Long-term trends, total numbers and species richness of increasing waterbird populations at sites on the edge of their winter range: cold-weather refuge sites are more important than protected sites

Recent climate changes are most likely the major determining factor for the increasing importance of Central Europe for wintering waterbirds, given that most of the region is located on the edge of these species' wintering ranges. A few recent large-scale studies have demonstrated changes in species distribution at the flyway level, but detailed studies at the site level are still scarce. Using mid-January wintering waterbird counts from 532 sites taken over a 48-year period (1966-2013), we have assessed the role of 'cold-weather refuges', i.e. sites where the selective pressures of winter harshness are reduced (e.g. sites with ample sources of running water, urban areas, warmer sites, sites with a relatively higher proportion of wetlands), and a site's protection status on species richness, total numbers and trends at the site level. We found prevailing increasing trends in total numbers and species richness at the site level, which are in line with the area's increasing importance as wintering grounds. However, some sites have likely been affected by density-dependent regulation as mean total numbers per site have not increased since the 1990s. Density dependence may also be a reflection of an increasing number of sites with running water in traditionally cold areas. Factors affecting trends in species richness at the site level are however less predictable. We demonstrate the great importance of cold-weather refuges, where running water has an effect on total numbers, species richness and trend affect total numbers, warmer areas and higher proportions of wetlands in the surroundings affect total numbers and species richness and urban areas affect total numbers. However, we found that legislative safeguards for sites, such as the establishment of Nature Reserves and Special Protection Areas, have no significant effects. Our findings therefore suggest that the effect of temperature and presence of cold-weather refuges (as defined here) are more relevant to this observed increasing trend of wintering waterbirds on the edge of wintering ranges than a reduction in human developmental pressures.