In a recently published paper in the Biological Journal of the Linnean Society Shai has shown that the spread of ratios of hatchling or neonate masses to adult masses is very similar across the three classes of amniotes (mammals, birds and, of course, reptiles). This suggests that relatively large offspring are the ancestral and dominant mode of amniotes and have not evolved in response to the elaborate parental care of endotherms
The relative frequencies of the ratio of offspring size to adult size in mammals (grey), squamates (black) and birds (white). The peak at the smallest ratio is almost entirely composed of metatherians, but mammals (mostly bats) also dominate the highest ratio categories. Note that the range of values is narrower in birds than in either squamates or mammals.
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