Both breeding and wintering numbers of the Mallard Anas platyrhynchos have been decreasing in the last decades. The decline of the Dutch breeding population contrasts unfavorably with the trends in other European countries. In addition, this negative trend contrasts with the positive development of an ecologically closely related dabbling duck, the Gadwall A. strepera. Two other duck species, Tufted Duck Aythya fuligula and Common Pochard A. ferina are also decreasing in winter in the Netherlands. Both species are highly dependent on the network of Natura 2000 sites. It is important to know whether the causes for the observed negative trends lie within the Netherlands or elsewhere. This report is a compilation and review of existing knowledge on Mallard, Gadwall, Tufted Duck and Pochard.
The Dutch breeding population of mallard has decreased for a quarter of a century, by about 30% since 1990. This decline is visible in almost all types of landscape. De decline has no parallel in neighboring countries, indicating driving factors within The Netherlands. The wintering population, consisting of both resident and foreign birds, has also decreased recently (with 25-30% since 2000). A decline in wintering numbers is also visible on the scale of the Northwest European flyway. The demographic problem for the Dutch breeding population does not seem to lie in the nest success (proportion of clutches hatching), which is stable and comparable to that in populations elsewhere. The survival of full-grown birds has increased, especially that of first year birds. This suggest that factors in the chick period and/or the first months after fledging are most likely to play a role in the decrease. Another option, emigration of adult birds from the Dutch population, is less likely. The cause of the declining wintering numbers, as far as it does not apply to Dutch breeding birds, is unclear. However, a shift in wintering distributions within Europe, does not seem to play a major role.
The Dutch breeding and wintering populations of gadwall have been increasing for decades. This is part of a large-scale phenomenon which covers the whole of Europe. The increase is partly due to adaptation to new habitats or habitats becoming more suitable. As a water plant and algae eater this species is able to cope with both eutrophication of waters and the current improvement of water quality (de-eutrophication). Considering the continuing growth there do not appear to be any demographic problems. Nest success is similar to that in Mallard and is stable, while the survival of adult birds has increased. The more favourable trend in numbers compared to Mallard – with similar nest success and survival of full-grown birds – suggests that losses among chicks and/or fledged young are smaller in Gadwall. The increase in wintering numbers will partly be caused by the increased populations, but probably also to a decrease in migration distance as a reaction to on average milder winters.
The number of breeding birds of Tufted Duck has been rising for decades in the Netherlands, but the number of wintering birds seems to be slightly decreasing, though a stabilization has become visible recently. Although some decrease is also apparent on a European scale, the main driving factors are found within the Netherlands. Developments in the IJsselmeer area are steering, as this area is home to 40-60% of the wintering population of Tufted Ducks in the Netherlands. Improvement of the water quality has caused a decrease in the number and quality of Zebra Mussels. It led to a decrease in Tufted Ducks in this area, which was partly offset by increases elsewhere in the country. Evidence of large-scale impacts of climate change, such as a northward shift of the European winter distribution, is lacking to date. While the relatively small Dutch breeding population of Pochard is stable, the much larger wintering population has been decreasing substantially since the midseventies. Similar to Tufted Duck this decrease is Sovon-rapport 2015/65 6 part of a European wide phenomenon. However, developments in our own country played a leading role in the decline. This is particularly the case for the IJsselmeer area, which accounted for on average 50% of the wintering numbers. Also in this species the decrease coincides with a decrease of food sources, in the form of Zebra Mussels. Despite a partial switch to other food types (water fauna and aquatic plants) no recovery of the population has occurred so far.