Saturday, January 1, 2011

Black-fronted terns in crisis

Black-fronted Terns Tarapiroe - Agile birds of the river under threat.

Peter Langlands

For me one of the defining moments of spending time on the river is watching the wildlife. In the South Island black-fronted terns are one of the most conspicuous of riverbed birds. They are constantly in flight, and like giant swallows capture all of their food on the wing. Often it is the tern’s call, which will alert me prior to sighting the birds. Often the terns will be seen gliding effortlessly up the river and will invariably fly into the wind when feeding. For me as an angler the experience of watching the terns is a welcome sight when tramping long distances up the riverbed. Sighting the terns makes the trip go quicker, and the distance between fish shorter. Without them the river would seem an empty place.

Yet as I write the black-fronted tern is classified as a critically endangered species. While they still appear to be very conspicuous, the species is in decline in many areas. A few terns can seem like a lot of birds as they are constantly flying up and down the river’s course. Yet often the some bird will be sighted throughout the day, giving an inflated impression of the numbers of terns. Also the terns we are sighting on the river may also be an aging population. Black fronted terns may, if we look at closely related species, live for at least twenty years.

The reason why terns are so endangered is that they are highly vulnerable to a multitude of factors during their breeding phase. They are ground nesting birds. Their nests consist of inconspicuous scapes in the stones and are easily run over by four wheel drives, stepped on accidentally by anglers, or dogs. Yet of greater concern is the predation of tern nests by stoats, and other introduced mammals, such as hedgehogs, which appear not be as innocent as thought. Wild cats can also wreck havoc on the terns breeding colonies. As terns require areas of open gravel to nest on, vegetation encroachment is also a major factor and greatly reduces the available space for nesting birds. Increasing numbers of black-backed gulls on the rivers also reduce nesting space for the terns. If these factors weren’t enough the terns also have to contend with natural floods, which will often wipe out an entire colony. The eggs and chicks are highly vulnerable and the chicks are also often run over accidentally.

Often you know you are in a tern colony when the birds swoop and squawk relentlessly, and will even when highly agitated lightly hit you on the head. At such times whether walking, or in a four-wheel drive, the best thing to do is to immediately vacate the area. The terns are sensitive to disturbance, and if the colony is in the early stages the birds will readily dessert the colony, after only a low amount of disturbance. Given the fact that the terns are colonial nesters on the riverbed they also literally “put all their eggs in one basket”.

Yet it is in our broader interest that anglers and trampers show concern for the terns, along with other endangered riverbed birds, such as the wrybill and black-billed gull, for these birds act as important icons when advocating water conservation. Terns are a marker of the health of the river. Surely without their presence on our open braided rivers, they would seem like very empty spaces. In the winter the terns vacate the rivers of the South Island to feed on coastal waters on the East Coast, with the Kaikoura region being an important feeding ground. Many birds also feed on the estuaries around the lower North Island.

Black fronted terns are only found in New Zealand (primarily a South Island bird during the breeding season). An estimate published in 1996 is of a population of only 5000 birds. They are critically endangered and as outdoor enthusiasts we must minimise our presence on their breeding colonies when fishing or four wheel driving, and have a responsibility to alert DOC of any riverbed activities that we see that may be putting the birds at threat. Hopefully things will not “tern out” for the worse, for this bird, which characterises the wilderness spaces, open spaces and prolific mayfly populations on our East Coast riverbeds. The black fronted tern is a treasure that is uniquely New Zealand’s.

1 comment:

  1. Out of curiousity, how does the "critically endangered" status get determined? Is it linked to decline or functioning community? Or to vulnerability of the environment?

    Because you're listing these as "Critical" when there are 5000, yet in another article blue ducks are considered "vulnerable" with an estimated population of 3000 and over in Africa the Western Lowland gorilla numbers 150,000-200,000 yet is still considered "critical".

    Just curious!