Seabirds – Stunning aerobatics

The Australasian gannet is spectacular in many ways – a stunning bird to look at in flight, with its blindingly white plumage and glowing orange head, also stunning when diving from height for prey, which this bird excels at. Australasian gannets breed in dense colonies, with most gannetries here in the north on islands and stacks. The exception is the mainland Muriwai colony, the best known in our region. This is spectacular as much as for its location on Auckland’s wild west coast as for the gannet activity, which you can observe at close range. They breed in several places in the Hauraki Gulf, with Mahuki Island, on Great Barrier Island’s west coast being the largest. Other places include the Poor Knights Islands, Mokohinau Islands, Horuhoru Rock (near Waiheke) and islands off the Coromandel. A word of warning: gannetries are best viewed from the upwind side, allowing the breeze to carry the strong fishy and guano smell downwind.

Gannets have remarkable eyesight, not only to detect prey under the sea’s surface from high up, but also to spy activity at a distance. I remember one time looking out from Burgess Island and seeing gannets circling higher and higher. It looked like they were tracing the shape of a cumulus cloud. When you see this happening, you know there’s something going on under the water.

Some work-ups will see other seabirds, dolphins and even Bryde’s whales feeding, with gannets diving to spike the sea in rapid fire. These are truly gob-smacking events to witness from a boat close by – the sound of birds hitting the water, their calls, the maelstrom of activity. And then, almost miraculously, the activity can stop and the birds settle, the gannets sitting on the water, or drifting away to look for other work-ups. Recent research by University of Auckland Leigh Marine Laboratory scientists has shown that dolphins can be attracted to work-ups by the sound through the water of gannet activity.

Gannets are designed to dive. Watch one circling, from shore or from a boat, its head facing down searching for prey. Then, with a twitch of a wing, it will roll into a rapid dive. Close to the water its wings will fold back, right back, not folded against the sides of its body. Small air sacs in the base of the gannet’s neck and breast inflate to absorb the shock of the impact. At the same time, an opaque membrane drops down over the eyes to prevent any damage. This membrane has led to the myth that gannets go blind from diving too often. Gannets are also capable of swimming underwater in pursuit of prey using their wings and feet to propel themselves. They can stay down for up to a minute, though most dives are much shorter. They feed on pilchards, anchovies, jack mackerel, saury and squid. They can also take prey in very shallow water. One time in an estuary I watched in awe as a gannet skimmed the surface to dive underwater, its bubble-trail like that of a torpedo. Remarkable creatures indeed.

These birds can travel big distances to feed both themselves and their chicks. A recent tracking study of birds breeding on Mahuki Island, off Great Barrier Island, found that birds headed in all directions – out across the Gulf, around the top of Great Barrier or out through the Colville Channel. One bird flew all the way across to Pakiri Beach, then followed the surf-line all the way to Mangawhai, before heading back to Mahuki.

Catching prey is serious business for a gannet, but sometimes you see something that makes you think these birds do things for the sheer hell of it. At Rakitu, I saw three birds flying in graceful formation over the highest point of the island, over 200 metres high.

Another time, I watched a gannet fly low over a school of tightly-packed trevally causing the fish to erupt and the disappear below the surface – gannets don’t target trevally.

by Chris Gaskin
chris@nzseabirds.com