Helen Bain: Terns take priority over new houses

Fairy terns may be the most critically endangered birds in New Zealand, and perhaps even the world's rarest terns, new DNA evidence suggests.

But developers are proposing to build up to 2000 homes near Mangawhai Heads on the Northland coast at what the Department of Conservation describes as "the single most important breeding site in the world" for these birds.

Now new genetic research confirming the birds are unique to New Zealand means the threat posed to their survival is even more critical.

Research at Auckland University on DNA from New Zealand fairy terns has identified a unique genetic trait which shows they are a distinct population with characteristics different from Australian and New Caledonian populations.

The New Zealand fairy tern (Sterna nereis davisae) is already considered to be a separate subspecies of the Australian fairy tern (Sterna nereis) because of physical and behavioural differences, including a distinctive area of enlarged black feathering in front of the eye.

The Auckland University report recommends that New Zealand fairy terns should at least be considered a distinct population and that further research should be conducted to determine their species status.

The DNA research confirms the fairy terns are our most critically endangered birds, with only 35 adult birds left - less than half the number of kakapo, of which there are 86.

The IUCN (World Conservation Union) has produced a "Red List" of bird species threatened with extinction. It ranks the Chinese crested tern, with a population of 50, as critically endangered and the most endangered tern in the world.

If future research reveals that the New Zealand fairy tern is a separate species, it would overtake the Chinese crested tern for the dubious honour of being the most threatened species of tern in the world.

Either way, having thousands of new residents in the vicinity visiting the beach at Mangawhai would directly impact on the birds through disturbance or direct damage to their well-camouflaged eggs and nests.

Even brief disturbance of nesting parents leaves the eggs or chicks vulnerable to predation or over-heating by the sun.

The DoC report into the risks posed by the proposed development states that even the loss of one more tern as a result of human interference would be too much.

Given that New Zealand fairy terns breed only at four locations - Mangawhai Wildlife Refuge, Waipu Wildlife Refuge, Papakanui and Pakiri Beach - the proposed subdivision on the doorstep of their breeding grounds would pose a serious threat to their fragile existence.

And with only 10 breeding pairs left, the total population remains perilously close to extinction, even after a relatively successful breeding season during the summer, in which seven chicks were raised.

The last time such a healthy number of chicks were raised successfully was in 2002 when eight chicks survived to fledging. In 2004, only three chicks survived, and in 1984 numbers fell to just three pairs.

The terns nest between October and February and usually lay one or two eggs, which both parents take turns to incubate for between 22 and 24 days. The parents vigorously defend nests and chicks against human and bird intruders by dive-bombing and defecating on them - but even this interesting defence mechanism gives no protection against introduced predators.

Predator control is one factor behind this season's breeding success. Eight cats, seven weasels and five stoats have been trapped at Mangawhai Spit since October, and since the last feral cat was caught in December, no cat prints were seen for the rest of the season.

In 2004-2005, several chicks and a breeding adult fairy tern were eaten by a large ginger cat that evaded capture for the breeding season. As a result no chicks survived from the Mangawhai Wildlife Refuge that year.

The developers promise to ban cats and restrict dogs from the proposed subdivision, but these pledges are not enough to safeguard the terns from predation by these animals.

With up to 2000 houses planned, a ban on cat ownership and a requirement that dogs be kept on leashes, while well-intentioned, would be impossible to police.

Inevitably the regulations would be breached, either by the thousands of new residents or the increased number of visitors.

Human populations also attract feral cats, which are drawn to food sources and the presence of rats which are encouraged by human settlement.

Given the critically low number of fairy terns, just one extra cat or dog in the area would be enough to tip the balance and condemn these unique New Zealand birds to extinction.

* Helen Bain is Forest & Bird's Media Officer.