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Courting the Cahow

by Mark Reaves

Lab’s recordings attract Bermuda Petrels to a safer haven

Be careful when you jump onto Green Island. The limestone is so sharp, you could literally fillet a fish on it.” These were words of wisdom from Jeremy Madeiros, Bermuda’s chief terrestrial conservation officer, when he pulled his boat up to the inhospitable landing. As he carefully balanced the craft against the current ripping out of Castle Harbour and the waves crashing in from the Atlantic, I began to understand how a small number of cahow (Pterodroma cahow) could breed unnoticed for three centuries on these small but dangerous rock islets.

inspecting concrete nesting chamber for 

Bermuda Petrels

I arrived in Bermuda on November 15, 2003, hoping to obtain high-quality audio recordings of the critically endangered cahow, or Bermuda Petrel. These recordings would be used in conservation measures, as part of a collaborative effort by Bermuda’s Department of Conservation Services, Cornell Lab of Ornithology’s Macaulay Library, and Stephen Kress (director of Audubon’s Seabird Restoration Program). The cahow is a pelagic species with a rich history which includes its presumed extinction, its eventual rediscovery, and the work of many devoted people.

Fossil remains and early historical accounts suggest that this mysterious species once bred over the entire island of Bermuda and its surrounding islets. Despite the abundance of the cahow documented by the early settlers of Bermuda in 1612, the species was presumed extinct within nine short years of human occupation. The ready supply of cahows, both chicks and adults, had provided a dietary staple for the hungry settlers. In addition, introduced mammals such as rats, cats, and dogs contributed to the cahow’s rapid decline.

one of few natural burrows on the islets

The presumed extinction of the cahow was proved to be wrong in 1951 when 18 breeding pairs were discovered by biologists Robert Cushman, Louis Mowbray, and 15-year-old David Win-gate. The birds were nesting on a series of small islets that now compose the Castle Harbour Islands National Park. This rediscovery prompted conservation efforts in Bermuda, and Wingate has since devoted his life to working for the birds’ survival.

inspecting concrete nesting 

chamber for Bermuda Petrels

There are currently 70 breeding pairs of adult cahows in the world, and an estimated 65 juveniles remain at sea. The birds are only found on land on these islets during the breeding season. The strange experience of inspecting every nesting site of an entire species in a morning’s work gave me a frightening view of the cahow’s fragile existence. The islets have little soil for the burrowing birds to use and are considered suboptimal nesting sites, but they do provide isolation from the mainland and its predators. Conservationists have installed manmade burrows and concrete nesting chambers to encourage breeding.

Two serious issues threatening the cahow are rising sea levels and increased hurricane activity. Some of the nesting colonies close to the water suffered massive damage during last fall’s Hurricane Fabian. Waves swept over the islets during the storm and cleaved off huge pieces of limestone. Portions of long-established nesting sites were literally washed away. To help draw displaced or prospecting pairs to new nesting burrows on higher and safer ground, the restoration team needed recordings of vocalizations that could be broadcast to attract courting birds. This tactic, pioneered by Stephen Kress, has proven successful with other colonial nesting species.

My trip was timed to coincide with the initial phase of the breeding season, when adult cahows return to the islets to engage in aerial courtship displays. After sunset, the birds fly low and fast over the islets and ocean, emitting an eerie moaning vocalization. The sounds of thousands of cahows earned Bermuda the title “Isle of the Devils” by early sailors. I could imagine how amazing those sounds must have been as I sat motionless during nighttime recording sessions, listening to a mere dozen cahows pass swiftly within inches of my head. Their calls resonated across the landscape as their silhouettes appeared and vanished against the night sky.

A remote, solar-powered attractant system, designed and built by Susan Schubel of the Seabird Restoration Program, was installed during my stay and prepared for the cahow recordings. A January update from Jeremy Madeiros reported that the recordings were being broadcast from Horn Rock Island and that the birds were settling in nicely. Eight females had laid eggs, and Jeremy had even found tracks indicating that a cahow had investigated a nesting cavity right next to one of the audio speakers.

Mark Reaves is the assistant curator of terrestrial collections in the Macaulay Library of Natural Sounds.

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