Waterbirds: where wintering and breeding grounds overlap

In Switzerland, the only common and widespread duck is the Mallard, a species that is extremely adaptable in its choice of habitat. All other duck species are rare breeders. On the lakes and rivers that serve as wintering sites, individuals or small groups of ducks can regularly be observed spending the summer in Switzerland. These are often one-year-old juveniles that do not yet breed; some are injured birds or birds in too poor a condition to return to their breeding grounds. Breeding will occasionally occur, but it is mostly sporadic as in the case of the Common Eider. Only Red-crested Pochard and Tufted Duck have managed to establish themselves in Switzerland and form populations of several hundred pairs. The Common Pochard, on the other hand, which often associates with these two species in winter, has remained a rare and local breeder. The reasons are unclear, as Red-crested Po-chards and Common Pochards breed side by side in areas such as the Doñana in Spain, the Dombes in France or the fish ponds in the Czech Republic. The mating system may play a role. While Red-crested Pochards search for a mate at the wintering sites and are thus able to rapidly colonise new areas, Common Pochards often only form pairs during spring migration or at the breeding grounds. Ducks are not the only birds to have established themselves as breeding species after wintering in Switzerland. Other waterbirds to have done the same are Red-breasted Merganser, Goosander and Black-necked Grebe.

The simultaneous presence of late winter visitors and early breeders makes it difficult to es-timate the size of the breeding population. To minimise the risk of counting birds that spend the summer here without breeding, all Swiss atlases adopted strict requirements for records concerning the rarer duck species and other waterbirds: breeding must be confirmed. As the observers do not actively search for nests, the most frequent evidence of breeding are family parties (Atlas Code 13); the artificial islands on Lake Neuchâtel are the only sites where nests are counted systematically.

Such a conservative approach that limits records to confirmed broods is problematic when it comes to documenting population trends, as it only accounts for breeding success, which fluctuates more widely from year to year than the population itself. Breeding can fail for a number of reasons. Clutches are often lost to predators or because of variations in the water level, and many of the young disappear shortly after fledging. Duck families often keep to sheltering vegetation, where they are hard to detect. As an added difficulty, families often band together, especially Tufted Ducks. Brood parasitism, where females lay their eggs in the nests of the same or other species, is well documented especially for the Red-crested Pochard. The resulting mixed families further complicate monitoring efforts. For these rea-sons, the size of the breeding population is often underestimated. In the case of Red-crested Pochard and Tufted Duck, doubling the number of confirmed breeding records yields a real-istic estimate of the breeding population. A systematic census of breeding pairs at the start of the breeding season as it is done, for example, in Finland could improve the accu-racy of population trends.

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