Leach Storm Petrel Oceanodroma leucorhoa
On 27 April we found only one large immature with down on the belly. This bird represents the last of the winter breeding race which has been called O. l. kaedingi. Later in the summer, a race thought to be O. l. soccoroensis breeds on the island. This temporal separation of the two races was first described by Hubbs (Systematic Ecology 9 (3 & 4) 134-147, 1960.) However, the subspecies O. l. kaedingi is not accepted by some taxonomists as a distinct race from soccoroensis. But the fact remains that there are at least two populations, if not races, breeding here in the same environment at different seasons.
During our visit to Islote Negro a Burrowing Owl (Speotyte comicularia) was flushed from the rocks and was seen flying around the island several times. Where the bird was originally sitting we found remains of several Oceanodroma. And as the survey continued many other Oceanodroma remains were found. The owl has done considerable damage to breeding storm petrels. One downy young was found partly eaten in the back end of a burrow. Did the owl go into that burrow and kill and eat the storm petrel there, leaving the remains? We found no indication of the owl having preyed on any of the other breeding birds.
Cassin's Auklet Ptychoramphus aleutica Estimated nesting pairs: 200 ±
This little alcid is a burrow-nester on Islote Negro. They reportedly dig their own burrows. On Islote Negro the burrows were seldom more than six inches deep, but were up to four feet in length. In some areas the burrows are found among burrows of Black-vented Shearwaters and Xantus Murrelets. Fresh eggs, pipped eggs, and one large immature with traces of down were found.
We were using number three bands on these birds. The bands fit well on some birds, but on others they were too large and slipped off the tarsus. Was this tarsus size variance due to sex, age, or were we handling two different races of birds? There is a second subspecies, P. a. australis van Rossem, described from the San Benito and San Roque islands.
Xantus Murrelet Endomychura hypoleuca Estimated nesting pairs: 150 ±
This attractive small alcid was nesting on Islote Negro. It nests in burrows and cavities in the rocks. Nests were found with one, two, and three eggs, but two seems to be the normal clutch size. There is considerable color variation in eggs from the same clutch. One clutch was found with one buffy egg and one olive-brown egg. There were also a few burrows that contained adults tending small downy young. Fifteen Xantus Murrelets were banded with Dr. Hubbs' bands on Islote Negro on 27 April.
On nights of 30 April, 2 and 3 May, Xantus Murrelets flew aboard the R/V Ellen B. Scripps. On these three nights 61 birds were banded, 25 of these with POBSP bands. Fifty-nine were banded aboard ship while anchored in Melponime Cove, next to Outer Island where the birds breed, and two were banded in Northeast Bay. A breakdown of these birds follows: