The relationship of the Rapa shearwater, P. myrtae in regard to these species are also in need of research; this taxon was until recently placed in P. assimilis (Austin et al., 2004), but now raised to full species status by the IOC and the NACC (North American Classification Committee) of the AOU.

These cats usually prey on shearwaters immediately after landing, where a lack of understory renders them exposed.

Despite this, some breeding habitat remains to the north and north-west of Socorro's summit, where introduced animals have yet to ravage.

Further threat is posed by light pollution from a recently constructed airstrip and possible volcanic eruptions, the latter eradicated the San Benedicto population.

[1] The species is named for Charles Haskins Townsend (1859–1944), an eminent American ornithologist and ichthyologist.