It is probable, however, that cases of true sexual inversion--in which gratification is preferably sought in the same sex--may be found among animals, although observations have rarely been made or recorded. It has been found by Muccioli, an Italian authority on pigeons, that among Belgian carrier-pigeons inverted practices may occur, even in the presence of many of the other sex.[10] This seems to be true inversion, though we are not told whether these birds were also attracted toward the opposite sex. The birds of this family appear to be specially liable to sexual perversion. Thus M.J. Bailly-Maitre, a breeder of great knowledge and a keen observer, wrote to Girard that "they are strange creatures in their manners and customs and are apt to elude the most persistent observer. No animal is more depraved. Mating between males, and still more frequently between females, often occurs at an early age: up to the second year. I have had several pairs of pigeons formed by subjects of the same sex who for many months behaved as if the mating were natural. In some cases this had taken place among young birds of the same nest, who acted like real mates, though both subjects were males. 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 Zoölogical 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. It is probable that such observations will be multiplied in the future, and that sexual inversion in the true sense will be found commoner among animals than at present it appears to be.