Ardea
Official journal of the Netherlands Ornithologists' Union

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Westerterp K., Gortmaker W. & Wijngaarden H. (1982) An energetic optimum in brood-raising in the Starling Sturnus vulgaris: an experimental study. ARDEA 70 (2): 153-162
The cost of raising nestlings in relation to brood-size was investigated in the Starling. Data were collected on brooding, feeding, growth of the chicks and weight change of adults. The observations concentrated mainly on a total of 8 broods in two seasons, raised by pairs using specially designed nest boxes with a balance under the nest and a hide at the back. This allowed undisturbed watching and simultaneous weighing of the adults and their feeds. The smallest broods of 3 and the biggest of 7 were compared with the average size of 5 chicks per brood. A brood of five represents the optimum when cost of raising a chick is considered (Fig. 10). A chick in a brood of 5 needs 8-22 per cent less energy to reach a given body weight at fledging than in a brood of 3, a saving which is probably mainly based on huddling behaviour. This trend did not continue with a further increase to brood-size 7. Here a chick needed even 6-10 per cent more. Preliminary observations on energy expenditure showed that deterioration of the insulating properties of the nest in the biggest broods might explain this. Parents of a brood of 3 chicks hardly changed weight during the nestling period. Adults with 5 and 7 chicks all lost 4-6 g, their entire fat depot and representing only a fraction of the energy needed for themselves and the brood. Some parents did not manage to meet the demands of their nestlings, bringing in enough food in weight though poor in quality, resulting in mortality and low fledgling weight.


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