Bermuda Petrel Pterodroma cahow


Justification of Red List Category
Successful conservation has increased the population of this species, but it remains extremely small and the species consequently qualifies as Endangered. If the population continues to grow, which recent figures suggest it has, the species will warrant downlisting to Vulnerable in due course.

Population justification
In 2005, the population was thought to include 71 breeding pairs (J. Madeiros in litt. 2005). By 2011, the population reached 98 nesting pairs (Madeiros 2011).

Trend justification
The population has increased from 18 pairs in 1951 to 71 pairs in 2005. This is equivalent to an increase of well over 79% in three generations, given the species's long lifespan. Records of 40 chicks fledged in 2008 and 35 chicks hatched in 2009 suggested the population continues to increase, and indeed by 2011, the population reached 98 nesting pairs (Madeiros 2011).

Distribution and population

Pterodroma cahow once bred abundantly throughout Bermuda (to UK). It was thought extinct for almost three centuries, until reported (with specimens) during the first half of the 20th century. In 1951, 18 pairs were rediscovered breeding on suboptimal rocky islets (total area 1 ha) in Castle Harbour. Intensive management has resulted in slow but steady increases, and the population was estimated at 250 birds in 2005 (J. Madeiros in litt. 2005), with 70 pairs fledging a record 40 young in 2003 (Madeiros 2003), and 71 pairs fledging 35 young in 2005 (J. Madeiros in litt. 2005). More recently, 40 young fledged in 2008 from 85 established active nest sites and at least 35 chicks hatched in 2009 (Madeiros unpubl. data). By 2011, the population reached 98 nesting pairs (Madeiros 2011). 

From 40 chicks on all islands in 2008, 21 chicks were translocated in May to Nonsuch, forming the last year of a translocation project (Madeiros 2008). Fourteen individuals fledging from Nonsuch Island after translocation in 2005 and 2006 were observed in 2009 returning to the island and entering artificial burrows. One chick was born on the island in 2009 (Madeiros unpubl. data). By 2011, 22 translocated birds have returned to Nonsuch, and 8 more translocated birds returned to the original nesting islets. Non-translocated birds have also been recorded on Nonsuch indicating a sufficient nucleus of translocated birds to attract non-translocated individuals into the colony (Madeiros 2011). 

In the non-breeding season, birds probably move north into the Atlantic, following the warm waters on the western edges of the Gulf Stream (Wingate 1997). There are confirmed records off the coast of North Carolina, Virginia and Massachusetts, U.S.A. (D. S. Lee in litt. 1998, Wingate et al. 1998, Nisbet et al. 2013, Maderios et al. 2014, Adams et al. 2014, Sattelmeyer et al. 2015), and one bird was captured in the Azores in November 2002, in November 2003 and in December 2006 (Bried 2003, J. Bried in litt. 2010). 


It formerly nested in soil burrows, but such habitat is not available on current breeding islands and it now nests in suboptimal, natural erosion limestone crevices and artificial burrows. The breeding season is January-June, and breeding success has increased from less than 5% per year in the 1950s to more than 25% per year in the 1990s (Wingate 1997). Ringing recoveries have shown that birds first return to breed four years after fledging (Madeiros in litt. 2006). The breeding grounds are not visited by birds between mid-June and mid-October (Wingate 1997), with adults returning from mid-October (Madeiros 2008). Results from geolocator tags showed that individuals can cover in excess of 5,000 km during a foraging trip, following different courses from Bermuda but generally all foraging over the Gulf Stream (Madeiros unpubl. data). During the non-breeding season they have been recorded in the Gulf Stream, north to the Bay of Fundy, into the Gulf of St. Lawrence and over the Grand Banks, with one individual recorded 201-241 km off southwest Ireland (Madeiros 2009).


Of the threats to the species, predation by introduced species and direct hunting are likely to have been the main drivers of the historical decline, but the impact of area lost to development is unknown. Dogs, pigs, cats and rats were present and likely to be the main drivers of the dramatic decline that led to the species being considered extinct prior to 1951. Rats also represent a threat, because, since 2005, rats have managed to swim out to breeding islets on multiple occasions, including invasions of Nonsuch Island in 2015 and 2016 (N. Carlile in litt. 2016). So far these have been detected and the rats controlled with only minor chick mortality, but this is an ongoing threat that is only at a negligible severity due to intensive conservation interventions. A single, subadult, Snowy Owl Bubo scandiacus, the first ever recorded in Bermuda, on Nonsuch Island was eradicated after having eaten 5% of the population (Dickinson 2007). Swarms of feral European honeybee have taken over burrows and also killed at least one chick, but ongoing control using insecticide has been successful (Madeiros et al. 2012).

The threat of sea-level rise and increased storm activity appears real, with five or more major floods affecting burrows in the 1990s, after 25 years without significant problems (Wingate et al. 1998). Category three hurricane Fabian in 2003 overwashed three of the four breeding islets, damaging or destroying a significant number of nest burrows (J. Madeiros in litt. 2005). This, however, does not seem to have caused population level impacts.

Light pollution from the international airport continues, but those lights intense enough and orientated high enough to potentially cause disruption to Cahows on their breeding islets have been realigned or had hoods fitted to reduce the impact (Madeiros 2005). As a result of this, light pollution does not seem to be causing significant declines.

In the past, its recovery has been hampered by competition from White-tailed Tropicbird Phaethon lepturus for nest-sites. Successful use of wooden 'baffles' at the entrance to burrows prevents access by White-tailed Tropicbirds alongside the provision of artificial tropicbird nest sites has relieved the pressure on Bermuda Petrel.

Conservation actions

Conservation Actions Underway
CMS Appendix I. Since 1961, there has been management of nesting-grounds, including the creation of artificial burrows, periodic removal of rats and the elimination of nest-site competition from Phaethon lepturus (by installing baffles over burrow entrances). As part of the Bermuda Conservation Programme, potential breeding islands (e.g. Nonsuch) have been reforested with native flora in an attempt to attract nesting petrels (Wingate et al. 1998). The Castle Harbour islands are a National Park and Nature Reserve (J. Madeiros in litt. 2005). In 2004 and 2005 efforts were made to attract adult Bermuda Petrels, displaced from low-lying nest burrows destroyed by hurricane Fabian, on the main breeding islet to a new artificial burrow complex built on a more elevated section of the islet. Using a combination of a sound attraction system set up among the new nests and physical translocation of adult individuals from the destroyed sites, three pairs occupied burrows in the new complex by March 2005 (J. Madeiros in litt. 2005). A project has also been conducted to establish a new nesting colony on the Nonsuch Island Living Museum, which is much larger and contains more suitable breeding habitat than the present suboptimal breeding islets. The project involved physical translocation of chicks from the present breeding islets to a new complex of artificial burrows on Nonsuch, so that they would imprint on the new site and return when mature to establish their nests at the new location. In 2004, the trial year of the project took place with 14 chicks moved to Nonsuch, where they were fed and monitored every other day until departure, with all fledging successfully. In 2005, 21 chicks were translocated, with all again fledging successfully by mid-June. In 2009, the first adult-fed Bermuda Petrel for 400 years hatched on Nonsuch Island (Dobson 2009). So far, 79 relocated chicks have fledged on Nonsuch, with only one fatality (Dickinson 2007). A Sound Attraction System was also set up in 2007 to help encourage birds to stay and prospect on Nonsuch, and overcome any tendency for young birds to be attracted back to the activity at the original nesting islets (Madeiros 2008). A total of 171 fledglings was ringed between 2002-2007, with 31 confirmed having returned (including 4 recaptured on Nonsuch Island), representing cohorts from four nesting seasons, 2002 to 2005 (Madeiros 2008). Rats have managed to get to Nonsuch (N. Carlile in litt. 2016), but so far these have been detected and the rats controlled with only minor chick mortality. Additionally, a single, subadult, Snowy Owl Bubo scandiacus, the first ever recorded in Bermuda, on Nonsuch Island was eradicated after having eaten 5% of the population (Dickinson 2007).

Conservation Actions Proposed
Maintain all management activities at current nesting-grounds. Investigate whether contaminants are increasing egg failure (Wingate 1997). Continue to manage the new breeding colony on Nonsuch Island (Wingate 1997, Madeiros 2003, J. Madeiros in litt. 2005). Investigate the pelagic and foraging range of the species using new data logger technology (J. Madeiros in litt. 2005).


38 cm. Medium-sized, long-winged, brownish-grey and white gadfly petrel. Brownish-black cap extending to eye, but interrupted by pale eyebrow. Brownish nape extending towards upper breast to form partial collar. Brownish-grey mantle, upperwing and tail. Pale uppertail-coverts may form narrow whitish band. Entirely white underparts. White underwing with narrow black trailing edge, black tip, extending narrowly onto leading edge. Black bill. Pink legs, pink feet proximally, black distally. Similar spp. Black-capped Petrel P. hasitata is larger with white hindneck and more extensive white rump, but sometimes separation at sea may be impossible. Great Shearwater Puffinus gravis is larger, darker and less contrasting above, lacks black edge to underwing and has slower wing-beats and less erratic flight.


Text account compilers
Moreno, R., Stuart, A., Temple, H., Wege, D., Clay, R.P., Anderson, O., Fjagesund, T., Calvert, R., Hermes, C., Isherwood, I., Mahood, S., Martin, R.

White, T., Carlile, N., Madeiros, J., Lee, D.

Recommended citation
BirdLife International (2021) Species factsheet: Pterodroma cahow. Downloaded from http://www.birdlife.org on 20/10/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 20/10/2021.