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Journal Article

Adaptation vs. Constraint: Intraclutch Egg-Mass Variation in Birds

Javier Vinuela
Journal of Animal Ecology
Vol. 66, No. 6 (Nov., 1997), pp. 781-792
DOI: 10.2307/5995
Stable URL: http://www.jstor.org/stable/5995
Page Count: 12
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Adaptation vs. Constraint: Intraclutch Egg-Mass Variation in Birds
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Abstract

1. Intraclutch egg-mass variation may be determined by nutritional constraints during the laying period, or may be adaptive. These hypotheses are not mutually exclusive, and may act simultaneously on the same species. For this study, egg mass variation was analysed in the black kite, Milvus migrans (Bodd.), a mid-sized Falconiforme. 2. Egg size showed consistent patterns of variation with laying order and clutch size (2-3 eggs); egg size was larger in three-egg clutches, smaller in last-laid eggs, and relatively smaller in last-laid eggs of three-egg clutches. 3. Inexperienced breeders laid smaller eggs. There were no significant effects of year or laying date on egg size, once the confounding effect of breeder experience was removed. 4. Egg size affected hatchability only in two-egg clutches, and it affected the probability of survival of last-hatched chicks, after the confounding effects of hatching order and breeder experience were removed. There was no significant effect of egg size on growth rates or asymptotic body size. 5. The relative sizes of last-laid eggs and of first-laid eggs were not correlated with hatching asynchrony. 6. Factors reflecting the degree of nutritional constraint during laying (year, laying date, breeder experience) did not affect the degree of intraclutch egg-size asymmetry. The slight intraclutch egg-mass variation in this species (less than 0.5% of female weight) cannot impose serious energy constraints. Pairs nesting in high-quality territories laid clutches with relatively large last-laid eggs, which could be an adaptive variation of egg size to food availability. 7. Although there may be nutritional constraints on egg size at the time of laying, intraclutch egg-size variation in black kites cannot be explained exclusively by this factor. Rather, the variation may reflect distribution of resources among eggs, based on their reproductive value relative to laying order. This is determined by higher mortality of last-hatched chicks and lower hatchability of first-laid eggs in three-egg clutches. Also, it may be a component of the brood reduction strategy of raptors. 8. Relative importance of nutritional constraints or adaptation in determining intraclutch egg-size variation may depend on life history traits of each species, mainly on how the resources for laying are gathered. Adaptive patterns could exist in species storing most of the resources in the prelaying period, while nutritional constraints could hamper the appearance of any adaptive variation in species gathering those resources during the laying period.

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