A CASE OF APPARENTLY ADAPTIVE ACCELERATION OF EMBRYONIC GROWTH-RATE IN BIRDS

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Abstract
In the cowbirds, a group of 3 genera and 6 species, the period of incubation is longest in the most primitive species and successively shorter in each succeeding species, the species being arranged in phylo-genetic order. The total difference between extremes amounts to a decrease of about 20% of the total time of incubation of the most primitive species. The most primitive bird is not parasitic, but the others are; i.e., they leave their eggs in nests of other birds to be hatched and cared for by them. The degree of perfection of the parasitic habit seems correlated with diminution of the period of incubation or, in other words, with an acceleration of the rate of embryonic development. It is very important for a parasitic bird to have a short incubation period, as its eggs may be laid in nests in which the eggs have already been incubated slightly; and yet to survive, the young parasites must hatch not later than their nest-mates. As a matter of fact, in the great majority of cases the cowbird eggs hatch first.

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