Seeking explanations for recent changes in abundance of wintering Eurasian Wigeon (Anas penelope) in northwest Europe

Abstract

We analysed annual changes in abundance of EurasianWigeon (Anas penelope) derived frommid-winter InternationalWaterbird Census data throughout its northwest European flyway since 1988 using log-linear Poisson regressionmodelling. Increases in abundance in the north and east of the wintering range (Norway, Sweden, Denmark, Germany, Switzerland), stable numbers in the central range (Belgium,Netherlands,UKand France) and declining abundance in the west and south of the wintering range (Spain and Ireland) suggest a shift in wintering distribution consistent with milder winters throughout the range. However, because over 75% of the population of over 1 million individuals winters in Belgium, the Netherlands, UK and France, there was no evidence for a major movement in the centre of gravity of the wintering distribution. Between-winter changes in overall flyway abundance were highly significantly positively correlated (P = 0.003) with reproductive success measured by age ratios in Danish hunter wing surveys and less strongly and inversely correlated (P = 0.05) with mean January temperatures in the centre of the wintering range, suggesting thatwinter severitymay also contribute to influence survival. However, adding winter severity to a model predicting population size based on annual reproductive success alone did not contribute tomore effectivelymodelling the observed changes in population size. Patterns in annual reproductive success seem therefore to largely explain the recent dynamics in population size of northwest European Wigeon. Summer NAO significantly and positively explained 27% of variance in annual breeding success. Other local factors such as eutrophication of breeding sites and changes in predation pressure undoubtedly contribute to changes in the annual production of young and differences in hunting pressure as well as winter severity affect annual survival rates. However, it seems likely that the observed flyway population trend since 1988 has been mostly influenced by climate effects on the breeding grounds affecting reproductive success and marginally on the winter quarters affecting survival. We urge improved demographic monitoring of the population to better assess annual survival and reproductive success.We also recommend development of an adaptive management framework to remove uncertainties in our knowledge of Wigeon population dynamics as information is forthcoming to better informmanagement, especially to attempt to harmonise the harvest with annual changes in demography to ensure sustainable exploitation of this important quarry species now and in the future.