significance of which is not yet well 

 understood. The Cocos Island population, 

 however, comprises only uniformly grayish- 

 brown birds, and we saw no other type on our 

 voyage . 



Sula leucogaster brewsteri . Brewster's 

 booby. The case of this booby and the next 

 poses interesting biological and bio- 

 geographical problems. Both are subspecies 

 of the cosmopolitan brown booby, and both 

 are confined to the west coast of America 

 and outlying islands . From the topotypical 

 brown booby the two races differ in a 

 similar manner, notably in that the heads of 

 adult males have pale or whitish feathering. 

 The physical distinctions between the sub- 

 species brewsteri and etesiaca are slight 

 but are constant and readily recognized. 



Physiologically, however, the differences 

 between these two races may be relatively 

 profound because brewsteri lives in an area 

 of high aridity, whereas etesiaca extends 

 from some unknown point N of western Panama 

 southward to the coast of Colombia. It 

 includes also Cocos Island. Whether there 

 is a hiatus between the coastal ranges of the 

 two races is yet unknown. 



We first met Brewster's booby on Nov. 12, 

 midway off the mouth of the Gulf of 

 California. The birds were flying in pairs 

 or in groups of three. Next day the first 

 specimen was collected. Thereafter examples 

 were observed, sometimes standing on the backs 

 of turtles, as far as waters off the Gulf of 

 Tehuantepec . On the return voyage we saw this 

 booby again off the entrance of the Gulf of 

 California on Dec. 10. 



Sula leucogaster etesiaca . Columbian booby. 

 The presence of boobies of this type in the 

 "Dome" area was inconclusive because of the 

 difficulty of discriminating, without 

 specimens in hand, between etesiaca and 

 brewsteri . 



When we approached the coast of western Panama, 

 large flocks of Colombian boobies became a 

 familiar sight. Sometimes they were feeding 

 with other sea birds, such as cormorants, 

 Jaegers, and terns. A particularly large 



concentration was passed on Nov. 25 off the 

 Islas de Ladrones, where they doubtless nest. 

 Later we found them in the Gulf of Panama and 

 along our course toward Cocos. On Dec. 1 a 

 female in breeding state was collected at 

 05°59' N, 79°l48' w. 



While approaching Cocos on Dec. 3> we met a 

 movement of Colombian boobies 50 miles from the 

 Island. At Cocos they were nesting principally 

 on the outlying islets, particularly on 

 Manuelita or Nuez, where their nests, with 

 eggs and young in all stages, were underneath 

 tall shrubs in which red-footed boobies were 

 nesting. Although confined to the ground for 

 nidif ication, the Colombian boobies perch 

 freely on good-sized branches of trees, but 

 perhaps never on twigs. 



The surface water at Cocos proved of slightly 

 lower temperature than that in the range of 

 brewsteri . far northward. 



Phalacrocorax penicillatus . Brandt's 

 cormorant. The cormorants are all coastbound 

 birds in the part of the world under 

 consideration. This species was noted along 

 the coast of Baja California. 



Phalacrocorax olivaceus . Bigua cormorant. 

 Observed in western Panama and in the Gulf of 

 Panama. 



Phalacrocorax pelagicus. Baird's cormorant. 

 Although named pelagicus , this species is also 

 confined to the narrow continental platform. 

 It was noted only within a few miles of San 

 Diego and the Coronados Islands. 



Fregata magnificens . American man-o'-war bird. 

 This species is common to both Atlantic and 

 Pacific sides of tropical and subtropical 

 America but, except at the Galapagos, it is 

 replaced by the following species as an off- 

 shore bird in the Pacific. Our most seaward 

 records were made near the breeding station of 

 Alijos Rocks, on Nov. 9. This is presumably 

 the northern limit of the nesting range on the 

 Pacific coast. Two adult males were collected 

 here. 



Thereafter we saw this species regularly to the 

 Gulf of Panama, always interested in 



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