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.
Petrels in New Zealand
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.
Flying and feeding
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.
Food for Māori
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.
Threats to mainland colonies
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.