Habitat niches in shallow-water zones

Because of their different foraging strategies, different types of ducks occupy different areas of a waterbody. Diving ducks, for example, mostly use zones of deeper water, while dabbling ducks mainly forage in shallow-water zones and close to the shore. Dabbling ducks favour plant food or a mixed diet, while diving ducks feed mostly on animal food. An exception among the latter is the Red-crested Pochard, which feeds on stoneworts.

There are also important differences within the group of dabbling ducks regarding habitat use. These differences reduce competition for food resources and make it possible for mixed groups of ducks to overwinter in the limited space available. Eurasian Wigeon primarily forage on land, often in neighbouring grassy areas. Eurasian Teal favour shallow water and mud banks. They are subject to large fluctuations, being strongly affected by ice cover and high water levels. Gadwall are also often found near the shore, where they favour deeper water zones than Eurasian Teal. Northern Shovelers prefer shallow, nutrient- rich waters, where they feed on plankton. Wetlands created through the silting up of dammed rivers led to a substantial increase in Northern Shovelers from the 1970s. Finally, the long-necked Northern Pintail is adapted to deeper water zones, where it feeds on aquatic plants.

Four dabbling duck species with strongly increasing January numbers since the start of the census in 1967.

Diving and dabbling ducks are strongly affected by climate change and the warmer temperatures in their stopover and wintering sites. As long as waterbodies do not completely freeze over, diving ducks will find the food they need, while dabbling ducks rely on ice-free stretches of shore. These differences help us to understand the current trends: the number of diving ducks recorded in Switzerland has been declining significantly for years, because they increasingly find areas of open water further north, which allows them to shorten the migration distance and save energy. At the same time, dabbling duck numbers are rising, as even the shores of our lakes now frequently stay ice-free and remain accessible to various species.

What happens underwater is reflected in the spatial distribution of ducks as well. They can only overwinter Eurasian Wigeon, Gadwall, Northern Shoveler and Northern Pintail all have different water-depth requirements; their order from left to right reflects a preference from shallow water to deeper water. successfully where there is a suitable food supply. In the eutrophication phase during the second half of the 20th century, over-fertilisation and wastewater led to a massive nutrient overload in many waterbodies; it was this nutrient overload that provided ample food for the introduced Zebra Mussel, leading to a strong increase, which in turn boosted the food supply for diving ducks. Restoring waterbodies to their natural low-nutrient state (re-oligotrophication) results in the return of pondweed and other aquatic plants, improving the food supply for herbivores.

Because many shorelines in Switzerland are heavily built up and ecologically impoverished, protecting existing shallow-water zones is extremely important. Ducks need sheltered coves where they are protected from the wind and from disturbance through humans and dogs. Reserves for wintering and migratory birds are hotspots for many species’ wintering populations; 85 % of the Swiss population of Northern Pintail, for example, spent the winter of 2022/23 in these reserves.