14 PACIFIC COAST AVIFAUNA No, 12 
Colonization of the islands by birds may have been brought about in two, or 
possibly in three, ways. First, through those species which originally lived in 
the territory at the time when the islands were part of the mainland; these would 
seem to constitute the bulk of the species now resident there. Second, through 
such instances as where a pair or more of a species had wandered from the main- 
land during fogs, or, having been blown to an island by storms, settled there 
permanently. This theory, has, I believe, been accepted as an explanation of the 
presence of some few resident birds on other islands. A third theory, which J 
consider rather improbable, is that a few individuals of a species regularly or oc- 
casionally visiting the islands in winter or during migration, have remained to 
breed. 
Conditions bearing upon the bird life of the islands differs from those on 
the mainland in a variety of ways, as one would expect. Here we have the sur- 
vival of the fittest carried to the extremest degree. If a species cannot readily 
adapt itself to changed conditions, it is unable to seek more congenial surround- 
ings, as on most parts of the continent, but must get along where it is, or perish. 
If structural adjustments are possible, insular forms arise. Food on the islands 
must differ, in the case of many birds, from that available on the mainland; but 
to Just what extent, it is impossible to say. Notable cases are those of the insular 
Mockingbird and House Finch, which feed on the fruit of the opuntia eactus 
until their whole fronts are stained by the red juice. This may enter into the 
menu of their mainland relatives as well, but certainly not to a like extent. 
As for extreme change of habits, one has but to visit Santa Barbara Island 
to be impressed by the case of the Song Sparrow. This island is rather barren 
and without water; so, instead of a shady retreat among the dense brush of a 
damp ravine, we see the little fellows out-larking the Horned Larks themselves, 
among the iceplant and short grass of the mesa. The House Finches have taken 
to building in pockets of the conglomerate cliffs as well as in the cactus, and the 
Dusky Warbler, instead of building almost invariably on the ground, as does 
the Lutescent, prefers a vine, shrub, or even the branches of a tree fifteen feet 
above the ground. 
Different exposures would seem to give more widely varying results than do 
similar situations upon the mainland, judging by my experiences on Todos San- 
tos Island, near Ensenada, Lower California. This island, of course, is beyond 
the range of the present paper, but it is very similar to the smaller of the Santa 
Barbara Islands, and is comparable in climate as well as otherwise. From April 
15 to 20, 1910, I was on Todos Santos, and found that at the northern end the 
San Clemente House Finches were far advanced in nesting (Howell, Condor, 
xiv, 1912, p. 190). I found only two pairs having eggs far advanced in ineuba- 
tion, while a dozen were located with young in all stages, some of which were 
ready to leave the nests. On the southern end, fresh eggs and incomplete sets 
was the rule, no young at all being noted. This was an unusual state of affairs. 
The island is a mile and a quarter long and the northern end is windy and fog 
drenched, while the southern part is comparatively warm and sheltered. I am 
unable to offer any explanation of this. Certainly the food supply could not 
have had anything to do with it in such a small area, 
