22 NORTH AMERICAN FAUNA. 



species. Visitant land birds, 26 species or subspecies. Eesident water 

 fowl, 13 species. Visitant water fowl, 8 species. 



Of the 36 resident species or subspecies of land birds all but 5 were 

 observed by Colonel Grayson. These exceptions are: Melopelia leucop- 

 tera, Tyrannus melancliolicus couchi, Ornithion imberbe, Vireo flavoviridis 

 forreri, and Thryothorus lawrencii magdalence. 



Twenty-four of the 36 resident land birds are specifically or subspe- 

 cifically distinct from their mainland representatives. Of this number 

 12 were described from Grayson's collections, 1 from Ferrer's, and 11 

 from our own. A study of our collections from the islands, and near 

 San Bias on the mainland, brings out the interesting fact that several 

 species from the latter district show a decided approach to their island 

 representatives. This is very marked in Compsothlypis which is very 

 nearly the same at San Bias as on the islands. The Polyborus and 

 Platypsaris from that locality seem to be intermediate between the 

 island races and the birds of the mainland. Specimens of Thryothorus 

 felix from the same part of the coast are much nearer T. lawrencii than 

 they are to typical T. felix. 



Among the 24 species or subspecies of land birds peculiar to the 

 islands 15 are larger than their relatives of the nearest mainland. 

 These are Columbaf. madrensis, Leptotila capitalis, Psittacula insularis, 

 Dryobates s. graysoni, Nyctidromus a. insularis, Amazilia graysoni, 

 Icterus graysoni, Cardinalis c. maricc, Piranga b. flammea, Vireo /. 

 forreri, Vireo h. sordidus, Compsothlypis insularis, Granatellusfrancesca;, 

 Thryothorus lawrencii and Merula graysoni. 



Six of the island birds average smaller than their mainland repre- 

 sentatives. These are Polyborus c. pallidus, lache lawrencei, Platyp- 

 saris a. insularis, Myiopagis p. minimus, Melanotis c. longirostris, and 

 Trogon a. goldmani. The two first named are generally smaller, but 

 Platypsaris a. insular is has a longer tarsus, Myiopagis p. minimus a 

 longer bill and tarsus, Melanotis c. longirostris a longer bill, and Trogon 

 a. goldmani a longer bill and tarsus. 



Although Compsothlypis insularis also occurs in a limited area along 

 the coast, I have considered it as a typical island species. The 

 difference in size between island birds and their mainland repre- 

 sentatives varies greatly, being slight in some and very well marked 

 in others. Nyctidromus a. insularis is a larger bird than albicollis 

 proper, but has a shorter bill and tarsus. Among the birds peculiar 

 to the islands Thryothorus laicrencii magdalence and Myadestes o. insu- 

 laris are almost the only ones which do not show more or less well- 

 defined differences in size from their nearest mainland relative; a series 

 of the first named, however, may show that it also differs. 



One of the most puzzling features of the fauna of these islands is 

 the absence of various land birds found on the adjacent mainland. 

 Although the physical conditions appear so much like those of the 

 mainland, yet some change must have occurred to upset nature's fine 

 balance and render these isolated areas unsuitable for many species. 



