The Cahow: a petrel that was extinct for 300 years
The Bermuda Petrel was written off in the seventeenth century and rediscovered in 1951. Reaching a hundred nesting pairs took another sixty years.
The Bermuda Petrel was one of the most abundant birds in the archipelago when Europeans arrived in the early seventeenth century. Within about thirty years it was gone — hunted for food by settlers, and its ground burrows raided by the cats, dogs, rats and pigs that arrived with them. It stayed gone for three hundred years.
Then, in 1951, it was found again: a few pairs on small rocky islets off the Bermudian coast, which had survived unnoticed since before the English Civil War.
Why the rediscovery was only the start
Finding a species is not the same as saving it. The Cahow that reappeared in 1951 was a population in the low tens of pairs on habitat that was barely adequate: a few small, low islets with limited soil for burrowing, exposed to Atlantic storms, and shared with the White-tailed Tropicbird, a larger and more aggressive bird that takes over petrel burrows and kills the chicks in them.
The species also breeds very slowly. Petrels of this kind lay a single egg a year and take several years to reach breeding age, so a population starting from a few dozen pairs cannot grow quickly even under ideal conditions. There was no fast route.
What was actually done
The recovery programme is a good example of conservation as engineering. Artificial burrows were built to replace the inadequate natural ones. Baffles were fitted to burrow entrances, sized to admit a Cahow and exclude a tropicbird, which removed the competition without removing the competitor. Introduced predators were controlled and excluded.
Later the work extended to translocation: moving chicks to a larger island with more room and better protection, and hand-rearing them to fledging so that they would imprint on the new site and return there to breed. That is a slow method, because the birds spend years at sea before coming back, and its success cannot be assessed for the best part of a decade.
Passing a hundred pairs
Reaching more than a hundred nesting pairs was the milestone that mattered, and the language used about it was careful: the population was described as becoming self-sustaining. That is a specific claim. It means the population’s own productivity has become sufficient to carry its growth, rather than the numbers being held up by continuous intervention.
It does not mean the work stops, and it does not mean the species is secure. The Cahow remains Endangered. It nests on a small number of low-lying islands in a hurricane belt, and the exposure of that arrangement to severe storms and rising sea level is the obvious threat to sixty years of recovery.
What the case is good for
Recovery programmes of this kind are often cited as evidence that extinction is reversible. The more useful reading is what they cost. Three hundred years of absence, a chance rediscovery, and then sixty further years of burrow construction, predator control and translocation to get from a handful of pairs to a hundred.
That is what saving one seabird takes, when it is possible at all. The Echo Parakeet is the same story on a different island, and the Mangrove Finch is what it looks like when the limiting factor is still winning.
Common questions
What is the Cahow?
The Bermuda Petrel Pterodroma cahow, a nocturnal, burrow-nesting seabird endemic to Bermuda and the country's national bird. The local name comes from its call.
Why was it thought extinct?
It was abundant when Bermuda was settled, and within a few decades it had effectively gone. Settlers hunted it for food, and introduced cats, dogs, rats and pigs took eggs, chicks and adults from ground burrows. By the mid-seventeenth century it was presumed extinct.
When was it rediscovered?
In 1951, on small rocky islets off Bermuda, where a handful of pairs had survived for three centuries. It is one of the best-known examples of a species surviving unnoticed in a fragment of its former range.
Why did recovery take so long?
Because the species breeds slowly: one egg per pair per year, with birds taking several years to reach breeding age, and because the surviving nest sites were poor. The islets were too small and too low, competing with a larger tropicbird for burrows and exposed to storm damage.
What did the recovery programme involve?
Artificial burrows, baffles to exclude the larger White-tailed Tropicbird from petrel nests, predator control, and eventually translocation of chicks to a larger island to establish a colony on ground with more room and better protection.
What does 'self-sustaining' mean here?
That the population's own productivity, not continuous intervention, is carrying its growth. Passing a hundred nesting pairs was the milestone that marked that shift; it does not mean management has stopped.
Is it safe now?
No. It remains Endangered, and a species confined to a few small low-lying islands in a hurricane belt has an obvious and growing exposure to storms and sea-level rise, whatever its population trend.