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Résumé :
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Summary. 1 The nestling food and breeding of Jackdaws, Rooks, and to a lesser extent Carrion Crows were studied at Oxford from 1952 to 1954. 2 The average clutch-size in the Jackdaw is 4–4 and in the Rook 4–3. Late clutches are significantly lower than early in the Rook but not in the Jackdaw. Annual variation in clutch-size is small. 3 In both Rook and Jackdaw the survival of nestlings decreases as brood-size increases. Predation, disease, and nest-site are unimportant in producing this mortality; food-shortage is very important. 4 In Jackdaws and Rooks the eggs hatch asynchronously and this is an adaptation to a variable food-supply; by the speedy elimination of the small young in a brood when food is short, the brood is brought into closer correspondence with the food-supply. 5 In the Jackdaw and Rook the brood-size which effectively produces most young lies between four and five. This corresponds to the average clutch. 6 Rooks typically probe in grassland where they obtain earthworms which form the bulk of the food brought to nestlings. April is on average the best month for obtaining earthworms on grassland (or suitable alternative food) and this is the month when most young Rooks are in the nest. Jackdaws feed on the surface of open grassy places and have their young in the nest when many of the varied invertebrates in this habitat are most abundantly available. Carrion Crows feed partly in woods and partly on grassland. The nestlings are fed mainly on earthworms, nestling birds and small mammals. 7 The breeding season of the Jackdaw and Rook varies little from year to year. The food-supply is unlikely to be a major factor in initiating ovulation since its season of abundance is variable.
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