16 PACIFIC COAST AVIFAUNA No. 12 



eral island climate has a slightly northern or more humid tendency, and this has 

 its expected effect in that the majority of forms are slightly darker than are 

 their relatives on the adjacent mainland. In connection with the effect of cli- 

 mate on the birds as they were and as they now are, it is interesting to speculate 

 in regard to a number of suggestions and theories set forth. The previous integ- 

 rality of the islands and continent, and their subsequent separation at an uncer- 

 tain date; the effects of the encroaching ice cap in glacial times; some of the 

 tendencies of Pleistocene times as indicated by the avifauna of the Rancho La 

 Brea beds, and the finding there of conifers which do not now occur in the con- 

 tigous territory : all this makes interesting food for thought and speculation, but 

 is not closely enough related nor well enough understood for me to dare to set 

 down any conclusions. Each of these changing factors has undoubtedly had an 

 influence, however. 



Both food and the quest of it have probably been contributing factors in th(5 

 forming of island races. It might readily be that in the course of time an arbor- 

 eal form inhabiting an island tliat had few trees and few predaceous forms, wo aid 

 show a shortening of the wdngs, and corresponding increase in the size of the 

 lower extremities. Wliether or not, this has had any effect, it is a fact that prac- 

 tically all the island subspecies whose feet differ from the forms of the main- 

 land, have those parts lieavier, but without the correlated shortening of the wing. 

 Habits, as previously illustrated in the Song Sparrows of Santa Barbara Island, 

 may have an active influence in this connection, and the effects of inbreeding 

 must also be considered. It is a well known fact among breeders of domestic ani- 

 mals that continual inbreeding will result in loss of vigor, lessening of size, and 

 accentuation of any defective points; and that it will help to bring out latent 

 atavistic tendencies. This surely does not concern us in the majority of in- 

 stances ; but where a stray pair of a species has reached an island and remained 

 to breed, I believe that the resultant inbreeding would have a strong tendency 

 to form characters differing from the original type. 



The island species and subspecies, including those that have been deemed 

 unworthy of recognition by the A. 0. U. Committee, number nineteen. When 

 differing at all in dimensions from their relatives of the mainland, it is in the 

 following respects. Wing : in three races shorter and in two longer. Tail : in one 

 shorter and in two longer. Bill : in two smaller and in ten larger. Tarsus : in 

 one smaller and in seven heavier. Toe : in one shorter and in seven heavier or 

 longer. Eleven of the races show darker markings and three show lighter mark- 

 ings. Taking a composite of the lot, we find that the influences of this group o£ 

 islands tend to produce a bird of greater total length, with larger, heavier bill, 

 and heavier tarsus and foot. The length of wing and tail remains about the 

 same, and coloration becomes darker, with brighter colors and heavier streaking. 



There seems to be a well used line of migration through the islands. Prac- 

 tically none of the passerine transients occur in large numbers, but the seasonal 

 waves of migration are well marked. There are several records from these and 

 the Farallon Islands, of birds that are decidedly rare in any part of the far west. 

 Winter visitants are abundant, but the species which are absent during the win- 

 ter, and return to the islands for the purpose of breeding, number only five. 



