On Tue, Jun 13, 2006 at 09:54:35PM +1000, harry clarke wrote:
> Olivia Judson asks this question on a New York Times site today but doesn't
> answer it. http://judson.blogs.nytimes.com/?th&emc=th
> Does anyone know?
Because they are are careful?
I can't access the times article but if the question is why aren't
there viviparous birds then there isn't a good answer according to
Bennett and Owen's Evolutionary Ecology of Birds. They suggest cavity
nesting has similar advantage to mammalian gestation and they refer to
this paper: "Why are there no Viviparous Birds?" Daniel G. Blackburn,
Howard E. Evans American Naturalist, Vol. 128, No. 2 (Aug., 1986):
Here is part of the abstract:
... 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. ...
Andrew
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