Page 4 – Prions, gadfly, storm and diving petrels
Appearance and feeding
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 four 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 hang from 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.
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 13 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. Those that nest in southern New Zealand 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.
Walking on water
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 the 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 Kermadecs in the north, to the subantarctic Auckland Islands.
Appearance and feeding
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.