Wednesday, April 13, 2011

Banding Black-billed gulls

Gull on the nest

New Zealand has three species of gulls – the Red-billed gull which is strictly coastal and two species which are equally common inland or on the coast. These are the larger Black-backed gull and the Black-billed gull. Because they are very different in habits, I will treat each as a separate blog beginning with the Black-billed gull.

Gulls at Queenstown

These birds are insect eaters, useful for controlling porina and grassgrubs, and they are the birds commonly seen following a plough to get the worms. For thousands of years they had few predators except falcons, harriers and wekas. Now stoats, ferrets, cats, dogs, off-road vehicles and illegal shooting are threats to the survival of the species which is classified as ‘endangered.’ Gull banding is an interesting experience. The chicks crèche, which is an uncommon habit, shared with some penguin species but few other birds. The banding team makes a corral out of chicken wire and herds the chicks in. Sounds easy? It’s even easier than it sounds. Because the gulls synchronise their laying, all the chicks are the same age. The captives are boxed up and then removed singly. A metal band and several coloured bands are fastened around the legs. The band combination – perhaps red/green/metal left and green/white right – is the cohort combination unique to that colony. In a banding session on the Aparima River last year we took mouth swabs as well, part of a study to test for the remote possibility of bird flu. Results were negative fortunately.

Chick banding on the Waiau

Each year there are perhaps 20 Black-billed gull colonies in Southland rivers, mostly the Oreti, Upper Mataura and Waiau. For several years colonies were photographed from the air and the number of gulls counted that way. Thousands were banded as well but there have been few returns. We noticed a steady creep of colonies upriver, possibly because the encroachment of broom and other weeds made nesting risky in the lower reaches. Perhaps gravel extraction had channelled the river more that normal and removed the gravel islands that the gulls favour.

On one of the flights we photographed a colony in the Aparima River. A few days later we visited it and it had been abandoned, nests, eggs and young chicks. Perhaps they had a fright from a predator. We then did a detailed count of the nests in the colony and matched this with the gulls in the photo. From this we derived our formula for assessing the number of nests at about 1 nest per 1.5 white dots in the photo. The gulls become unsettled easily and abandon a nestsite in its early stages. They then settle on another location and begin again. They have from about September to January to nest and some pairs may nest more than once in this time.

When handled, the chicks tend to leak from both ends. In one colony, they may all regurgitate aquatic insects or whitebait at another. One lot all regurgitated worms, the produce of their parents’ following the plough in a nearby paddock.

Following a plough

Any reports of banded birds, unusual species or odd behaviour are welcomed. Freshly dead birds are useful for museum specimens. Observers can assist by reporting the band colours of birds they spot, and if the bird is found dead, the metal band should be removed and sent to the address on it. It usually says Send National Museum NZ. I heard a tale of an American who returned a band from a crow. It had a number and Wash. Biol. Surv. on it, short for Washington Biological Survey. He said. “I shot one of your crows. I followed the cooking instructions on the band and I want to tell you that it tasted awful!”