BIRDS OF THE TRES MARIAS ISLANDS. 23 



The death by starvation of the Louisiaua Tanagers on Maria Mailre 

 Island (p. 52) is an example of the manner in which the ishind fauna 

 may be maintained in its present state. As the climatic conditions on 

 the islands and on the mainland are very similar and the vegetation 

 nearly alike, this paucity of species jjresents one of the curious prob- 

 lems of distribution. 



It would be hard to find an equal area of similar country on the 

 mainland, near San Bias, where so few species of land birds could be 

 found. The only reasonable explanation seems to be the scarcity of 

 water and the long, dry season, which combine to reduce the food sup- 

 ply and perhaps render the country unsuited to some species. It was 

 very surprising to find a total absence on the islands of such com- 

 mon and widely spread mainland genera as Conunis, Momotus, Fiaija, 

 CampephUns^ Melunerpes, Mj/io^etetes, Cissolopha, Cyunospha^ Pipilo, 

 ri/raisoma, Saltator, and others. The absence of Pipilo is especially 

 unexpected, for this genus is represented on Socorro Island, which 

 lies very much farther at sea off the same part of the coast. 



ANNOTATED LIST OF SPECIES. 



? Brachyrhamphus brevirostris (Vigors.) Short-billed Murrelot. 



Brachyrhamphus brevirostris was described from San Bias and B. 

 hypoleucus from Cape St. Lucas. Colonel Grayson mentions having seen 

 'guillemots' at Isabel Island (Mem. Boston Soc. Nat. Hist, II, p. 318, 

 1874) and off theTres Marias group (Proc. Boston Soc. Nat. Hist., XIV, 

 p. 288, 1871). This led me to anticipate finding at least one of the 

 species there, and it is with some disappointment that I have to record 

 our failure to see either species about the islands, although I watched 

 for them constantly. From this experience I am inclined to think that 

 they breed only along the coast of Lower California, and visit these 

 islands sporadically. 



Larus argentatus sniithsonianus Coues. American Herring Gull. 



A single immature specimen was taken on San Juanito Island May 

 22, and a few others were seen. These birds were flying back and forth 

 along a strip of beach where a large colony of blue-footed gannets 

 were breeding, and the gulls probably had an eye on the nesting 

 ground for the purpose of capturing any unprotected eggs. They were 

 noted singly a few times along the shores of the Tres Marias and at 

 Isabel Island. No fully adult individuals were seen. 



Larus heermaiini Cassin. Heeruumn's Gull. 



On April 23 a fine adult bird of this species was shot on the shore of 

 Isabel Island. In company with its mute it had harried a blue footed 

 gannet into disgorging a number of small fish upon a rock at the edge 

 of the water, and was picking up the spoils by a series of little down- 

 ward swoops and hoverings. The gannet had shuftied into the water 

 and was making off, with backward glances at its tormentor, when I 

 drew near. These gulls are bold and noisy aggressors when they wish 



