Egg and clutch size in the mallard as related to food quality

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Abstract
The effect of food quality on egg and clutch size was studied in captive wild-strain and game-farm mallards, Anas platyrhynchos, and comparisons were made with various wild Swedish populations from different habitat types. Captive mallards laid larger eggs than wild birds. In captivity, game-farm ducks laid larger eggs than wild-strain birds, but when they were exposed to wild conditions the size was reduced. Captive ducks supplied with high-protein food laid larger eggs than ducks fed low-protein food. In the wild, there was a negative correlation between egg size and population density. Various wild Swedish populations showed similar egg-size variations among years, indicating some common factor operating outside the breeding season. In the food quality experiments, clutch size was positively correlated with egg size, but no correlation was found in wild populations. A negative correlation between clutch size and laying date, obtained in the wild, was not found in captivity. Egg and clutch sizes were both significantly smaller in a lake with food competition from fish than in an area rich in lakes empty of fish. When comparing egg characteristics of mallard populations it is important to distinguish between wild, captive wild-strain, and captive game-farm birds.

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