Food limits survival of breeding boreal mallards Anas platyrhynchos

• During the breeding period dabbling ducks (Anas sp.), and especially newly hatched ducklings, face heavy mortality. The reasons are not clear, but suggested factors contributing are scarce food, predation, chilling and disease. We highlighted the question why many of the boreal lakes in Scandinavia are without breeding ducks and why mortality is considerable during brood period. In an experimental study in 2002 we hypothesized food being a major factor deciding duck survival during breeding. One wing-clipped hen mallard with her own newly hatched brood (10 ducklings) was introduced onto each of 10 lakes. Food was added ad libitum at 5 of these lakes (experimental lakes), whereas the other five lakes (controls) did not get any extra food added. Survival of hens, broods as well as each individual duckling was monitored regularly until 24 days after introduction. At all three levels (i.e. duckling, brood and adult), survival was significantly higher on lakes with food added than on control lakes, although the difference was most pronounced on the duckling level. Direct consequences (i.e. dying from starvation) as well as indirect (e.g. undernourished individuals being more easily taken by predators) may both explain this pattern. Our results clearly show that food is a major limiting factor on mallard survival in this environment. Also, our results may explain why many boreal lakes in Scandinavia do not have any breeding waterfowl at all. It seems likely that such lakes are too poor to raise broods and even to sustain adults.

Ämnesord

NATURVETENSKAP  -- Biologi -- Ekologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Ecology (hsv//eng)

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