l )2 



THE CONDOR 



Vol. XIX 



of which the most important are those anatomical peculiarities, which give the 

 b.rd a long and flexible neck. In birds there are generally more neck vertebrae 

 than among many other vertebrates, and these are longer than usual and have 



remarkably flexible joints. Fig. 40 shows the 

 articulatory surfaces of a crow's vertebra. These 

 do not form a ball and socket joint, which theo- 

 retically would be the most flexible possible artic- 

 ulation, but each is convex in one direction and 

 concave, or saddle-shaped, in the other. Several 

 such joints in series make a neck as flexible as a 

 similar number of ball and socket joints. 



Next to the flexibility of the neck as a factor 

 in increasing the usefulness of the bill, is the rath- 

 er flexible articulation of the mandible itself. In 

 reptiles and birds an extra bone, the quadrate, is inserted between the articu- 

 latory tip of the mandibular ramus and the base of the cranium. This bone in 

 the mammalia forms the incus of the middle ear according to a generally ac- 



Fig. 40. Neck vertebra of 

 Crow showing the saddle- 

 shaped ARTICULATION. 



Fig. 41. a. Diagram of the monocular vision of a chicken, b. 

 Diagram of the binocular vision of an owl. c. Diagram of a 

 Hummingbird's head. Each eye with two centers of acute sight 

 giving both monocular and binocular vision. 



cepted view. In most reptiles the insertion of this quadrate in the articulation 

 of the mandible makes a true double joint, but in the birds it is so firmly fused 

 to the cranium that the hinge of the jaw lies between the quadrate and mandi- 

 ble. Nevertheless, the joint is perhaps freer, at least than in mammals, as it is 

 farther removed from the base of the cranium. 



With this, but of considerably more importance, is the length of the bill, 

 which removes the grasping tip far enough from the eyes at its base to bring the 

 work of the tip within the range of vision. As one result of this we find that the 

 very finest nests are built by such birds as hummingbirds and the longer-billed 

 insectivorous species, rather than by the conical-billed sparrows, whose nests are 

 usually loosely constructed as compared with the nests of the preceding. ' 



The smooth horniness of the bill, and the lack of teeth, are characters that 

 permit the use of the bill as a shuttle or needle, making it an organ that can pro- 

 duce the beautiful woven nests of such forms as the orioles, or the interesting 

 nests of the tailor bird, with leaves sewed about them. But even with these char- 



