Cahow  (Pterodroma cahow)

The cahow is classified as Endangered (EN) on the IUCN Red List (1) and is listed on Appendix I of the Convention on Migratory Species (CMS or Bonn Convention) (3).

Having disappeared for over 300 years following human settlement and the catastrophic introduction of mammals to the Bermuda islands in the sixteenth and seventeenth centuries, the cahow was re-discovered in 1951 and only survives as a result of careful conservation management (4). This medium-sized, long-winged gadfly petrel (seabird in the genus Pterodroma) has a greyish-black crown and collar, dark grey upper-wings and tail, white upper-tail coverts and white under-wings edged with black (2). The underparts are completely white. It has a short, yet robust black bill with a tube nose, and pink legs and feet, with the outer two-thirds of the webbing being black (2) (4). This petrel is also famed for its calls; frightened sailors formerly named Bermuda the ‘Isles of Devils’ after hearing the bird’s haunting nocturnal mating calls (2).

The cahow nests only on Bermuda, but ranges throughout the North Atlantic within and north of the Gulf Stream. It has been seen off the coast of North Carolina, USA, as well as in the Azores (2) (4) (5). The finding of Bermuda petrel bones on Crooked Island in the Bahamas suggests that the species may have once been a more widespread breeder (6).

Previously nesting inland under the forest throughout Bermuda, the cahow used to burrow into soft soil to nest. Having been driven to local extinction on Bermuda’s main islands, it is now restricted to four small, predator-free, soilless islets totalling less than one hectare, where it could only nest in crevices and caves in seacliffs prior to rediscovery and conservation management (2) (4) (5). 

The cahow feeds primarily on small squid, but also eats shrimp and small fish (6). Pre-established adult pairs return to the same nest site each year, with courtship and mating taking place in the nest site between October and late November. Both birds then take to the sea for up to six weeks to fatten, before returning to the nest site to lay a single white egg in early to late January. Incubation duties are shared in 8 to 12 day stints over a period of 51 to 54 days. Hatching occurs between late February and late March, and the downy chick is fed erratically, with gaps of four days between feedings not unusual (7). The chick fledges from late May to late June, peaking in early June, some 80 to 100 days after hatching (6). The breeding grounds are vacated till the following October, but young adults do not return to the breeding grounds until at least three years old. Upon arrival, the young, unpaired birds perform aerial courtship over the islets at night involving paired flights and moaning calls. Once the male has found or created a nesting burrow, it attracts a female to it, and breeding begins usually by the fourth or fifth year. Pairs of birds may remain together for about 30 years (7).

The huge pre-colonial cahow population was rapidly decimated by human hunting for food, and by introduced pigs, rats, dogs and cats, which eliminated it from all the larger islands (2) (4) (5). Surviving only on the smallest offshore islets, where lack of soil prevented burrowing, it was forced to compete for nest sites with the larger, cliff-hole-nesting white-tailed tropicbird (Phaethon lepturus), which caused 70 percent breeding failure every year. Rat predation on new-hatched chicks remains as an occasional threat. During the 1960s and early 1970s, before DDT pesticide use was banned, DDT residues caused a 50 percent decline in breeding success due to eggshell thinning and embryo mortality (7). From the late 1960s through the 1990s, light pollution from a nearby NASA tracking station and U.S. Naval air station inhibited new pair formation on the breeding islets (2). Erosion damage to the nesting islets from hurricane overwash has become a major problem due to global warming and sea level rise since 1995 (7).

The rediscovery of the Bermuda petrel’s nesting grounds in 1951 enabled concerted conservation efforts, and since 1961, all of the breeding sites have been intensively managed by a conservation officer employed by the Bermuda Government Ministry of the Environment. Periodic invasions of rats are controlled by trapping and baiting with anti-coagulant poisons, and nest competition with tropicbirds has been eliminated by the installation of wooden entrances, which exclude the larger tropicbirds. In addition, man-made concrete burrows have been provided on the level tops of the islets to accommodate the cahow population increase. Furthermore, since 1962, the six hectare, soil-covered Nonsuch Island has been restored and kept predator-free as a much larger future home for the expanding population (7).

Between 1961 and 2010, the breeding population has grown from a low of 18 pairs to 95 pairs, and fledgling production has risen from 8 per year to 50 per year (7). From 2005 to 2008, 104 fledglings were translocated from the smaller islets to man-made burrows on Nonsuch Island (7), resulting in the establishment of a colony of 7 pairs by 2010 (2). These success make the conservation of the cahow one of the most successful endangered species recovery programs in the world (7).

View information on this species at the UNEP World Conservation Monitoring Centre.

For more information on the conservation of the Bermuda petrel, see:

• West Indian Breeding Seabird Atlas:
  http://www.wicbirds.net/bope.html

To find out more about conservation in Bermuda, see:

• The Government of Bermuda, Department of Conservation Sciences:
  http://www.conservation.bm/
• Procter, D. and Fleming, L.V (1999) Biodiversity: the UK Overseas Territories. Joint Nature Conservation Committee, Peterborough, UK. Available at:
  http://www.jncc.gov.uk/page-3045