Tristram's Storm-Petrel (Hydrobates tristrami) — Vorstman's Guide to the Birds of North America

Voice

At the colony, which like all storm-petrels it visits only in darkness, moaning and purring sounds come from the nesting burrows under shrubs and among the rocky rubble. At sea, where it spends most of the year, it is silent.

Listen: SongXC441709

Recording: Drew Lindow — CC BY-NC-SA 4.0 · Midway Island, United States Minor Outlying Islands, United States · xeno-canto

Identification

With its long, angular wings, this is the largest of all storm-petrels. The plumage is mostly dark brown, with a paler, greyish rump and a pale grey bar across the upperwing. The flight is stronger and broader-winged than that of the much smaller storm-petrels of the American west coast, gliding more between beats. Male and female look alike.

Confusion species

The most useful feature is size: no other storm-petrel in the Pacific is as large, and the pale wing bar and rump are far less pronounced or absent in smaller species. Bulwer's Petrel, which occurs in the same waters and is likewise mostly dark, can cause confusion, but it has a longer, wedge-shaped tail and a stiffer, more gliding flight with high arcs, rather than the fluttering, direct flight of a storm-petrel.

Habitat

It spends most of the year over open tropical and subtropical waters of the North Pacific, catching squid and small fish. For breeding it uses short burrows under shrub vegetation or rock crevices on low, uninhabited islands and atolls. At the main colonies the ground is sandy to rocky, with little tall vegetation.

Where the bird is most often found, arranged by the USNVC — the vegetation classification of the United States and Canada — and CMECS for sea and coast.

Range and movements

It breeds on Nihoa and on most atolls of the Northwestern Hawaiian Islands, and on a few islands south of Japan; on Midway Atoll, where it once bred, birds have been returning since introduced rats were removed in 1996, though breeding has not yet resumed there. It does not breed on the main, inhabited Hawaiian Islands. Outside the breeding season it disperses widely over the tropical and northern Pacific.

Breeding (summer)

A map of the archipelago itself: on the map of North America this bird would be a speck in the bottom left, and with island birds the question is exactly which islands it holds. The islands where it is absent are shown in grey. Measured island by island from the records held by GBIF: the records of the species divided by all bird records there, season by season, just as on the ordinary map. On Niihau and Kahoolawe so little birding is done that the measurement is silent and the standard references decide. Island outlines: Natural Earth (public domain).

Where and when to look

The Northwestern Hawaiian Islands are essentially inaccessible to visitors; look for this species instead on pelagic boat trips from the main Hawaiian Islands, especially during the winter months around the breeding season, and watch for its large size and pale wing bar.

Conservation

The IUCN lists the world population of Tristram's Storm-Petrel as Least Concern (LC). Introduced rats and cats have wiped out entire colonies in the past, as at Midway; on islands where these predators have been eradicated, the species is slowly recovering. Invasive plants that choke the ground under shrubs remain an ongoing problem, as they make burrows inaccessible.

Scientific name

Hydrobates tristrami | (Salvin, 1896)

Salvin described the species in 1896 as Oceanodroma tristrami, in a genus later merged into Hydrobates, hence the parentheses around author and year. The species name honours Henry Baker Tristram, a nineteenth-century English clergyman and naturalist. The type locality is Sendai Bay, Japan.