Cahows bounce back as Bermudans build burrows
Breeding numbers of Bermuda Petrel Pterodroma cahow have recovered from the impact of Hurricane Fabian in 2003. But the four islets on which the tiny world population depends were severely damaged, and traditional nesting burrows are increasingly vulnerable to future storms.
Now an ambitious project to persuade the birds to move to safer alternative sites is showing early signs of success.
Believed extinct for almost 300 years, numbers of Bermuda Petrels (known locally as Cahow) have been slowly recovering since the species rediscovery in 1951, when just 18 pairs were known. But the total population, including non-breeding birds, stands at barely 250 birds.
Fabian struck while the Cahows were at sea, but many birds returned to find their burrows destroyed. In 2004 Bermuda’s Department of Conservation, with the help of the New South Wales Department of Parks and Wildlife, trialled a translocation programme, moving 14 chicks to a complex of artificial burrows on nearby Nonsuch Island. This island is larger and higher, and offers much greater potential for growth of the Cahow population. It is also managed to exclude rats and other predators and has been the site of a project to restore endemic forest cover. The island now closely resembles the original nesting habitat as described by the island’s early settlers.
"We decided to go ahead with the translocation because of the probability of further damage to the islets from future hurricanes, as most forecasts now predict increases in both frequency and intensity of these storms." —Jeremy Madeiros, Bermudan Department of Conservation Services
The translocation programme uses techniques developed to establish new populations of Pterodroma petrels in Australia and New Zealand, taking advantage of the tendency of petrel chicks to return upon maturity to the point they fledged from. Fed every other day on fresh squid and anchovies, the birds all fledged and departed for the open sea, where they are believed to follow the warm waters on the western edge of the Gulf Stream. This marked the first of five planned years of translocation, with a target of 90 to 100 chicks to be moved in total.
In 2005, with a larger number of chicks available thanks to the breeding recovery, 21 birds were moved to Nonsuch. Again, all birds fledged successfully. "So far, the results of this project have exceeded our best expectations," said Jeremy Madeiros of Bermuda’s Department of Conservation Services. "Of course, now we have to wait several years until the chicks mature and return to nest, so I will not consider the project to be a success until that happens."
There were 71 active nest burrows in 2005, more than in the year before the storm. The number of fledged chicks, which dropped from 39 in 2003 to 29 in 2004, recovered to 35 chicks this year.
"We have also been successful in attracting Cahows to nest in an entirely new complex of nest burrows built on the most elevated section of the largest present nesting islet. These were built to replace a group of low-lying burrows that were destroyed by Fabian, and are eight metres higher, which should render them safe from storm waves." —Jeremy Madeiros
For two years Madeiros and his team used a solar-powered sound system to play back Cahow courtship calls, and moved pairs of adults found scrambling in the debris of destroyed nest burrows up to the new site. "We were rewarded with three pairs occupying and building nests in the new burrows. One pair were identified from their band numbers as birds which had been physically moved to the new burrows, so the combination of techniques seem to have worked."
The last chicks departed from the translocation site and breeding islets around mid-June. Preparations were already underway for the next breeding season, with the first Cahows due to return to the nesting sites by late October.