Grey-backed storm petrels are tiny grey-and-white seabirds found throughout the southern ocean in subantantarctic to temperate waters. They do not migrate, but range further away from the breeding islands after the season is over. Although one of the smallest seabirds known from the New Zealand region, they thrive in the often wild seas of the roaring forties, the furious fifties and even reaching the screaming sixties.
There is a long history of grey-backed storm petrels being attracted to lights in Fiordland and South Westland, including adults with bare brood-patches, and recently fledged young with down adhering. The provenance of these birds is unknown, but there has been speculation that there may be a breeding colony somewhere inaccessible to predators in a remote area of Fiordland.
Grey-backed storm petrels are dark-grey on the head, breast and neck, slightly paler grey on the back and ashy grey on the rump. The flight and tail feathers are black. The belly and under the wings are white excepting a black leading edge to the wing. They are compactly built, with short tails and legs, with the feet barely reaching past the tip of the tail. In flight they flutter more and use their legs less than other storm petrels, but at times do run off the surface of the water.
Voice: a monotonous wheezy, cricket-like chirp with two syllables is given by birds on the ground at the breeding colonies.
Similar species: In harsh light, grey-backed storm petrels can look very dark, but they never have the white rump that the black-and-white storm petrels do. White-faced storm petrels also have grey backs, but they are larger with longer legs and have a characteristic bounding flight.
Distribution and habitat
Grey-backed storm petrels breed on many islands around the southern ocean. In the New Zealand region there are breeding colonies on the Chatham, Antipodes, Campbell and Auckland Islands. They burrow under mats of vegetation and into the bases of tussocks to nest.
At sea, they forage over deep water near the shelf edge and beyond in subantarctic waters as far north as the subtropical convergence, dispersing further away from the breeding islands after the season has ended. Some birds regularly feed in southern New Zealand waters, and in winter they may occur anywhere around the coast, occasionally in substantial numbers.
The breeding population of grey-backed storm petrels in New Zealand probably runs into the tens of thousands of pairs, including 10,000 to 12,000 birds on the Chatham Islands, and tens of thousands of pairs on the Antipodes Islands. On Campbell and the Auckland Islands, grey-backed storm petrels are restricted to offshore islands by predators (since eradicated on Campbell Island). The Campbell Island population is probably small but the Auckland Island population may be substantial.
Threats and conservation
Grey-backed storm petrels are small and vulnerable on land. They are harvested at their breeding colonies by subantarctic skuas, and on the Antipodes Islands some breeding adults may be killed and eaten by Antipodes Island parakeets. Nests, eggs and chicks may be inadvertently damaged by larger petrels and penguins especially when seeking nest sites.
Grey-backed storm petrels have gone completely from islands with rats and cats but survive on the Antipodes Islands in the presence of mice. As the storm petrels are dependent on vegetation for nesting, fire and large herbivorous mammals could destroy breeding habitat rapidly.
Grey-backed storm petrels breed as pairs, and share parental duties. Nest sites are tunnels under dense low vegetation such as mat plants, tussocks and flax. A few nests with two eggs seen on the Chatham Islands were probably due to two females laying in the same nest, as a single egg is normal for all petrels. Incubation is poorly synchronised and the egg may be left untended for several days, but may still hatch after up to three days without incubation.
Behaviour and ecology
Grey-backed storm petrels are usually solitary but may be seen in small groups. They nest in colonies.
The diet of the grey-backed storm petrel consists mainly of barnacle larvae but with a variety of other small crustaceans and fish. They are often seen hovering over floating kelp at sea and this may be where they catch the barnacle larvae.
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Grey-backed storm petrel
- Social structure
- Breeding season
- Nest type
- Nest description
- Burrow into or under dense plants.
- Nest height (mean)
- 0 m
- Maximum number of successful broods
- Clutch size (mean)
- Mean egg dimensions (length)
- 31 mm
- Mean egg dimensions (width)
- 23 mm
- Egg colour
- Dull white with sparse reddish-brown speckles usually confned to the larger end.
- Egg laying dates
- Interval between eggs in a clutch
- Not applicable days
- Incubation behaviour
- Incubation length (mean)
- About 45 days
- Nestling type
- Nestling period (mean)
- About 50 days
- Age at fledging (mean)
- About 50 days
- Age at independence (mean)
- About 50 days
- Age at first breeding (typical)
- Maximum longevity
- Maximum dispersal
- 2,000 km