Why are there no Viviparous Birds?

The absence of viviparity in birds is typically explained by invoking morphological or physiological factors putatively incompatible with live-bearing reproduction. Examining these factors in terms of falsifiable predictions and underlying assumptions, we suggest that no single avian feature is known to be inherently incompatible with viviparous production of small clutches and that the absence of the live-bearing mode is a consequence of the lack of selection for the intermediate evolutionary stage of egg retention. Birds have achieved most of the advantages that potentially could accrue from egg retention and viviparity by such specializations as endothermy, egg incubation, nest construction, uricotelism, shell pigmentation, parental care, altricial hatchlings, albumen provision, and calcareous eggshells. A theoretical model is presented in support of our contention that the costs of egg retention associated with decreased fecundity, increased maternal mortality, and decreased paternal investment outweigh the potential benefits for most birds.