The Leach Petrels 



feathered inhabitants of these islands will lie rather with American for- 

 bearance and good faith than with Mexican piety. 



Four species of birds, namely, the Socorro Petrel, the Black Petrel, 

 the Xantus Murrelet, and the Frazar Oyster-catcher, have here their 

 northernmost stronghold; and their occurrence upon islands of the Santa 

 Barbara group or the waters of San Diego is rather incidental to their 

 occupation of Los Coronados. 



Mr. Alfred Brazier Howell, being for some months resident on the 

 islands in 1910, enjoyed unusual facilities for the study of their bird-life; 

 and it is chiefly to his account, published in Pacific Coast Avifauna, Num- 

 ber 12, 191 7, that I am indebted for information regarding both the Socorro 

 Petrel and the Black Petrel. 



The Socorro Petrel breeds only in burrows of its own excavation, and 

 appears to require fresh ones each year. For this purpose light loamy 

 soil is selected, and the birds resort alike to open situations or to brush- 

 covered areas. The nesting tunnels, usually about two feet in length, 

 have narrow entrances, much wider than high, and they twist sharply 

 either to right or left within a few inches of the mouth. The nesting 

 chamber is considerably enlarged and is usually lined indifferently with 

 twigs or rootlets. The single egg varies, as do all petrels' eggs, from pure 

 white to those having faint wreaths of lavender dots about the larger 

 end. 



The season of fresh eggs is late June or early July; but there is reason 

 to suppose that the nesting burrows are provided as much as two or even 

 three months in advance of final occupation. 



Unlike its larger relative, 0. melania, this form but rarely vomits oil 

 when removed from the burrow; but it will often do so on the wing imme- 

 diately after being released. In addition to the staple diet of rock lobsters 

 in the juvenal stage, young squid an inch or so in length have been found 

 in the crops of freshly killed specimens. 



0. I. socorroensis, as is well known, exhibits a great variety of plumage 

 as to its rump, ranging from a phase which has a sooty black rump, nearly 

 concolor with the back, to a phase whose rump is as white as any beali. 

 This variety is not correlated with season, sex, or age, inasmuch as dark- 

 rumped and light-rumped birds are found in the same burrow. This 

 variability, moreover, appears wherever the subspecies occurs; and 

 appears also in the closely related 0. leucorhoa (?) monorhis of eastern Asi- 

 atic coasts. Howell believes that he has evidence of a historic increase in 

 the number of white-rumped individuals; and surmises that we may have 

 here a highly plastic form which is undergoing a total, or totospecific, 

 change from a uniformly dark to a white-rumped species, rather than an 

 incipient "splitting" into races. Whatever be the significance of this 



2023 



