In the hand, the Guadalupe storm petrel could be distinguished by slightly larger size and the paler underwing coverts.

[3][4] 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)[note 1] cloud forest on top of Mount Augusta.

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.

[7] Three species of lice were found to parasitize the Guadalupe storm petrel: the menoponids Longimenopon dominicanum and Austromenopon oceanodromae, and the ischnoceran Halipeurus raphanus.

By the end of the 1906 breeding season, it was still considered "abundant",[6] though the "large numbers" of birds present there and then must have been nearly the entire population of this species.

Still, it was noted that: the mortality among these birds from the depredations of the cats that overrun the island is appalling – wings and feathers lie scattered in every direction around the burrows along the top of the pine ridge.

In the words of the expedition's primary researcher, Exequiel Ezcurra of the San Diego Natural History Museum, We searched thoroughly for the Guadalupe storm petrel, and failed to find it.

In any case, the precautionary principle would probably require a few years of follow-up surveys, possible now that restoration of Guadalupe's ecosystem is underway.