BIRD. 



467 



motion except in bringing the centre of action to the 

 tree, or removing- it to a greater distance ; the second 

 or prehensile part, which consists of the posterior 

 part of the spine and its apparatus, the legs and feet, 

 and the tail ; the former of which act as double hooks 

 in holding- on, and the latter as a prop or strut ; and 

 the third, or immediately acting part, consists of the 

 head and neck, which move with great rapidity with- 

 out in the least disturbing the other parts. The wings 

 do not come into play, unless when the bird shifts its 

 position laterally, or rises from the tree ; but they 

 are, as in the case of all climbing and perching 

 birds, which have much action of the bill when 

 holding on with the feet, always held in readiness, 

 so as to come into action whenever any sudden jerk 

 may require it. 



The wryneck, which adheres to the bark of trees 

 something in the same manner as the woodpecker, 

 though its action upon them is different, has the 

 sternum very much of the same structure ; only as the 

 wryneck does not hew into the wood of trees, but 

 merely searches the crevices for bark insects, it has 

 a different motion of the head, and the furcal bone 

 differently formed to suit this motion. It cannot 

 strike so forcibly with the bill, or so repeated! v in 

 one place, as the woodpecker ; but the wryneck has 

 the joints of that organ remarkably quick and free in 

 their motions, so that when it is searching the cre- 

 vices, now on the one side now on the other, the well 

 defined mesial line of rich deep brown, which marks 

 the neck and shoulders, appears to be twining up the 

 tree like a little snake. 



Zygodactylic birds, which are a very numerous 

 tribe, and the characteristic tree or forest birds of 

 the warmer parts of the world, have many differences 

 of habit, according as they depend "more upon 

 Hying, walking, or climbing, and according to the 

 nature of their food and the places of the trees, or 

 between them, where they seek it. The sternum is of 

 course modified to suit these differences ; and thus 

 no one bird can in that respect be considered as 

 typical of the whole order. That of the woodpecker, 

 which we have given, may be perhaps considered as 

 the one extreme, while the following, which is half 

 the lineal dimensions of nature, may be regarded as 

 the other. 



Jacko Parrot. 



The posterior part of this sternum bears some 



^semblance to that of the diurnal birds of prey, but 



interior portion and the attached bones are very 



different. Its relative length is also nearly the same. 

 But all this similarity of the hinder parts of the two 

 sterna merely shows that parrots and diurnal birds 

 of prey have both a very powerful and firm action in 

 those muscles which move their tarsi or toes, while 

 the different actions depend on the structure of the 

 toes themselves. It is, however, of no inconsiderable 

 importance in facilitating our knowledge of the rela- 

 tion between structure and use in those parts of birds, 

 even those parts which are not immediately con- 

 nected with each other, to find that the same quan- 

 tity and power of motion (however different the 

 object) are accompanied by the same general form of 

 the posterior portion of the sternum ; and there is 

 still this other relation between the action of the 

 feet in these otherwise dissimilar tribes of birds, that, 

 though they are both powerful cltitchers, the one in 

 killing game, and the other in climbing among twigs, 

 they are both very imperfect walkers on the ground. 



When we come to examine the anterior part of 

 the sternum that on which the character of the flight 

 more immediately depends, we find that, though the 

 wing of the parrot must, from both the depth of the 

 keel and the breadth of the sternum, be a ready wing, 

 yet it must be comparatively feeble and unsteady. 



The lower ends of the coracoids do not form an an- 

 gle of about sixty degrees, with these edges attached 

 to the sternum, as is the case in ail the falconidae 

 of powerful wing ; neither are their axes directed 

 toward the strongest part of the keel of the sternum, 

 either when viewed in front or on the side, as those 

 of the falcons are when viewed both ways. Their 

 union with the sternum, though not quite a straight 

 line, is upon the whole at right angles to the axis of 

 the body when seen in front, and they are nearly 

 parallel to each other ; and though enlarged at the 

 heads, not nearly so robust as those of the falcons, 

 although rather longer in proportion. In their 

 articulation on the sternum, and their position with 

 regard to each other, they have thus very little stiff- 

 ness in resisting the approach of the shoulder joints 

 toward each other ; thus in this respect they are to a 

 great extent the very opposite of those of the falcons. 

 And this flexibility which the coracoids have for 

 bringing the shoulder joints together, is not counter- 

 acted by any great strength of the furcal bone ; for, 

 though the two extremities of that bone are enlarged, 

 the branches are slender and straight, which renders 

 them as feeble as the coracoids. Even the blade 

 bones are not formed for taking a very firm hold in 

 their embedment. 



Thus when we examine the point of articulation to 

 which the wing of the parrot is attached, we find it a 

 very feeble one, and one which could not be used in 

 long flight, without great fatigue to the bird. The 

 coracoids and branches of the furcal bone are placed 

 in that position in which they are the least fitted for 

 resisting that strain on the shoulder joint which is 

 necessarily given when a bird flies rapidly ; and they 

 even appear to be loaded with an additional quantity 

 of bone at the joint, for no other apparent purpose 

 than that the joint may be the more unsteady, and 

 the flight of the bird the more feeble and laborious. 

 We would thence, if we reasoned only from the rela- 

 tive structures of the sternal apparatus, be apt to 

 conclude that nature has given to the parrot tribe 

 similar power of foot with the birds of prey, but shorn 

 and crippled them in those characteristic organs of 

 birds, the wings. 



O O2 



