FAMILY LARroAE 449 



ing birds in immature dress remain in Panama through the period 

 of northern summer, when they are found especially in tidal areas 

 on the lower courses of rivers of the Pacific side. In June 1953 

 I recorded many on the lower Rio San Pablo, below Sona, where 

 they moved back and forth with the tides from the head of the Gulf 

 of Montijo. Hundreds more ranged the shallow waters of the head 

 of the Gulf itself between Isla Leones and Isla Verde, They come 

 inland also when rains fill the channels in the swamps of the La 

 Jagua marshes. 



A principal haunt here at the northern edge of their wintering area 

 is in the Gulf of Panama, where I have found them regularly as far 

 down as Punta Mala and also to the eastward toward Colombian 

 waters. They range in flocks of 30 or 40 to 200 or more, sometimes 

 rafting in fairly close formation, sometimes spread over wide areas. 

 A floating board or stick may have one or two standing on it, and 

 a log of driftwood may support several. They join other larger 

 birds in hovering over the great schools of sardines to snatch at 

 the smaller ones as the rush of predatory fish below drives them to 

 break at the surface. I have seen black terns at various points in 

 the Archipielago de las Perlas, but they are found more commonly 

 in the open sea. It is interesting to compare this winter habit with 

 that of their breeding grounds, which lie entirely in fresh waters 

 inland. 



GELOCHELIDON NILOTICA ARANEA (Wilson): GuU-billed Tern; 

 Gaviotin Piquigordo 



Sterna aranea Wilson, Amer. Orn., vol. 8, 1814, p. 143, pi. 72, fig. 6. (Cape May, 

 New Jersey) . 



A tern of medium size with strong, heavy bill like that of a gull; 

 tail notched, not deeply forked. 



Description. — Length, 330 to 355 mm. Bill short and stout. Adult, 

 breeding plumage, crown and nape black ; back, wings, and tail light 

 gray ; entire under surface white. 



Winter dress, head and neck white, with the auricular region and 

 a crescent in front of the eye dusky gray ; otherwise as in summer. 



Measurements. — Males (9 from Maryland, Virginia, Bahamas, 

 Cuba, Haiti, and Veracruz), wing 285-304 (295), tail 108-122.8 

 (113.7), culmen from base 38.2-42.5 (39.7), tarsus 29.5-32.7 (30.8) 

 mm. 



Females (8 from Virginia, Florida, Alabama, Bahamas, and Haiti), 

 wing 286-297 (292), tail 105.6-118.8 (110.4), culmen from base 

 35.6-40.6 (38.2), tarsus 28.3-31.9 (30.5) mm. 



