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 Mar., 1915 NOTES ON MURRELETS AND PETRELS 77 breeding ground on the islands. Just why the Socorro Petrel should be forced out by the above mentioned vermin (if such be the case), while the Black Petrel persists in abundance, is a question which for the present must remain unanswered. Black Petrels nest on all of the group with the exception of South Island and in aggregate numbers probably exceed the Socorros, though their habit of nesting in isolated pairs or at best small scattered colonies renders them much more difficult to obtain. Another factor which adds to the difficulty of collecting them is that very frequently the nesting site is under some great boulder or in a narrow, inaccessible crevice in a rock wall. I am morally cer- tain that, on these islands at least, although the Black Petrels occasionally do use burrows, these are not excavated by the birds themselves, but are merely abandoned ones of Socorro Petrel or Cassin Auklet. The normal site is a nat- Fig. 28. ADULT AND YOUNG PETRELS: Oceanodroma socorroensis AT LEFT; 0. melania AT RIGHT. THE LARGER SIZE OF THE BILL OF THE LATTER IS EVIDEITj EVE I TIrE YOUNG ural one, such as a cranny in a rocky wall, beneath misplaced slabs of rock, in dark caves, or even under heavy bushes, sharing this last situation with socorroensis, birds of each species sometimes being found within a few inches of each other. The young of these two petrels are very similar in appearance. When first hatched the bills, tarsi and feet are a dirty pinkish, gradually darkening as the birds grow older till by the time they are the age of those figured (about two weeks) the parts mentioned are the solid, shiny black of the adults. Even in newly hatched chicks the difference in bills is plainly visible (see fig. 28), and no difficulty should be experienced in distinguishing the young even in the field should a young Black Petrel be taken from a Socorro burrow. The natal down is at first a light smoky gray, changing as the bird develops to