Gulls suffer from marine changes

By: Blogger

For many people, red-billed gulls are very much New Zealand’s ‘fish and chip’ bird. While they can be aggressive scavengers, they have an intriguing range of foods. One of the biggest colonies these days is at Marsden Point, nesting within the refinery. In June, my wife and I were driving through to Whangarei and we saw thousands of them feeding on earthworms in the wet paddocks, either side of SH1, at Raukaka. I’ve even seen red-billed gulls feeding on taupata berries on Burgess Island, Mokohinau Islands, small groups moving from bush to bush across the slopes. Last year I was asked by Auckland Zoo’s keepers about what could be done with gulls that were harassing the macaws when they were let out of their cages for flights. Of course, the issue was that the gulls had become hooked on food left out for animals and leftovers in the open-air café, and they could have seen the macaws as competition. Another favoured technique is ‘paddling’ in sandy shallows on beaches or tidal pools of estuaries to disturb small crustaceans.

But red-billed gulls are, of course, a seabird and small fish are on the menu. Garfish, for example, or fish larvae. I see them in small flocks on the Whangateau Harbour when I’m out rowing or kayaking – birds on the surface feeding, or fluttering up and diving, usually where tide flows sweep or eddy around mangroves. Out in the Hauraki Gulf their main prey are krill, euphausiids (Nyctiphanes australis), which they mainly catch in association with fish schools. Gulls will gather (with terns, shearwaters and sometimes gannets) over shoaling fish (called work-ups, or boil-ups), paddling around the fringes of the tightly-packed fish, scoffing the krill or picking them up with dipping dives.

Many red-billed gull colonies will be close to where they can feed over shoaling fish, such as at Tiritiri Matangi, Tawharanui and Goat Island. They make their nests on rocky headlands and stacks along the mainland coast and on offshore islands. They defend their nests aggressively, driving away intruders with screaming dives.

Red-billed gulls are found only in New Zealand – the silver gulls in Australia and New Caledonia are a close relative – but despite their seeming abundance, they are in trouble. For those who think of them a nuisance, that might sound like a good thing. But these gulls, like seabirds generally, can tell us a lot about what is happening in our marine environment. Up until the mid-twentieth century there was a very large colony on Burgess Island – some 20,000 plus birds. This, and the colony at the Three Kings Islands, were the largest in New Zealand (and for an endemic species that means globally). Today, only 250 pairs breed at the Mokohinau Islands. That’s a massive change. A recent New Zealand-wide survey by the Ornithological Society of NZ found that red-billed gulls are in long-term decline, not just here in the north, but nationally. Does this mean there’s less food at sea for them? Are the schools of fish disappearing, schools that concentrate krill at the surface? A major study of red-billed gulls at Kaikoura showed that euphausiid abundance is linked to environmental conditions, being highest in years during La Niña events with the gulls’ breeding attuned to these events. Are we also seeing changes in our marine environment? So, like with our bird of the month in June, the white-fronted tern, we’ll be keeping a close eye on their foraging activity and the dynamics of fish schools.

Chris Gaskin chris@nzseabirds.com

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