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The Consequences of Brood Size for Breeding Blue Tits I. Adult Survival, Weight Change and the Cost of Reproduction

Nadav Nur
Journal of Animal Ecology
Vol. 53, No. 2 (Jun., 1984), pp. 479-496
DOI: 10.2307/4529
Stable URL: http://www.jstor.org/stable/4529
Page Count: 18
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
The Consequences of Brood Size for Breeding Blue Tits I. Adult Survival, Weight Change and the Cost of Reproduction
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Abstract

(1) It has been assumed that the cost of reproduction increases with an increase in the size of the brood reared by a breeding individual. To identify and quantify these costs, brood size was manipulated in a population of blue tits Parus caeruleus breeding in Wytham Wood, near Oxford, England. In 1978, 1979 and 1980 breeding pairs (n = 215) were randomly assigned broods of 3, 6, 9, 12 or 15 young at the time of hatching. Possible costs of reproduction were measured by comparing adults which reared different size broods with respect to (i) the weight of males and females during the nestling period (8-13 days after the exchange of nestlings) and (ii) the male and female survival rate (proportion of adults recaptured in the subsequent winter or spring). (2) In each year female weight loss increased linearly as brood size increased. Females rearing large broods lost on average 15-36% more weight than those rearing small broods. In 1978 (but not in 1979 and 1980) male weight decreased as brood size increased: in 1978 males rearing the largest broods were on average 5% lighter than those rearing the smallest broods. (3) For broods manipulated in 1978, an increase in brood size from 3 to 15 was associated with a decrease in female survival of 54% (0.41 v. 0.19); the relationship was non-linear (second derivative positive). That is, the effect of brood size on apparent female survival was greater at small brood sizes than at large. Male survival was not significantly related to brood size nor to male weight. Only the survival rate of low weight females (<=10.5 g) and medium weight females differed significantly; female weight loss and survival were weakly negatively correlated. (4) Female weight loss occurred, for the most part, early in the nestling period; between days 8 and 16, females, but not males, lost additional weight. The negative correlation between female weight and brood size was already well established before nestling feeding frequency reached its peak but while the female was still brooding her young. (5) These results demonstrate that (i) the energetic stress of feeding nestlings is not sufficient to account for the observed pattern of weight loss, and (ii) differences in weight loss reflect real differences in reproductive costs incurred. These results suggest weight loss is regulated so as to reflect benefits and costs to the breeding individual.

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