Vor. II, Pt. 11] LOOMIS—A REVIEW OF THE TUBINARES 115 
enlargement of the testes. In following “the feed” when the 
sea was calm, they would dive from wing, flying directly into 
the water, disappearing wholly from view. When resting on 
the placid surface, they took wing readily, differing in this 
respect from the Murres and Rhinoceros Auklets, with which 
they were sometimes associated. In coming down the wind 
during a strong breeze, they changed their rapid wing strokes 
and short sail to an undulating sailing flight, much elevated for 
a shearwater. 
The Galapagos Expedition first encountered Black-vented 
Shearwaters on July 2, 1905, off San Miguel Point, Lower 
California. On the 7th they were abundant off Banda Point, 
and the last were seen on the 14th near West Benito Island. 
At Natividad Island on the 19th, only deserted burrows closed 
by cobwebs were noted. In 1903, the Revilla Gigedo Expedi- 
tion found them breeding sparingly on San Benito Islands on 
May 6 and abundantly on Natividad Island on May 9. No 
nestlings were discovered, only incubated eggs. 
Like Cooper’s Shearwater, the present species has a double 
style of coloration. In the extreme dark phase, the gray pre- 
dominates over the fore-neck, strongly contrasting with the 
white of the rest of the lower parts (plate 14). It also inter- 
rupts the white lining of the wings and is much extended on the 
axillaries and sides of the body. In No. 9491 C. A. S. the gray 
in faint spots invades the breast and abdomen, becoming less 
distinct posteriorly. In the extreme light phase, the fore-neck 
is immaculate white (plate 14), the lining of the wings dis- 
plays few if any traces of gray, and the axillaries and sides of 
the body have the gray much restricted. White nearly sup- 
plants the dark color on the shorter lower tail-coverts in Nos. 
9474 and 9516 C. A. S. Between the dark and light extremes 
of the two phases, lies the great majority of the Academy’s 
one hundred and thirty-nine specimens. 
The explanation of the double form of coloration prevailing 
in this shearwater is apparently found in dichromatism rather 
than in age variation, for in the worn breeding plumage of both 
phases the growing feathers on the fore-neck are like the old 
ones they are replacing. Specimens passing from the natal 
down into the definitive feathers would present conclusive evi- 
dence. 
