Behind this phenomenon lies the special circulatory system of waterfowl, which ensures that cooler blood flows through their feet than through the rest of their body.
Heat exchangers in the legs
This is a clever trick of nature, because: “If ducks had warm feet, they would thaw the ice and then freeze in the meltwater. However, this is rarely the case — for instance during prolonged frost or when an animal is sick or injured,” says wildlife biologist Prof. Klaus Hackländer. Instead, the arterial blood, at around 40 degrees, flows like a heat exchanger on its way from the heart to the feet directly past the much colder venous blood returning from the legs, cooling down to the point where the webbing of the feet can drop to zero degrees.
And the duck has even more survival tricks for winter: its contour feathers overlap like roof tiles, trapping air between them and forming an insulating layer. “In addition, the duck oils its feathers with an oil-containing, water-repellent secretion from its preen gland,” says Hackländer. The birds carefully distribute the oil through their plumage with their bill, so that water simply beads off. An insulating layer of fat beneath the skin provides the waterfowl with additional protection against the cold.
Why feeding bread is harmful
Despite these strategies, ducks must conserve their strength and save energy in winter. It is therefore advisable to avoid startling them and to prevent dogs from flushing them. Furthermore, ducks depend especially in winter on food and safe roosting spots. As a rule, the animals find sufficient food year-round — feeding them bread is not a good idea: “Too much bread not only attracts other ducks, but also pigeons and rats. There is also a significant risk that excessive nutrients will accumulate in standing or slow-moving bodies of water — a process known as eutrophication,” explains Hackländer. High nutrient levels in the water cause algae to grow more prolifically in spring, crowding out other aquatic plants the light essential for survival. When the algae die off later, so much oxygen may be consumed that the body of water can collapse.
For this reason, feeding ducks is usually prohibited. Disregarding a feeding ban is considered a regulatory offence and can be costly: depending on the fine schedule, the well-intentioned act can cost between 25 and 1’000 euros. Those who truly mean well by the animals should therefore limit themselves to observing them — in complete peace and quiet, with warm feet and without bread crumbs in their pockets.
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