Tuatara: Volume 21, Issues 1 and 2 (New Zealand Albatrosses and Petrels: an Identification Guide)
Black-capped Petrel (P. externa cervicalis)
Juan Fernandez Petrel (P. externa externa)
The two subspecies are easily distinguishable from each other at sea and are therefore described separately. For the Juan Fernandez Petrel (P. externa externa) see 54.