LIFE HISTORIES OF NORTH AMERICAN PETRELS AND PELICANS. 183 



was some sixty feet deep. It was at its widest point some seventy or eighty 

 feet, and oval in shape as a wliole. The bottom was covered with coarse sand 

 and gravel, and boulders of varying size, evidently having fallen from above, 

 were scattered thickly over this floor except at the extreme back of the 

 caveirn farthest from the sea. The height of the roof or ceiling, which was 

 of an uneven rough surface, was about twenty-five feet, and many bats were 

 hanging wherever the projections or inequalities afforded them opportunity. 

 Toward the back of this chamber five birds were secured, each one sitting on 

 a single egg. The place chosen for the nesting site, for this is all it can be 

 termed, was in all these cases where two boulders on the gravelly floor lay 

 close together, just leaving room on the ground for the birds to crawl between 

 them. Two birds were obtained in like situations that had not laid, and may 

 have been simply resting. The females were in every case the birds that were 

 sitting on thei eggs, and it was quite evident upon dissection that the single 

 egg forms the complement in these cases. The birds taken from the holes 

 in the cliff, and also those taken in this cave, were very tame, and were cap- 

 tured readily without attempting to escape. Later on the same day a bird 

 was found with a single egg laid at the bottom of one of the holes in the face 

 of the cliff. 



Mr. Karl Plath (1913) states that he found nests, on the Bermu- 

 das, " in the deep grass and also under small cedar bushes." The 

 data sent out with certain eggs would seem to indicate that bulky 

 nests were made of grass, moss, and sea weed, but it seems more 

 likely that the egg was laid on a bed of such material already in 

 the cavity than that the material was carried in by the bird for nest- 

 building. 



i^ggs. — The single egg of the yellow-billed tropic-bird is variable 

 in appearance and often quite handsome. The shape is practically 

 ovate or short ovate; some eggs are more pointed and some more 

 rounded at the small end. The ground color is pinkish or dirty 

 white, but this color is more or less completely (generally more) con- 

 cealed by profuse, fine specklings or cloudings of various shades of 

 "livid browm," "vinaceous brown," or "purplish drab" in an endless 

 variety of patterns; some eggs are very light in color, some very 

 dark and some are very much variegated ; some are marked with 

 heavy blotches or splashes of the above colors. 



The measurements of 40 eggs, in various collections, average 54 by 

 38.9 millimeters ; the eggs showing the four extremes measure 60 by 

 40, 56 by 41, 50 by 37.5, and 51 by 36.5 millimeters. 



Apparently two young birds are raised by each pair in a season, 

 the first eggs being laid early in April and the second sets late in 

 June. The period of incubation is 28 days. Doctor Gross (1912), 

 says: 



Both the male and female birds take part in incubation, and during this 

 period the egg is seldom left uncovered for more than a few minutes. The 

 birds take their turns at the nest, thus giving each other an opportunity to 

 feed. In one case an adult was seen feeding its mate while the latter was 

 brooding the egg. 



