16 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



Because the two quadrates are held in comparable positions by the 

 jugal and palatal struts and therefore cannot move independently, 

 one of the mandibular rami must slide backward along the quadrate 

 while the other remains in place during displacement of the jaw to 

 the side. The possibility of backward displacement of the lower jaw 

 in the domestic chicken, although less obvious during normal feeding 

 of the bird, can be demonstrated by manipulation of fresh heads. 

 I conclude that the shift of the ligamentary fulcrum toward the jaw 

 articulation is one means of overcoming strict "coupling" by the 

 postorbital ligament. 



In addition to the experiments already described, three instances of 

 independent jaw action in birds possessing a well-developed post- 

 orbital ligament have come to my attention. Two of these represent 

 jaw motions during yawning — one in the night heron (Calherodias 

 leuconotus) observed by me (see p. 24) and the other described by 

 Yudin (1958, p. 168), who gave the sequence of jaw motions in a gull 

 as follows: (1) lower jaw maximally depressed, (2) upper jaw raised, 

 (3) upper jaw lowered, and (4) lower jaw raised. The third instance 

 appears in a series of photographs of the great snipe (Capella media) 

 taken by P. O. Swanberg (1956). In these birds, protraction of the 

 upper jaw is restricted to the tip of the bill, where it would be readily 

 evident by comparison with the immovable base-line provided by 

 the rest of the upper jaw. Several photographs show the closed bill 

 and its slight downward curve. Plate 74 shows the lower jaw slightly 

 open and in plate 73 quite widely open, but in neither case is there 

 any change in the downward curve of the tip of the upper jaw and 

 thus no protraction or coupling. 



Jaw Articulation 



One important question remains to be answered: how did the de- 

 pressor mandibulae effect protraction in the evening grosbeak and in 

 the domestic chicken after removal of the postorbital ligaments? In 

 addition to coupling of the two jaws by the postorbital ligament, 

 Bock (1964, p. 18) briefly referred to another type of coupling through 

 an interlocking of the condyles of the quadrate and articular with the 

 result that the mandible cannot be depressed without forward motion 

 of the quadrates. He listed certain families of birds in which this 

 interlocking is well or moderately developed, citing Balaeniceps as a 

 prime example, but he offered no explanation of this action. Although 

 neither of the species used for experiments in the present study is 

 coupled through the jaw articulation to a high degree, a discussion of 

 the mechanism in Balaeniceps will serve to introduce the general 

 problems of coordination of the jaws through the jaw articulation. 



