Bermuda is seeing the largest number of nesting pairs of cahows for more than 400 years.

While the numbers of chicks are down this year owing to adverse nesting conditions affected by Hurricane Fay and Hurricane Gonzalo, the number of pairs, or bird couples, has risen to 112, from 108 in 2014. That is a 600 per cent increase from the time of the species’ rediscovery in the early 1960s, when there were just 17 or 18 pairs.

Although the chicks numbers are down, the 57 or 58 are being successfully reared, according to Jeremy Madeiros, senior terrestrial conservation officer at the Department of Conservation Services.

However, in giving warning, he said: “Although this sounds like a large increase, it is still a dangerously tiny number, as it is the entire population of the species on Earth.”

For a species to move from the “critically endangered” category to “threatened”, the number of pairs must be at 1,000.

Mr Madeiros explained the breeding season is now in its final phase and nearly five dozen chicks will be ready to fledge between late May and late June.

The conservation officer explained the cahow remains one of the rarest seabirds on the planet and that there are still multiple threats to its survival.

“One of the biggest of these threats is the impact of powerful hurricanes on the tiny nesting islands, some of which are mere rocks only a half-acre in size,” he wrote in the Nonsuch Island blog.

“These islands are so small and low that the huge waves and storm surge experienced in hurricanes can completely submerge these islands, tearing off huge sections of rock, causing massive erosion and damaging or destroying even the solidly built, artificial concrete nest burrows.”

Mr Madeiros explained that last October’s hurricanes, which made direct hits on Bermuda five days apart, caused widespread damage, with Gonzalo’s winds reaching 145mph as the eye tracked over Bermuda.

“Unfortunately, this hurricane was late enough in the year that some of the male cahows had already returned to claim and clean out their nest burrows,” Mr Madeiros said.

“Cahow pairs mate for life, returning every year to the same nest burrow, and the males consistently return seven to ten days before the female birds.

“After checking and repairing all active nest burrows after the hurricane, I was able to confirm that at least four to five males had disappeared during the hurricane, and had evidently been drowned and washed out of their burrows as huge waves submerged two of the nesting islands.

“The female cahows, returning and finding no males at the nests, then abandoned the sites, causing a net loss of at least four to five nesting pairs.”

Other threats are rats, which kill and eat the cahow chicks or eggs if they get on to an island, along with longtails, which can occasionally compete with cahows for nesting sites.

“The cahow chicks are defenceless against the more aggressive longtails, as their parents are usually out to sea during the day to gather food to feed their chicks.

“This problem has been largely managed for many years by fitting the entrances of nest burrows with wood ‘baffles’ with a precisely shaped entrance hole, which allow the cahows to enter and leave, but normally prevent the longtails from doing so because they have a different shape and slightly larger body.

“However, in early April, I unfortunately recorded the first death of a cahow chick by a longtail in many years, as an evidently smaller than normal Tropicbird managed to squeeze through a baffle and kill the chick.

“I immediately built and installed a smaller baffle, but this demonstrates the fact that if management of the cahow population was reduced, that the population would immediately plunge again, possibly to the point of extinction.”

Useful website: Nonsuchisland.com