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

-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. Received 5 December 1983, accepted 19 April 1984. BIRDS possess several mechanisms by which they can adjust the magnitude and pattern of their breeding effort in relation to environmental conditions and to their own breeding condition. The most important of these factors is clutch size (Lack 1954, O'Connor 1978, Lundberg and Vaisanen 1979). Other proposed mechanisms are the sex ratio of the brood (Trivers and Willard 1973, Howe 1976, Fiala 1981), egg quality (Schifferli 1973, Howe 1978, Ricklefs et al. 1978, O'Connor 1979, H6gstedt 1981, Birkhead and Nettleship 1982), hatching pattern (Lack 1954, Ricklefs 1965), and intraclutch egg-size variation (Parsons 1970, 1976; Howe 1976; Ryden 1978; Ojanen et al. 1981). When clutch-size adjustments and the influence of nest predation are excluded, a pronounced variation still is found among birds in the proportion of eggs that are successful. The hatchability of the eggs (Koenig 1982) and nestling mortality vary significantly. In many passerine birds, for example, nestling mortality from starvation is rather low (Nice 1957, Ricklefs 1969, Slagsvold 1982a), compared with that of some birds of prey in which one of the two young in the brood always dies (Stinson 1979, Edwards and Collopy 1983). It has been assumed that birds that lay relatively many eggs in relation to the number of young that they are normally able to feed (e.g. raptors) have adopted a brood-reduction strategy that enables them to adjust the number of offspring they rear in relation to the environmental conditions that prevail during the nestling stage. This is brought about by hatching asynchrony, a phenomenon that results in a size hierarchy within the brood (Lack 1954, Ricklefs 1965, Hahn 1981, Slagsvold 1982a). Clark and Wilson (1981), however, cited data on the hatching patterns of several species of altricial birds and claimed that, rather than supporting the broodreduction hypothesis, these data supported predictions adduced from a model based on the probabilities of nest failures occurring. In many species of passerine birds egg size increases with laying order, and this phenomenon is indeed difficult to explain by means of the broodreduction hypothesis (Clark and Wilson 1981). In the present paper, we report the results of an interand intraspecific comparison made with regard to such within-clutch egg-size variation. A large body of data now exists for a wide variety of bird species, and the time is ripe for a comparative analysis. The patterns of such egg-size variation seem fundamental to an understanding of overall reproductive strategies in birds.