LIFE HISTORIES OF NORTH AMERICAN PETRELS AND PELICANS. 141 



space a yard square, aud one must tread cautiously indeed to escape breaking 

 through tlie burrows at every step. 



The burrow leads inward from the entrance for varying distances, two feet 

 being about the average length. In extreme cases tunnels have been opened 

 having a length of fully s!x feet, aud from two to five birds occupy this in 

 common, each nest being placed in one of the lateral offshoots from the main 

 trunk. Such extensive residences have evidently been vacated by Cassiu auk- 

 lets, as one young bird of this species was found in a burrow with five petrels. 

 The nest is a flat, thin pad composed of fragments of grass, bits of moss, and 

 small twigs of spruce or salmon berry. 



Eggs. — The Leach petrel lays a single egg and raises onl}^ one young 

 bird each season. The Q.gg is much like that of others of the same 

 genus, varying considerably in shape and size. The shape is elliptical 

 ovate, elliptical oval, or nearly oval. The texture of the shell is 

 smooth, but not glossy. The color is pure white, dull white, or dirty 

 cream white and it is often much nest-stained. Many eggs are spot- 

 less, but many others are finely sprinkled or more or less conspicu- 

 ously wreathed with small and usually faint spots or fine scrawls 

 of reddish, purplish, or lilac about the larger end. 



The measurements of 55 eggs, in the United States National Mu- 

 seum collection, average 32.5 by 24 millimeters; the eggs showing 

 the four extremes measure 35 by 23, 33 by 26, 30.5 by 23, and 33 

 by 22 millimeters. 



The period of incubation has been variously estimated at all the 

 way fi'om two weeks to a month, but, as the stormy petrel egg has 

 been demonstrated to require 35 days to hatch in an incubator, it is 

 probable that the time required for the Leach petrel to incubate is not 

 far from five weeks. Both sexes incubate, relieving each other dur- 

 ing the night; the sexes of specimens taken on the nests show that 

 this duty is shared about equally by both. It has been stated by sev- 

 eral observers that one of the pair feeds its mate on the nest, but I 

 think this hardly likely ; it seems more reasonable to suppose that 

 the bird which incubates during the day is relieved early in the even- 

 ing and returns again to relieve its mate in the morning after having 

 been feeding during the night. It is a weird experience to spend a 

 night in a petrel colony during the breeding season. Night is their 

 season of activity, birds are coming and going all the time, dark flit- 

 ting, ghostlike forms, hardly discernible in the darkness, uttering 

 their loud and peculiar cries, as they call to or greet their mates. 

 They are awkward at first on leaving their burrows, stumbling about 

 in the grass in their efforts to get on the wing, as they must find some 

 little eminence from which to launch into the air. It is a wonder that 

 the incoming birds can find their mates or their burrows in the dark- 

 ness and the confusion of thousands of fluttering birds. 



Young. — The young bird when first hatched is brooded by one of 

 its parents for three or four days, after which it is left alone in the 



