On the Adaptive Value of Intraclutch Egg-Size Variation in Birds

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Abstract
Using data from the field and the literature on 67 species of birds, we analyzed intraclutch variation in egg size, especially the deviation of the last egg from the clutch mean (D). Values of D are closer to zero in precocial than in altricial species; D is negatively correlated with body size in interspecific comparisons, i.e. large birds, including precocial species, lay small final eggs; and D is higher in open-nesting passerines (on average D = +3.56%, 17 species) than in hole-nesting species (on average D = -0.05%, 13 species). Within populations of birds, a negative relationship exists between D and clutch size, particularly in species that have a generally low value of D. The results support the view that intraclutch variation in egg size has an ultimate, adaptive value. We suggest that birds adopting the "brood-reduction strategy" have a small final egg, particularly those birds with large clutches, whereas birds adopting the "brood-survival strategy" have a relatively large final egg, particularly those birds with large clutches.