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.