164 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I50 



generated by the heat of the sun. These migrant movements in 

 Panama were reported first by Frank M. Chapman (My Tropical Air 

 Castle, 1929, pp. 147-148 ; Auk, 1933, pp. 30-34) in his observations 

 at Barro Colorado Island. In the course of my studies I have found 

 that the main lines of flight are not isthmus-wide at random but follow 

 a definite pattern. Most of the birds that come from South America 

 cross from the lower Atrato basin in Colombia to the Pacific side 

 of Panama in eastern Darien. Then they travel northwest, between 

 the sea and the inland mountains, quartering against the steady north- 

 east trade winds, a course that they follow until they reach the central, 

 low depression crossed from sea to sea by the Canal Zone. Here the 

 vultures move across Gatun Lake, and then swing over the Caribbean 

 slope, turning gradually northwestward again, to continue over Bocas 

 del Toro, and on to Costa Rica. They travel sometimes in small, 

 separated bands of 25 to 400 or more individuals, but during the 

 period of greatest movement join in continuous lines in which thou- 

 sands pass during the course of an hour. The flights vary from 150 

 to 500 meters or more above the earth, with the birds moving steadily 

 forward, supported by slightly bowed, stiffly held wings that carry 

 them steadily without flapping. Occasionally they encounter rising 

 thermals in which they may circle, sometimes to gain altitude, but 

 ordinarily they pass quickly and silently across the sky. 



So far as I have observed, the movement is entirely by day, and at 

 dusk the vultures come down to sleep in open trees, where they re- 

 main in companies. Periods of rain, when the wind drops or changes 

 direction, may leave them stranded. This happens particularly in the 

 heavier rainfall found along the north coast. At Almirante in early 

 morning I have seen several hundred perched near together in dead 

 trees during rains that held the birds motionless for hours. Not until 

 past noon did the heat of the sun build up the necessary rising air 

 currents to enable them to move. There is no pause for feeding in 

 these migrations, but regularly some of the birds drop down to open 

 stretches along the rivers to drink. 



In addition to the main line of flight that I have described in 

 detail I have seen small groups moving north near Jaque on the 

 coast of Darien. Small flights in the Tonosi valley near the southern 

 end of the Azuero Peninsula may be indication that turkey vultures, 

 like numerous small birds, may cross the open Gulf of Panama from 

 the Choco or adjacent Darien. Small groups pass north through 

 Chiriqui. 



At present data on movement during the southward flight that be- 

 gins in October are not available in detail. 



