Guadalupe storm petrel

It bred only on Guadalupe Island off Baja California, Mexico, and presumably ranged throughout the region. The breeding season was set between the two other breeding storm petrel species of Guadalupe, the winter-breeding Ainley's and the summer-breeding Townsend's, in accordance with Gause's law.

The single egg, white with a faint ring of reddish-brown and lavender speckles around the blunt end, was laid in burrows maybe 15 in (35–40 cm) long, below the Guadalupe pine (Pinus radiata var. binata )-island oak (Quercus tomentella ) cloud forest on top of Mount Augusta. By mid-June, almost all young had already left the burrows. Though little data is available on Hydrobates breeding, incubation was presumably 42 days or so in this species, just as in similar-sized relatives. Time to fledging must have taken between 60 and 75(−85?) days, most likely around 65 days. This would mean that egg-laying took place from early February to March, and that in April–May, unfledged young were present in most active burrows. Just as in their relatives, the egg was incubated a few days by either parent, after which the other took over, the relieved bird taking to the sea to feed itself for the next incubation stint. The young were fed only at night, also like in other storm petrels.

Its call was described by Walter E. Bryant as sounding something like "here's a letter, here's a letter", with repeated interjections of "For you, for you".

Three species of lice were found to parasitize the Guadalupe storm petrel: the menoponids Longimenopon dominicanum and Austromenopon oceanodromae, and the ischnoceran Halipeurus raphanus. The second also occurs on some other storm petrels, and the third was also found on the ashy storm petrel. L. dominicanum, though, has to date not been found on other birds and seems to be a case of coextinction.