"In the case of robins, both sexes share in building the nest, which is composed of a mixture of mud and grass. Incubation is done solely by the female. This is typical of species in which the male is more colorful and the female more drab. Presumably, the drabness is less conspicuous to nest predators. To ensure that all four to six eggs hatch together, she does not begin to incubate until her entire clutch has been laid. The eggs hatch after only two weeks of incubation." http://www.mdc.mo.