Efforts to save the critically endangered cahow hit another milestone on Nonsuch Island this year as the population continues to slowly regrow.

Jeremy Madeiros, terrestrial conservation officer, said on the Nonsuch Expeditions Blog that for the first time since the species was believed to be wiped out, a cahow couple created their own burrow on Nonsuch Island.

While conservationists have successfully reintroduced the seabirds to Nonsuch Island, the species have until this year used man-made burrows.

Mr Madeiros said, as far as records show, it is the first time cahows dug a natural soil burrow on Nonsuch and successfully raised a chick in it since the 1620s.

The development was another sign of progress in a record-breaking season.

Mr Madeiros said: “In 1960, the entire population of cahow consisted of 17 to 18 breeding pairs that produced a combined total of seven to eight successfully fledged chicks annually. These nested on four tiny, isolated islets with a combined area of only 2.4 acres.

“In 2023, we reached a new record number of 164 breeding pairs with 76 successfully fledged chicks, nesting on six islands, including Nonsuch Island, totalling 22.5 acres.

“The two nesting colonies on Nonsuch, established in two projects involving the translocation of chicks from the original islets to artificial burrows on Nonsuch and hand-fed on squid and anchovies until they imprinted on their new site and fledged out to sea, are increasing the fastest, having risen from the original pair of returned translocated birds, that produced their first chick in 2009, to nearly 40 breeding pairs producing 19 successfully fledged chicks in 2023.”

Cahows, also known as Bermuda petrel, were largely wiped out by introduced predators and hunting by early English colonists, and by the 1620s the species was believed to be extinct.

However, the species was rediscovered in 1951 with a handful of the birds found nesting on four rarely visited rocky islets, sparking efforts to rebuild the population.

The seabirds spend the bulk of their life at sea, but return to Bermuda in the autumn for their courtship period and to nest, with couples laying a single egg.