[Illustration: 98. Gannets. × 3. An enlarged detail of No. 99.] The Kittiwake population of the Rock probably numbers between six hundred and eight hundred birds; of Gannets, there are perhaps left only fifteen hundred of the more than one hundred thousand birds which Dr. Bryant writes of as living on the top of the Rock alone; and of Petrels, not more than fifty. When on the Rock I should have said that it was tenanted by at least ten thousand birds, and I was not a little surprised to find that the evidence furnished by my photographic records gave a total of about four thousand birds. However, the sight of four thousand birds domiciled in one small islet is sufficiently impressive to increase the pulse beat of the most phlegmatic traveler; and even if this estimate be too large, the Rock’s merits as a bird resort are too substantial to be affected by any decrease in it which truth demands. To return to an account of the day’s doings, the light, as has been said, was unfavorable for photography, and the time was devoted to collecting and preparing specimens and making a hurried survey of the bird rookeries on the Rock, with results briefly set forth above; but late in the afternoon the sun gave indications of its whereabouts behind the clouds, and I immediately substituted the camera for the scalpel, and had Keeper Bourque lower me in the crate in order that I might secure photographs of the birds observed on our ascent. Neither the stability of the crate nor its constant turning were conditions which a photographer would choose, and, without the twin-lens it would have been impossible to secure pictures of the Kittiwakes^{85} and Murres, who in a surprised but unalarmed manner regarded me from their nests on the Rock, in some instances at a distance of not more than six feet. At ten o’clock at night I visited the west end of the Rock to see and hear the Petrels that nest there. The casual visitor to Bird Rock would be quite unaware of the presence of these birds; indeed, one might live there for years without knowing that Petrels made it their home. As far as the Rock is concerned, the birds are strictly nocturnal; but as usually only one bird—either male or female—is found on the nest, it is supposed that its mate is at sea feeding. If this supposition be true, I am at a loss to account for the entire absence of the birds during the daytime. Why should they not return to their nests before nightfall? And if, as stated, the sea bird takes the place of the nest bird, does the latter always feed at night and the former by day, or do they sometimes change about, thus making the same individual both nocturnal and diurnal in habit?