36 BILL— BIRD 



position, but the I'ight nostril, and still more the groove, are 

 perceptibly slanting towards the right, as can be ascertained by 

 viewing the bill from the dorsal side. 



Sexual Dimorphism is mostly restricted to peculiarly shaped 

 bills ; for instance, the horn of the male Hornbills is often larger, 

 and differs in shape from that of the female. In the males of 

 Pelicans several unpaired excrescences are formed entirely by the 

 horny coating of the premaxilla ; they sometimes reach a height of 

 three to four inches, and are again cast off after the breeding 

 season, resembling in the latter feature the Auks, as described 

 above. 



The most striking example of dimorphic bills is that of the New- 

 Zealand HuiA, Heterolocha, the bill of the female being slender, 

 about four inches long, and much curved, while that of the male is 

 nearly straight, stout, and scarcely half that length. The knobs or 

 swellings in the Gallinse are mostly restricted to the males ; the 

 same applies to Qi^demia (Scoter). Sexual diflerences in colour 

 are common. For instance, in the male Scoter the bill is black and 

 orange, in the young and in the female It is simply grey, and with- 

 out the knob. The bill of the adult male Blackbird is orange- 

 yellow ; that of the young of both sexes and of the adult males of 

 Buceros malayanus (Hornbill) is white, but becomes black in the 

 adult female, forming thus an interesting exception to the general 

 rule that the young agree with the females, and that aberrant 

 coloration is confined to the males. The colour of the bill is 

 deposited as a dift'used pigment in the horny cells of the epidermal 

 coat, but is occasionally restricted to the deeper layers, or even 

 to the Malpighian layer itself, then shining through the outer 

 transparent layers. 



In connexion with the bill is to be mentioned the " egg-tooth," 

 which is developed in the embryos of all birds as a small whitish 

 protubei'ance or conglomeration 'of salts of calcareous matter, 

 deposited in the middle layers of the epidermis of the tip of the 

 upper bill, without being connected with the premaxilla itself. 

 The sharp point of this " tooth " soon perforates the upper layers 

 of the horny sheath, and then files through the eggshell, a slight 

 crack in the latter being sufficient to enable the young bird to 

 free itself. A similar egg-tooth exists in Reptiles, and is, as in 

 Birds, cast off after hatching. The wearing away of the growing and 

 constantly renewed horny layers of the bill can be easily observed 

 in the pealing beak of a Parrot. 



BIRD (etymology unknown ; but in Old English Brid), origin- 

 ally the general name for the young of animals ; ^ then, as the 



1 As ill Wyclif's translation of Matth. xxiii. 33, " eddris, and eddris briddis " 

 (A.V. "serpents" and "generation pf vipers"); Trevisa, Barth de P. E. xii. v. 



