With around 10 breeding pairs doing their bit for the species' survival, Northland's fairy terns are again at the centre of a heart-in-mouth monitoring programme.

The first of the season's chicks hatched on December 5 in a scrape in the sand on Mangawhai sandspit.

So far this season 13 eggs have been laid at the world's most endangered shorebirds' only four breeding sites. Two of those eggs were taken to be incubated at Auckland Zoo's rare bird breeding programme.

The eggs are expected to hatch between now and New Year but it is not yet known if all are fertile as some are too recently laid for rangers to "candle" them; that is, use a light to check for infertility.

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There are fewer than 44 fairy terns known to exist and a lack of breeding-age females is currently limiting breeding.

"The situation is changing just about daily at the moment and with the king tides coming up [in the coming] week, numbers might be changing again," a Department of Conservation (DoC) spokeswoman said.

DoC is imploring people to stay away from nesting sites, and dogs are banned from Waipu and Mangawhai sandspits.

But time and tide could yet undo the best efforts to keep New Zealand's rarest shorebird, also called tara-iti, in existence.

In a quirk of nature, fairy terns do not lay their eggs in safe places but in shallow scuffs in the sand called scrapes.

Hard to spot by the human eye, these nests are at the mercy of predators, wind drift, weather and high tides, as well as beach traffic, dogs and other dangers introduced by people.

Sand bagging, trench digging and trap setting are among methods used to protect the birds from a range of killers, the encroaching sea only one of them.

There are only three other sites where the bird breeds and they are at Te Arai, Waipu and Papakanui, the latter a spit below Kaipara Harbour's South Head.

Thanks to a discreet nurturing hand from DoC and a host of volunteers who trap predators, mark off nest sites from public access and closely monitor the chicks' progress, there has been a small revival in numbers in the past 30 years.

In 1983 the number dropped to an all-time low of three or four breeding pairs. DoC (then the New Zealand Wildlife Service) initiated a protection programme resulting in a population turnaround, albeit a still precarious one.

Fairy terns were once widespread around the North Island coast and South Island river mouths.