In order to mate them productively we have had to separate them and shut each of them up for some days with a female."[11] In the Berlin Zooelogical Gardens also, it has been noticed that two birds of the same sex will occasionally become attached to each other and remain so in spite of repeated advances from individuals of opposite sex.

This occurred, for instance, in the case of two males of the Egyptian goose who were thus to all appearance paired, and always kept together, vigorously driving away any female that approached.

Similarly a male Australian sheldrake was paired to a male of another species.[12] Among birds generally, inverted sexuality seems to accompany the development of the secondary sexual characters of the opposite sex which is sometimes found.

Thus, a poultry-breeder describes a hen (colored Dorking) crowing like a cock, only somewhat more harshly, as a cockerel crows, and with an enormous comb, larger than is ever seen in the male.
This bird used to try to tread her fellow-hens.

At the same time she laid early and regularly, and produced "grand chickens."[13] Among ducks, also, it has occasionally been observed that the female assumes at the same time both male livery and male sexual tendencies.