Bonin Petrel vs Cook's Petrel

In short

Bonin Petrel and Cook's Petrel are both small, dark-capped petrels with similar plumage, making them challenging to distinguish at sea. The most reliable field mark is the underwing pattern: Bonin Petrel has a more extensive dark trailing edge on the underwing, while Cook's Petrel shows a narrower dark edge. Observing these birds in their respective habitats can also aid in identification.

Identification tips

• Underwing pattern: Look for the extent of the dark trailing edge; Bonin Petrel has a broader dark edge.
• Geographic location: Consider the region; Bonin Petrel is more likely in the North Pacific, Cook's in the South Pacific.
• Overall appearance: Cook's Petrel may appear slightly paler and more slender.

Size

Both species are similar in size, but Cook's Petrel may appear slightly more slender.

Plumage

Bonin Petrel has a more extensive dark trailing edge on the underwing, while Cook's Petrel's underwing has a narrower dark edge. Both have a dark cap and white underparts, but Cook's Petrel often appears paler overall.

Behaviour

Both species exhibit typical petrel flight patterns, but there are no distinct behavioral differences noted between them.

Habitat

Bonin Petrel is primarily found in the North Pacific, particularly around the Hawaiian Islands, while Cook's Petrel is more associated with the South Pacific, especially around New Zealand.

Voice

Vocalizations are not a reliable field mark as both species have similar calls, typically heard at breeding colonies.