THE INCUBATION PATTERNS OF BIRDS.

Abstract

Summary. A classification of the known methods of incubation of the birds of the world, on the basis of the division of labour between the sexes and the apportionment of time on the eggs, shows that practically every feasible pattern has been adopted by one species or another. A review of the evidence suggests that incubation by both sexes was the primitive method among birds. Among passerines, there appears to be little correlation between incubation by the male and his possession of a brood‐patch. Some males which regularly incubate lack it, whereas others which fail to incubate develop it. There is also poor correlation between the form of the nest (whether open or covered), the coloration of the two sexes, and their participation in incubation. Participation of the sexes is not generally correlated with the environment. The non‐incubation by the female Emperor Penguin and the sharing of incubation by the male Gould's Hummingbird, seem to be instances of response to rigorous climates. Except possibly in migratory birds whose breeding season is short, incubation by the male seems compatible with efficient territorial defence. Some males and even females sing loudly from the nest, apparently without betraying it to predators. When the sex‐ratio is strongly unbalanced, the failure of the less numerous sex to incubate makes it possible for a larger proportion of the adults to reproduce. But in a number of species in which polygamy or polyandry occurs, the sex‐ratio is not known to be unbalanced. Many modifications in the incubation pattern, especially those involving the participation of the sexes, appear to be non‐adaptive, and to persist because they do not decrease reproductive efficiency.