LIFE HISTORIES OF NORTH AMERICMsT PETRELS AND PELICANS. 163 



as both male and female can be found in a burrow long before the time for eggs ; 

 but there are exceptions to this rule, for A. van Rossem took several birds 

 from fresh eggs out of burrows from which he had collected the original occu- 

 pants but a few days previous. After laj'ing, but one of the parents remains 

 on duty, and I have found that, among birds incubating in the daytime, the 

 males are slightly in the majority. Birds with partially white rumps are found 

 in the same holes with unicolored ones and every degree of variation in this 

 respect occurs. 



On one of the small Coronados Islands about 200 pairs in 191^3, and not 

 quite so many in 1910. were nesting. Most of these occupied a' small amphi- 

 theater in a compact colony, but this colonizing may have been because there 

 is very little soft earth to be found on the remainder of the island. In this 

 powdery loam the bird excavates a burrow about two feet long and a short 

 distance below the surface, and turns iisually either to the right or left a few 

 inches from the opening. The entrance seems very small for the size of the 

 bird is more than twice as wide as high. The tunnel is slightly enlarged near 

 the end and here a flimsy platform is constructed of any bits of twigs and 

 rootlets that are handy, though occasionally the egg rests on the bare ground. 

 Observers have usually found that the nesting time of the Socorros is slightly 

 in advance of that of the black petrels, and such A. van Rossem and I found 

 to be the case in 1913, for during the latter half of June, slightly Incubated 

 eggs of the former was the rule, while those of the latter were nearer fresh. 

 However, in 1910, on the Coronados. an egg of the present species was not 

 found until June 22, while incubated eggs of melania were taken nearly a week 

 before this. 



Messrs. Grinnell and Daggett (1903) have given us a good descrip- 

 tion of Middle Island, in this group, where the petrels' nests were 

 most accessible, as follows: 



This island presents two jagged peaks about a hundred feet high, with a 

 sag between the two. To one side of this saddle is a basin perhaps two hundred 

 feet across, unevenly edged with ragged ledges. The bottom of the basin farthest 

 from the saddle has been undermined by a subterranean channel connecting 

 with the surf on the outside of the wall. Here one can look down thirty feet 

 or more and see the water surging back and forth with the swell. The rest of 

 the basin sloping up to the saddle is covered by disintegrated rock from the sur- 

 rounding walls, and supports a scanty growth of dwarfed " buck-thorn " 

 bushes. Where this bush is thickest a few inches of peaty soil has accumulated 

 and this we found to be a favorite burrowing place for the petrels. Other parts 

 of the island were also occupied, but in those places the burrows usually ended 

 underneath or between heavy fragments of rock and so were mostly impossible 

 to reach. We were first made aware of the presence of the colony by the 

 strong and characteristic odor of petrel oil, for, of course, not a bird is to be 

 seen above ground during daylight. Following the scent we soon found open- 

 ings, generally more or less hidden by weeds or stones. A cursory survey 

 showed that the basin was honeycombed with burrows. In the loose talus of 

 the slopes they extended directly down into the ground, turning aside here 

 and there to avoid pieces of rock, and ending, where further excavation had 

 become impossible, in a cavity about twice the diameter of the main burrow. 

 Those in the more level ground were often entirely concealed by wide-spreading 

 bushes which had to be cut away before the entrance could be reached. Other- 

 wise these latter were easy of access, for the peaty, fibrous nature of the soil 

 rendered shallow burrows possible, and such were easily uncovered by sliding 



