The petrels include prions, shearwaters, storm petrels, diving petrels, albatrosses and several other groups, and they comprise the seabird order Procellariiformes. They range in size from the tiny storm petrels weighing 20–80 grams, to the largest albatrosses that weigh 7–11 kilograms.
Collectively the petrels are known as the tube noses, distinguished from all other birds by the tubular nostrils on the top or sides of their bill. The nostrils drain excess salt, which has been removed from the bloodstream by glands above the eyes. These glands act as auxiliary kidneys, excreting the salt that accumulates when the birds take in seawater. The prominent nostrils also reflect an acute sense of smell, which is used by some species to find food, and to help locate their own burrow among the many that make up petrel colonies.
New Zealand has a rich diversity of petrels. Excluding albatrosses, 37 of the world’s 114 petrel species breed in the New Zealand region. Fourteen of these breed only in the New Zealand region, although they travel beyond the country’s exclusive economic zone (of 200 nautical miles) to feed. There are other species that breed elsewhere but visit New Zealand waters.
As true seabirds, petrels feed at sea. Apart from coming on land to breed, their life is spent either in the air or on the water.
In the marine environment food tends to be in dispersed patches and can be far from breeding sites. Most seabirds have to fly large distances to find food, so the design of their wings is very important. Gliding flight uses little energy, and most petrels have long slender wings that are adapted for gliding. Only the small diving petrels, with their short stubby wings, fly by flapping their wings at all times. The tiny storm petrels combine fluttering and gliding, and most of the larger species fly by gliding when conditions permit. Moderate to very strong winds are used to power their gliding flight.
It is not surprising that a large number live in the windiest seas of the world, the Southern Ocean, at the edge of which sits New Zealand and its offshore islands. However, New Zealand’s most northern petrels breed in the subtropical Kermadec Islands, and there are petrels in all oceanic regions of the world, including the Arctic and the tropics. Many are migratory, breeding in one part of the world, then flying to places nearly halfway round the globe, where they stay until they breed again.
Petrels were important food for Māori. In the past, various species bred in most parts of New Zealand, and the chicks of many were harvested (as muttonbirds, or tītī) before they fledged. Muttonbirds provided much-needed fat and protein just as winter set in. Today only two species may be harvested by Māori with traditional rights: sooty shearwaters (Puffinus griseus) from islands around Stewart Island, and grey-faced petrels (Pterodroma macroptera gouldi) from islands off the north-eastern coast of the North Island.
The huge numbers of petrels that used to breed on the New Zealand mainland were important in transferring nutrients from the sea to the land. Their nitrogen- and phosphate-rich guano (excrement), and the remains of eggs, chicks and adults that died ashore nourished the forest ecosystems in which they lived.
In the past, at least 20 species of petrels bred on the North and South islands, but in the early 2000s only five species remain. The others have been hunted to extinction on the mainland by introduced mammalian predators.
The Westland petrel (Procellaria westlandica) is large and fierce enough to resist most predators, but even these birds are susceptible to dogs and cats. Hutton’s shearwaters (Puffinus huttoni) bred at a range of altitudes. Now, they remain high in the Kaikōura mountains where rats are scarce. However, this refuge has been invaded by stoats, and the birds’ future is uncertain.
There are a few remaining mainland colonies of sooty shearwaters and grey-faced petrels, but the birds are under threat from stoats, ferrets, rats and cats, and their numbers continue to decline. The last known mainland-breeding fairy prions (Pachyptila turtur) are restricted to a ledge part-way down a sheer cliff near Dunedin. Three further species have been translocated or attracted to mainland sites fenced to exclude predators.
Many petrel species also face dangers at sea, where large numbers are accidentally caught by fishing vessels.
The life cycle of all Procellariiformes is similar. They lay only one egg each breeding season, and the egg is unusually large. The tiny storm petrel produces an egg that is up to 29% of the female’s body weight, the highest ratio of any bird species.
Between copulation and egg-laying, most petrels spend from a few days to a month at sea. The female needs to obtain the nutrients required to produce such a demanding egg, and the male needs to put on weight in preparation for the many days he will spend incubating the egg while the female recovers. She will then return to relieve her mate. The species with the longest pre-laying exodus is New Zealand’s grey-faced petrel, which spends 50–80 days at sea between courtship and egg-laying.
Most of the smaller petrels breed in burrows, but a few nest in crevices. Cape petrels (Daption capense) nest on cliff ledges and some giant petrels and albatrosses breed in the open.
The burrow-breeding petrels come ashore only after dark and depart for sea before sunrise. They dig their own burrows or will occupy existing ones. The burrows are renovated or extended from year to year, and some sooty shearwater and Chatham Island tāiko (Pterodroma magentae) burrows are over 2 metres long. If both birds survive, pair bonds are usually retained from one year to the next, and pairs most often occupy the same burrow in successive years.
Procellariiformes have long incubation periods: 40 or more days for the tiny storm petrels, 51–53 days for the medium-sized shearwaters and 69–79 days for the albatrosses. Both parents take turns incubating. Those species that feed furthest from land, such as the mottled petrel (Pterodroma inexpectata), have incubation spells that may last 10–14 days, whereas the diving petrels (Pelecanoides spp.) that feed inshore have shorter incubation spells and both adults tend to visit the nest each night.
Chick rearing is an equally drawn-out affair. Their unusual growth pattern and feeding regimens highlight the challenges petrels face in exploiting scattered food sources far from their breeding colonies. The nesting period is 55 days for the small diving petrels, almost 100 days for the sooty shearwater and 278 days for the giant wandering albatross (Diomedea exulans).
Once hatched, the chick is guarded by one of its parents. Some burrow-nesting species watch over the chick for a few days, while surface-nesting birds such as albatrosses and the Cape petrel protect the chick for several weeks. After that, the parents return only to feed the chick.
In all species, both parents feed the chick. The hatchling is fed once every few days in most species, and the interval between meals gets longer as the chick gets older. Species that forage near the colony may feed their chick at regular intervals throughout development, but for species that feed far offshore it can be a week or more between visits. To compensate, each meal can be large. There are records of sooty shearwater and mottled petrel chicks doubling in weight overnight. Such gargantuan meals are no doubt the result of both parents returning on the same night.
This feast and famine lifestyle lasts throughout the birds’ lives. Food for most species is widely dispersed, and birds often have to travel hundreds of kilometres between feeds. Petrels (except diving petrels) have a unique ability to convert their bulky, heavy food into lightweight, energy-rich oil, which is produced in the proventricular gland – a part of their stomach. The oil is a far more concentrated energy source than body fat, which is the way most animals store energy. This allows petrels to feed far from their colony even while provisioning their chicks.
Chicks grow quickly, and they eventually weigh from 15% to 50% more than their parents, depending on the species. In many species the parents stop feeding their chicks some time before fledging, and the accumulated fat is used to complete feather development and growth. By the time chicks fledge, their weight is similar to that of their parents. With some migratory species, the parents depart before the chick has completed its development. Once the chick goes to sea it must learn to feed by itself, and find its own way to non-breeding grounds that may be more than 10,000 kilometres away.
The life of a petrel is complex, and it takes years to develop the foraging skills necessary to breed successfully. Shearwaters don’t breed until they are about seven years old, and albatrosses start breeding between 8 and 12 years of age, depending on the species. Diving petrels are the youngest breeders. Most breed at two years old, and a few when only a year old. Petrels live longer than most birds, with even the small species living several decades.
Shearwaters are long-winged birds that typically fly in broad sweeping arcs, slicing through the air just above the sea surface. They have long, slender, hook-tipped bills, which they use to seize fish, crustaceans and squid. They mostly feed by plunging into the water from less than a metre above the surface. Once underwater they use their half-extended wings to pursue fish and squid. The sooty shearwater regularly dives to depths of 60 metres.
Shearwaters are some of the most abundant seabirds. Many millions of sooty shearwaters breed on islands in southern New Zealand, although numbers are declining. In southern New Zealand, Kaikōura and the outer Hauraki Gulf, huge flocks of shearwaters can often be seen feeding in tight groups close inshore. Ten species breed in New Zealand.
Buller's shearwater (Puffinus bulleri), which breeds only in New Zealand, has characteristics unusual in a shearwater. For their size they are light in weight, with soft plumage. Instead of diving, they feed while sitting on the water’s surface, reaching down with their long necks. This species breeds only on the Poor Knights Islands but may be seen around much of New Zealand. The birds migrate to the North Pacific.
Petrels in the genus Procellaria are heavy-set birds that feed at the surface or make shallow dives. They also scavenge waste from fishing vessels. This extra food has benefited the Westland petrel (Procellaria westlandica) whose population appears to have increased. However, fishing boats pose a threat to grey petrels (Procellaria cinerea) and other species, which get caught by tuna long-line vessels.
Two species are endemic to New Zealand and now have very restricted breeding ranges. The black petrel (Procellaria parkinsoni) breeds only on Great Barrier and Little Barrier islands, and the Westland petrel only near Punakaiki, although in the past the black petrel also bred in other parts of New Zealand. The other species breed on subantarctic islands in the New Zealand region and elsewhere.
The Westland petrel and white-chinned petrel (Procellaria aequinoctialis) are the largest burrow-nesting species. Both weigh over a kilogram and their burrows are large, dank and cavernous.
In 2001, Campbell Island (700 kilometres south of the South Island) became the largest island in the world to have rats eradicated. Arriving on sealing vessels in the 19th century, Norway rats multiplied and ate their way through colonies of burrowing and crevice-nesting petrels. Now the rats have gone, remnant bird populations are likely to get bigger.
The fulmars and their kin are the least specialised of the petrels. They have broad diets and use a wider range of feeding methods than other petrels, but are restricted to surface or near-surface prey. All are enthusiastic scavengers. Giant petrels (Macronectes spp.) even come ashore to feed on dead whales or seals, offal or carrion close to the water’s edge, and they sometimes prey on smaller seabirds. Cape petrels mostly eat small plankton.
Fulmarine petrels typically live at high latitudes, but two species, the northern giant petrel (Macronectes halli) and the Cape petrel, are commonly seen around much of New Zealand. Both of these are more numerous in the south than the north, and both breed on the Chathams and some of New Zealand’s subantarctic islands. Giant petrels breed on open ground, while the other fulmars breed in crevices or on cliff ledges.
Prions are small birds (125–200 grams) with blue-grey colouring. Their plumage has similar reflective properties to the sea, so they seem to appear and disappear as they bank just above the water. At sea, prions are usually seen in flocks. Fairy prions can often be seen from the Wellington–Picton ferries.
They mostly eat planktonic crustaceans, which they take at, or very close to, the water’s surface. Two of the eight New Zealand breeding species, the fairy prion (Pachyptila turtur) and broad-billed prion (Pachyptila vittata), are common around New Zealand, and they are at opposite ends of the prion spectrum.
This is the smallest prion and has a narrow pincer-like bill. Fairy prions eat krill and other small crustaceans, which they either peck while sitting on the water or take while in flight, making only momentary contact with the water. Fairy prions breed, often in very large numbers, on many islands around New Zealand.
This is the largest prion and, as its name suggests, it has a broad bill with lamellae (comb-like fringes), which line the sides of the upper beak. These sieve planktonic copepods from the water’s surface.
Broad-billed prions breed on islands off southern New Zealand and on the Chatham Islands, where the birds are abundant. A severe storm in July 2011 killed over a quarter of a million broad-billed prions, which washed up on West Coast beaches in their thousands.
Gadfly petrels (Pterodroma spp.) are small- to medium-sized, lightly built birds with soft plumage. A quarter of all Procellariiformes belong to this genus, and 11 species breed on islands in the New Zealand region. This is the only group of petrels that has more species in tropical and subtropical seas than in cooler waters.
These agile, fast-flying petrels usually feed many kilometres offshore, well beyond the continental shelf. They are solitary at sea, but when searching for food they swoop high above the water in great arcs, which may allow them to see feeding birds that are kilometres away. Gadfly petrels mostly eat small squid, much of it snatched from the surface while the bird is in flight.
Mottled petrels (Pterodroma inexpectata) are one of the gadfly genus. They nest on islands off southern New Zealand but may feed in the Antarctic pack ice, over 2,200 kilometres south of their colonies.
Gadfly petrels approach their breeding colonies only after dark and leave for sea well before dawn, so they are seldom seen by land-based observers or inshore boaties.
These are the smallest and most delicate of the petrels. They are sufficiently distinctive to be placed in their own family – Hydrobatidae. They use their large feet, long legs and relatively broad rounded wings, along with upthrust from the waves, to ‘walk’ or bounce across the water’s surface, pecking at individual planktonic crustaceans.
Storm petrels seldom settle on the water when feeding. This seems a challenging way to feed, yet storm petrels, prions and gadfly petrels all take much of their food in this way. Circumstantial evidence suggests that storm petrels obtain a large proportion of their food at night. They are usually solitary at sea but may congregate along oceanic fronts or eddies where plankton is concentrated.
When storm petrels feed, they fly near the water and tap across the surface on their webbed feet. This may be where the name ‘petrel’ came from, as they appear to walk on water like the biblical St Peter.
The most common New Zealand species is the white-faced storm petrel (Pelagodroma marina), which breeds on many islands, from the Three Kings Islands in the north to the subantarctic Auckland Islands.
The New Zealand storm petrel (Fregetta maoriana) was thought to be extinct until it was rediscovered in 2003. In 2013, it was found to be breeding on Little Barrier Island.
Diving petrels are quite unlike other petrels. They are small, dumpy birds with relatively short wings, and they fly with a rapid, whirring wing beat. They can ‘fly’ through the sea’s surface and continue beneath the water using a scissoring action of their partially open wings. They generally feed on small crustaceans and fish.
Diving petrels breed on small islands scattered along the length of New Zealand and are sometimes seen from boats quite close to land. They breed at a younger age than other petrels, and as they feed inshore close to their breeding colony, their incubation spells are short and their chicks are fed most nights.
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