Common merganser or goosander

Autor: Magdalena Zadrąg    

We can find a close cousin of ducks and swans by the Vistula. It is the common merganser or goosander. Its shape really resembles a duck: the body is stumpy, the legs are rather short, with webbing between the fingers, the legs are closer to the back of the body so that the bird walks in a distinctive, heavy way, swaying from side to side. One feature clearly distinguishes the merganser from ducks – a quite differently built bill.

Merganser’s bill is adapted to take in different food, than, say the bill of a mallard duck. Mergansers catch mainly fish, frogs and sometimes also insect larvae or other aquatic invertebrates. Therefore, their bill has a sharp ending, and it is also bent at the end in the shape of a hook. Additionally, the inner edges of the bill are covered with a series of sharp, serrated teeth to help them grip the caught fish. Mergansers prey for fish by diving even 10 metres under water to actively search for victims. During preying the birds are guided by sight, so that is also why it is easier to find them on clean water reservoirs with clear water.

Identification
Mergansers are medium-sized birds, not much bigger than the mallard duck. An adult male in its mating plumage has dark green glossy head, which from a greater distance appears to be black. The entire body is in a black and white contrast, although in a fresh mating plumage the belly gets a lightly pink shade. The colours of the female and the young birds are more inconspicuous – the body is greyish, and the head is brown with a visible crest of rumpled feathers at the back. In resting plumage both sexes look alike, although the males have more white on the wing.

Common merganser. Photo M. Łukawski

Common merganser. Photo M. Łukawski

Occurrence
Merganser is a species that appear widely, it lives not only in Europe, but also over large areas of central Asia, as well as in North America. In Poland, they can be found throughout the year, but if the reservoir on which the birds live freezes over, they move to look for open waters. Some birds are migratory and leave their breeding area in late October and early November, but also for wintering they often choose the Vistula around Warsaw. When observing the results of monitoring within the LIFE+ Vistula Warsaw project, the largest numbers of mergansers in winter can be found here. In spring the birds live, establish their territories, later when females rear the chicks males gather in groups and can often be observed in joint hunts. In autumn young birds and often females gather in little groups and sometimes in larger flocks. In winter this species most often is seen in flocks.

Breeding
The merganser’s courtship can start as early as December, males dance, stretching their necks and ruffling their crests. However, couples do not mate until spring, around March. It is the female that chooses the place for a nest. It is usually a hollow of the black woodpecker. That is why mergansers primarily inhabit water reservoirs surrounded by forests. The nest can be very high, even at some dozen or so metres above ground. As a result, the eggs and the incubating female are safer than on the ground, they are not threatened by some predators, such as foxes, and also the nest will not be flooded with the raising water level in the river. Once the female lays the eggs, the male usually leaves his chosen one. The female incubates the clutch alone for about a month, and one day after they hatch the chicks jump out of the hollow and safely land on the ground. In one clutch there may be even a dozen or so chicks, which as soon as they leave the nest follow their mother as befits real nestlings. From their second day little mergansers can prey for small insects, little fish and other aquatic creatures. They can run and also swim and dive. Females, if possible, choose small oxbow lakes for the chicks’ first days, or sections of rives with weaker currents, so that the chicks are not taken by a current that may be too strong. It may be that such a peaceful reservoir is a pond in an urban park. Often one can see the chicks jumping on the back of the mother and are transported by her around the reservoir. They often rest in this way after going for their prey and diving, as well as basking in the sun. The chicks stay under the mother’s care for about two months. Many of them do not reach old age, because many predators prey on them, both on land, such as martens and foxes, as well as in the water, such as pikes and herons.

Photo M.Elas

Photo M.Elas

Threats
The merganser is not a very numerous species in Poland, occurring mainly in the west and northern parts of the country, and its breeding population is estimated at only 900-1,000 pairs. The population wintering in Poland is a little larger, although difficult to estimate, as it changes from year to year depending of the weather and the freezing over of individual water reservoirs. Main threats to mergansers are the predation of American monk and other predators and water pollution. As the bird is guided under water by its sight it requires quite clear waters so that it can prey easily.

Literature:
Chylarecki P, Sawicki G., Ostoja ptaków Dolina Środkowej Wisły, Wydawnictwo Askon, 2003.
Kruszewicz A.: Ptaki Polski Tom I., Warszawa, Wydawnictwo Multico, 2005
Svensson L., Ptaki Europy i obszaru śródziemnomorskiego, Multico Oficyna Wydawnicza, 2011.
Tomiałojć L., Stawarczyk T.: Awifauna Polski. Rozmieszczenie, liczebność i zmiany. Wrocław. Wyd. PTPP „pro Natura”, 2003. www.learner.org

The project ‘Protecting the habitats of priority bird species of the Vistula Valley under conditions of intensive pressure of the Warsaw agglomeration’ (wislawarszawska.pl) has received a grant from the Financial Instrument for the Environment (LIFE+) and from the National Fund for Environmental Protection and Water Management.