Monotypic
Also known as Hornby's Storm-petrel. A synonym of the specific name is collaris.
 

Identification

A large and distinctive forked-tailed and gray-backed storm-petrel of the Humbold Current off western South America. Gray above with a dark cap, dark tail, dark primaries and conspicuous white carpal bar on the upperwing. Underparts white except for a solid grayish-black breastband. The dark cap includes the eye and ear coverts but is separated from the dark bill. Underparts white from the collar to the undertail coverts. Dark tail is forked and the feet do not project in direct flight. The pale underparts make them especially conspicuous in low light.

Normal flight comprises slow and deep wingbeats interspersed with erratic sailing glides, often with legs dangling. Observing birds flying in a stiff breeze, Chapman described the species as 'the most erratic flier I have ever seen....like a bat, swift and nighthawk all in one. They were skimming here, flitting there, then suddenly swung off with the wind a hundred yards or more so quickly one nearly lost sight of them.' Fairly greagrious at sea. On water can be confused with basic-plumaged phalaropes. Diet includes small fish.
 

Where and When

Breeding grounds poorly known, but thought to be the coastal deserts of Peru and northern Chile.

Most occur in the Humbolt Current from 35 deg. S off Chile to the equator off Ecuador where numerous from August to December. Most common off Chile and Peru during July to November.
 

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