N0 - 36 °7 KINESIS OF AVIAN SKULL — ZUSI 21 



the mandible, the presence of ligamentous adductor muscles, and the 

 resistance to depression of the lower jaw by environmental forces. 

 Other features may be involved as well and a variety of factors 

 probably contribute to jaw motion in each species. The mecha- 

 nisms are developed to varying degrees in different groups of birds. 

 Only by understanding their functional properties can we recognize 

 convergence, independent divergent solutions to similar problems, 

 and adaptive radiation in the kinetic mechanism. A survey of such 

 trends in birds or in any group of birds has not yet been made in 

 detail, but a few cases of variation in the depressor mandibulae 

 complex should be mentioned here. 



Certain birds that habitually probe into flowers or fruit show a 

 combination of features that suggest a special role of the depressor 

 muscles for protracting the upper jaw. Representatives of the 

 "Coerebidae" (Coereba flaveola), Zosteropidae (Zosterops annulosa), 

 and Drepanididae (Vestiaria coccinea) display large depressor muscles, 

 well-developed retroarticular processes of the mandible, and strong 

 postorbital ligaments (Beecher, 1951a, pp. 277, 283; Moller, 1931, 

 p. 126). Beecher (1951a, p. 2S5) states that "The Neotropical nectar- 

 adapted tanagers are non-gapers. In none of them is M. depressor 

 mandibulae more highly developed than in Cyanerpes . . . and it is 

 apparent that these species simply insert the bill into flowers and sip 

 nectar." Moller (1931, p. 110), however, illustrates Dacnis cayana 

 as having a large depressor mandibulae, which, combined with its 

 strong postorbital ligament, long retroarticular process, and relatively 

 straight cidmen and gonys strongly suggests that gaping is among its 

 feeding patterns. Various members of the Icteridae display these 

 same features and, in addition, have the depressor muscles angled 

 well forward (Beecher, 1950, 1951b). Associated with this highly 

 developed gaping adaptation is the straight culmen and gonys so 

 characteristic of many icterids, essential for effective parting of the 

 grass, earth, or flesh of a fruit with the outer edges of the bill. The 

 starling (Sturnus vulgaris) is convergent with the icterids, especially 

 Sturnella, in some features related to gaping but divergent in others 

 (Zusi, 1959). Certain parrots have the depressor mandibulae muscles 

 enlarged and strongly angled forward (Nestor notabilis and Cyano- 

 rhamphvs novae-selandiae, illustrated in Hofer, 1950, pp. 457, 463). 

 The retroarticular process is well developed but the postorbital liga- 

 ment is lacking. Parrots are well known for their extraordinary 

 mobility of the upper jaw, but no comparative information on this 

 property in the Psittacidae is available. Among the probing Scolopa- 

 cidae, the woodcock (Scolopax) shows special development of the 

 depressor mandibulae and retroarticular process of the mandible 

 (see Marinelli, 1928, fig. 8), possibly in relation to the need for 



