LIFE HISTORIES OF NORTH AMERICAN PETRELS AND PELICANS. 157 



that we spent at the island Mr. Horace A. Gaylord and myself devoted most 

 of our time to digging for petrels. Both black and Socorro petrels were taken 

 from the burrows formerly occupied by the auklets, the former species out- 

 numbering the Socorro about five to one. There was no attempt apparently 

 on the part of the species to colonize by themselves, both being found in adjoin- 

 ing burrows. The Socorro petrel had evidently begun nesting somewhat earlier 

 than its neighbor, the black, for while fresh eggs of the latter were the rule, 

 very few fresh or even moderately incubated eggs of the Socorro were found, 

 and several downy young were taken. 



There was little, if any, attempt at nest building by either species, though 

 in several burrows a small nest-like platform of little twigs was found upon 

 which the egg was laid. But in most cases it rested upon the bare earth at 

 the end of a more or less winding burrow, about three feet in length. Several 

 eggs of both species were taken from under loose slabs of rock, but as a rule 

 they preferred the burrows, which were in all cases, I think, those of Cassin's 

 auklet. 



On our return to the island, September 8 and 9, we found that the Socorro 

 petrels had all left, but many young black petrels were found, as well as a few 

 eggs which the birds were still incubating. 



Eggs.^ — The single egg of the black petrel is nearly oval in shape, 

 with a slight tendency toward elliptical oval. The shell is smooth 

 and lusterless. The color is dull white, usually somewhat dirty and 

 generally unspotted; some specimens show a faint suspicion of fine 

 lavender or reddish dots about the larger end. The measurements 

 of 61 eggs, in various collections, average 36.6 by 26.7 millimeters; 

 the eggs showing the four extremes measure 38.5 by 25, 36.5 by 

 27.5, and 32.5 by 24.2 millimeters. 



Young. — The downy young looks much like that of the closely 

 allied species; the chin, throat and malar region are naked, but the 

 bird is elsewhere covered with long soft down, which is uniform 

 " fuscous " in color, darker basally. I have seen small downy young 

 only a few days old collected as early as July 4 and as late as Sep- 

 tember 5, on Los Coronados Islands. Subsequent phmiage changes 

 are probably the same as in other species of the genus. 



Food. — Like other petrels, this species feeds on what it can pick 

 up from the surface of the sea in the way of fatty, oily substances. 

 Mr. Howell found the stomachs, of those he examined, filled with 

 " oil and nothing else, except a small quantity of green, slimy stuff," 

 which he thought might be "the remains of some small crustacean 

 or a seaweed." 



Behavior. — Mr. Anthony (1900a) writes, of the night flight of 

 these petrels about their breeding grounds, as follows: 



Hauling the boat out on the shingle, a few steps places us in the city of 

 birds, a fact we discovered by breaking through into the burrows at almost 

 every step, but the birds themselves are very much in evidence. Hundreds 

 of Inky black objects are dashing about with bat-like flight, now here, now 

 there, with no apparent object in their wanderings. Like butterflies they come 

 and go, flitting so near at times that one attempts to catch them as they pass. 



