22 . NORTH AMERICAN FAUNA. 



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

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



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 melancholicus couchi, Ornitliion imberhe, Vireo fiavoviridis 

 forreri, and Thri/othorus latrrcncii ma(idalena\ 



Twenty-four of the 30 resident laud birds are specifically or subspe- 

 cifically distinct from their mainland representatives. Of this number 

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

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

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

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

 representatives. This is very marked in Gompsothlypis which is very 

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

 I'laty2>s(tris from that locality seem to be intermediate between the 

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

 feli.r from the same part of the coast are much nearer T. lawrencti than 

 they are to typical T. felU: 



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 Columba f. madrensis, LeptotUu capituUs, Fsittaenla innularis, 

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

 Icterus (/rnysoni, Gardinalis c. maria; Pirangn b. fammea, Vireo f. 

 forrcri, Vireo h. sordidus, Gompsothlyjns insula) is, OranaieUtis francescce, 

 Thryothorus laicrencii and Merula (jraysoni. 



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

 sentatives. These are I'oJyhorus c. palUdtis, lache laicrencci, Platyp- 

 saris a. insularis, Myiopagis p. minimus, Mclanotis c. lonyirostris, and 

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

 Platypsaris a. insularis 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 Gompsothlypis 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. Xyetidronuis a. insularis is a larger bird than albicollis 

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

 to the islands Thryothorus lawrencii magdalenw and Myadestes o. insu- 

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

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

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



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

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

 Although the ijhysical 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. 



