Autosexing is when pure bred day old chicks can be sexed by their different appearances when they have hatched. This is a characteristic that is highly desirable so that chicks can be raised as guaranteed pullets, keeping food costs down. There are a number of birds (note: that are pure bred birds) that will autosex, the Cream Legbar being an example. The offspring will breed true and remain autosexing.
Sex linked birds can also be sexed as day old chicks by their different appearances and this is what is used extensively in commercial farms. Sex linkage is when two different breeds of (pure bred) birds are crossed. For example, the Bovan Nera (AKA Black Rock, Black Star) are a sex linked cross of the Rhode Island Red X Barred Plymouth Rock. The offspring will not breed true from these crosses.
The following chickens are autosexing: Amrock, Ancobar, Barnebar, Bielefelder Kennhuhn, Brockbar, Brussbar, Buffbar, Cambar, Cobar, Cream Legbar, Dorbar, Gold Legbar, Hambar, Niederrheiner, Norske Jaerhon, Oklabar, Polbar, Rhodebar, Welbar, Whealbar, and Wybar.
Cream Legbars are autosexing. They are a pure breed of poultry but were originally created from a cross between the Brown Leghorn, Araucana and Barred Plymouth Rock. As can be seen, young chicks can be sexed by the colour of their down shortly after hatching. The 'barring' pattern is sex-linked, i.e. the males have two chromosomes for barring and the females only one. Day old chicks have a light patch on the top of the head. When the barring is combined with brown colouring, the light spot on the head of the females is small and well defined and there is a clearly defined dark stripe down the back. The stripe down the back of male chicks is much fainter and there is a light patch that covers most of their head. The down of the male chick is also much paler.
Female chick (left), male chick (right).