FAMILY CATHARTIDAE 



163 



Three geographic races of this species found on the isthmus are 

 described in detail beyond, one of them resident and the other two 

 present as visitors. The two visitors, which are migrants from the 

 north, have the bare skin of the head completely red in life. In the 

 resident form there are several lines of yellow across the nape, and 

 usually an irregular ivory-yellow spot in the center of the crown. 



Fig. 33. — Head of turkey vulture, noneca, Cathartes aura, with wrinkled skin 

 without papillae on the neck. 



The migrants arrive from the north during October and continue 

 to pass during November, often in tremendous flocks (see Loftin, 

 Caribbean Journ. Sci., vol. 3, 1963, pp. 63-64). While many remain 

 on the isthmus, thousands pass farther south into northwestern South 

 America. The return flight north begins in February, and continues 

 through March and early April, forming a notable addition to the 

 great migratory flight of the Swainson's and broad-winged hawks. 

 At Chiman on March 1, 1950, I estimated that about 15,000 passed 

 in the hour before sunset, an indication of the vast numbers con- 

 cerned. In these travels the turkey vulture moves mainly as a sail 

 plane, soaring by means of supporting air currents, with a minimum 

 of the greater muscular effort required by flapping flight. In this the 

 trade winds that blow steadily from the northeast through the dry 

 season are the main means of assistance, aided by the rising thermals 



