To help understand the demographic causes of the decline in population size, the Wildfowl & Wetland Trust has collaborated with an international team of researchers to assess how and why survival rates and breeding success have changed. Our findings suggest that changes in the Bewick’s Swan population have been driven primarily by changes in survival rates rather than breeding success.
A Bewick’s Swan being fitted with a plastic leg ring with a unique code to allow subsequent identification. © David Fotherby / WWT.
A capture-mark-resightings analysis of 3,929 swans showed that Bewick’s Swan survival rates were highest in the 1970s and 1980s, when the population was growing strongly, and lower in the 1990s, 2000s and 2010s during the periods of population decline (Wood et al. in press). However, the reasons for the changes in survival were unclear. Weather conditions in different areas across the flyway, food resources on the winter grounds, density-dependence, and the growth of swan numbers at a relatively new wintering site in southeast Europe (the Evros Delta in Greece), all performed poorly as explanatory variables of survival.
In a second study, we found no evidence that either the percentage of cygnets within the wintering population, or the mean number of cygnets per family, had changed between the 1960s and 2010s (Wood et al. 2016). Bewick’s Swans arrived with more cygnets in years in which the mean summer temperature on the Arctic breeding grounds had been higher, and fewer predatory Arctic Foxes (Vulpes lagopus) had been observed.
Wood, K.A., Nuijten, R.J.M., Newth, J.L., Haitjema, T., Vangeluwe, D., Ioannidis, P., Harrison, A.L., MacKenzie, C., Hilton, G.M., Nolet, B.A. & Rees, E.C. (in press). Apparent survival of an Arctic-breeding migratory bird over 44 years of fluctuating population size. Ibis. DOI: 10.1111/ibi.12521
Wood, K.A., Newth, J.L., Hilton, G.M., Nolet, B.A. & Rees, E.C. (2016). Inter-annual variability and long-term trends in breeding success in a declining population of migratory swans. Journal of Avian Biology, 47, 597-609.