カオジロウミツバメ 対 ハジロシロハラミズナギドリ

The most reliable field mark to separate White-faced Storm-Petrel from Cook's Petrel is size and flight style: White-faced Storm-Petrel is much smaller and shows a unique, buoyant pattering flight, often skipping over the water with dangling legs, while Cook's Petrel is larger, with a more typical petrel gliding and shearing flight. In plumage, White-faced Storm-Petrel has a striking white face with a dark eye mask and pale underparts, versus the more typical gadfly petrel pattern of Cook's Petrel, which has a pale grey upperwing with a dark 'M' and white underparts. Juvenile White-faced Storm-Petrels may show more obvious pale fringes above, but both age classes are much paler-faced than Cook's. Bill structure and overall jizz are also distinct. In any direct comparison, size and flight jizz are the fastest field clues.

1. Size is a primary clue: White-faced Storm-Petrel is much smaller than Cook's Petrel—about a fifth of the mass and with a much shorter wingspan. If you see both together, the difference is striking. This applies to all ages and sexes.
2. Flight style is diagnostic: White-faced Storm-Petrel bounces and patters over the water, wings out, legs dangling or skipping; Cook's Petrel glides and shears, rarely if ever pattering. Watch for this especially in windy conditions, when the storm-petrel's pendulum-like movement is obvious.
3. Face pattern: White-faced Storm-Petrel has a bright white face with a dark eye mask, very distinctive even at a distance. Cook's Petrel has a plain pale grey face, lacking any mask or white blaze.
4. Upperwing pattern: Cook's Petrel shows a dark 'M' across pale grey upperwings, a classic gadfly petrel mark. White-faced Storm-Petrel has pale brown to grey upperwings with black flight feathers but no 'M' pattern. This is visible in all ages.
5. Underparts: Both are white below, but the storm-petrel's white extends up to the face, while Cook's Petrel's white is limited to the underbody.
6. Bill structure: Cook's Petrel has a long, black bill with prominent tubular nostrils (as in all Procellariiformes), while White-faced Storm-Petrel's bill is shorter and less robust. This is best seen at close range.
7. Legs and feet: White-faced Storm-Petrel has long legs, often visible trailing behind in flight or when pattering; Cook's Petrel's legs are much shorter and rarely seen in flight.
8. Tail shape: Some White-faced Storm-Petrel subspecies (notably P. m. maoriana) have a forked tail, but this is subtle. Juveniles may show a shallower fork. Cook's Petrel has a more typical petrel tail, not deeply forked.
9. Juvenile plumage: Juvenile White-faced Storm-Petrels have broader pale fringes above and may look greyer or browner, but still show the white face and mask. Cook's Petrel juveniles do not mimic this pattern.
10. Context: In New Zealand, both can be seen offshore, but Cook's Petrel is strictly endemic, while White-faced Storm-Petrel is more widespread. If seen outside New Zealand, it's almost certainly not Cook's Petrel.
11. Sex and seasonal variation: Neither species shows strong sexual or seasonal plumage differences, so these marks are reliable year-round.
12. Plumage pitfalls: Do not confuse the pale underparts of both species—focus on face pattern and flight style. The storm-petrel's white face is never matched by Cook's Petrel.
13. Behavior at sea: If the bird is frequently touching or skipping off the water, it's a storm-petrel. If it's gliding and rarely contacts the surface, it's a Cook's Petrel.
14. Bill color: Both have dark bills, so this is not a useful mark.
15. Vocalizations: Not diagnostic at sea; both are generally silent away from breeding colonies.

White-faced Storm-Petrel is considerably smaller and lighter (body mass around 40 g, length 19–21 cm, wingspan 41–44 cm) than Cook's Petrel (length 25–30 cm, wingspan 65–66 cm, weight about 200 g) [wikidata][wikipedia]. This difference is obvious in the field: Cook's Petrel is nearly twice the mass and has a much longer wingspan. Even at a distance, the storm-petrel's daintiness and short body are apparent next to the more robust, long-winged Cook's Petrel.

Juvenile and adult White-faced Storm-Petrels are similar, but juveniles show fresher plumage with broader pale fringes and tips to upperparts feathers, and may appear greyer or browner depending on moult [wikipedia]. The tail fork is shallower in juveniles, especially in the New Zealand subspecies. Both ages have a distinctive white face with a dark eye mask, pale brown to grey back and wings, and white underparts. In contrast, Cook's Petrel (all ages) shows the classic gadfly petrel look: pale grey upperparts with a bold dark 'M' across the upperwings, white underparts, and a long, black bill with tubular nostrils [wikipedia]. The face is not white, and there is no dark mask. There are no age-specific upperwing patterns in Cook's Petrel that overlap with the storm-petrel's appearance.

White-faced Storm-Petrel is unique among petrels for its almost constant pattering flight: it skips or bounces over the water, wings outstretched, using its long legs to touch or push off the surface [wikipedia]. This creates a buoyant, erratic, almost phalarope-like jizz. Cook's Petrel, by contrast, flies with typical gadfly petrel grace—long, arcing glides, shearing low over the waves, rarely if ever pattering. The storm-petrel's flight is much more fluttery and active, while Cook's Petrel is more direct and powerful.

Both species are pelagic, but Cook's Petrel is endemic to New Zealand waters, while White-faced Storm-Petrel has a much wider distribution, including southern Australia, New Zealand, and several Atlantic islands [wikipedia]. In New Zealand, both can occur offshore, but Cook's Petrel is more likely to be seen further from land. Habitat is not usually diagnostic at sea, but the storm-petrel's habit of feeding close to the water surface and its unique foraging style can help.