44 HYBRIDISM 



Northern Eider and the Mergansers, which belong to different subfami- 

 lies. Mallards of the Old World are known to cross with many addi- 

 tional species. The Wood Duck has been crossed with 14 other species. 

 A rare cross between a Hooded Merganser and an American Golden-eye 

 is illustrated on Plate No. 36. In all, some 63 species of Ducks from 

 different parts of the world are listed as cross-breeding in the wild state. 



A feature of duck hybrids, and one by which the parentage of the 

 cross can generally be told, is that the characters of the parents, instead 

 of being "blended," usually show distinctly in the pattern of the mon- 

 grel bird. This is well illustrated on Plate No. 36, where some of the 

 distinctive markings of both parents are clearly discernible on each of the 

 hybrids. Sometimes, such striking parental characters as the white col- 

 lar of the Mallard, disappear in the first generation, only to re-appear in 

 some males of later generations. 



Though hybrids of kindred species, such as the Mallard and Black 

 Duck, are usually fertile and the sex ratio is normal, this is not the case 

 with crosses less closely related; even in the case of Mallard-Pintail 

 crosses the resulting hybrids are rarely fertile. 



As a rule, hybrids, particularly those from distantly related species 

 or genera, are sterile like the mule (horse-donkey cross). Even if such 

 hybrids are not sterile, they are inclined to be shy breeders and the fe- 

 males generally have infantile ovaries and oviducts, lay no eggs, and 

 refuse to mate. 



The great scarcity of females and an excess of males among hybrids 

 is another feature. Thus, in a list of 248 duck hybrids, 72 per cent are 

 males and 28 per cent are females; from a list of duck-merganser crosses 

 only 17 per cent are females. 



The prevalence of sterility and the unbalanced sex ratio among hy- 

 brids are the factors which control the perpetuation of hybrid birds. The 

 chances of two fertile hybrids mating with one another in the wild state 

 are remote. 



BREWER'S DUCK (mallard-gadwall cross). (See Plate 36.) This 

 famous hybrid was described by Audubon (1843) from a specimen shot 

 in Louisiana, in February, 1822. Audubon named it "Brewer's Duck," 

 saying, "I have named this Duck after my friend Thomas M. Brewer of 

 Boston, as a mark of the estimation in which I hold him as an accom- 

 plished ornithologist." When this was written the describer unques- 

 tionably thought that the specimen was a valid species, though when de- 

 scribing the bird (1843), he evidently had doubts on the matter for he 

 stated that it might possibly be an accidental variety, or a hybrid be- 

 tween the Mallard and "some other species, perhaps the Gadwall, to 

 which also it bears a great resemblance." The so-called "Brewer's 

 Duck" is now recognized as a mallard-gadwall hybrid. 



