INTRODUCTORY 15 



Where the arm (humerus) is long, as for example in the Alba- 

 tross, the gap between the innermost quills of the fore-arm and 

 the body, which would be formed during flight, is filled up by a 

 series of long feathers which are related to the humerus much 

 as are the secondaries to the fore-arm. But with this difference, 

 in the arm this series is double, one running along the upper, 

 and one along the under surface of the shaft. But for this 

 bridging, flight would be impossible. 



Glands 



Birds have no sweat glands. Indeed the only skin gland 

 they possess is that known as the Oil-gland or Uropygium, 

 which is situated above the bases of the tail feathers, and 

 secretes a fairly abundant quantity of clear oil. In some birds 

 this gland ends in a pointed nipple, e.g.. Owls; in others it 

 bears a tuft of feathers, e.g.. Ducks. 



As to the use of this gland : it may be remarked that it is 

 now universally believed to serve as a reservoir for the secretion 

 of oil which is used by the bird for the purpose of dressing the 

 feathers. It is supposed to be applied by the beak, the bird 

 turning the head backwards, and gently squeezing the nipple 

 of the gland. The oil thus expressed is then applied to the 

 feathers, individually, by drawing them through the mandibles, 

 and thereby it is believed aquatic birds make their plumage 

 waterproof. 



So deeply rooted is this belief, that he who would nowa- 

 days presume to question its truth would be branded as a 

 heretic, or worse. The late Charles Waterton had the temerity 

 to cast doubt upon this hypothesis, and was promptly scorned 

 for his pains; nevertheless, there are grave doubts as to the 

 function of this gland. And these can be briefly set forth by 

 one or two striking cases. 



To be of any use as a lubricant this oil must be very skilfully 

 applied : yet there are many birds wherein this gland is well 

 developed which could not possibly take up and spread so 

 much as one drop of this precious fluid. The Scissor-bill is 

 perhaps the most striking instance of this, for in this bird the 

 lower is not only longer than the upper jaw, but both jaws, from 

 the gape of the mouth onwards, are compressed to form a single 

 blade as flat as a paper-knife. The Pelican is tongueless, and 



