Bermuda Petrel is a medium-sized gadfly petrel, smaller and slenderer than the stocky Black-capped Petrel (Pterodroma hasitata) but markedly larger than the small Pacific petrels such as Cook's Petrel (Pterodroma cookii). In the North Atlantic Ocean, Bermuda Petrel should be critically distinguished from Black-capped Petrel, Zino's Petrel (Pterodroma madeira), and Fea's Petrel (Pterodroma feae feae, "Cape Verde Petrel" and Pterodroma feae deserta, "Desertas Petrel"); the last two, very similar taxa sometimes are considered to be separate species (Robb et al. 2008, Jesus et al. 2009). To confirm an identification of Bermuda Petrel, observers should note:
• a dark eye patch with deep gray plumage surrounding it, which merges with a gray crown and nape, rather than a well-defined dark cap;
• a very limited amount of white in a narrow fringe bordering the dark rump, with the rest of the uppertail area uniformly gray or dark gray—or in some cases, an uppertail area entirely dark;
• a diffuse area of pale gray or gray-brown extending down the side of the neck and breast, giving the bird a hooded or cowled appearance overall, rather than a narrow, dark, elongate mark at the side of the breast, as in Black-capped;
• a relatively small head and especially a small bill, smaller than that of Black-capped;
• an underwing pattern similar to Black-capped but with relatively more dark edging ("ulnar bar") relative to the white internal areas of the underwing (Zino's, "Desertas", and "Cape Verde" show a mostly slaty underwing); and
• an overall size noticeably smaller than, and flight behavior often different from, Black-capped, Bermuda Petrel being more buoyant, with wings held slightly bowed and often oriented closer to the horizontal than the vertical during arcing flight (Wingate et al. 1998).
Black-capped Petrel (Pterodroma hasitata) is superficially similar to Bermuda Petrel, being dark gray, gray-brown, or blackish above, white below, with a white underwing area broken by a black ulnar bar. However, most Black-capped Petrels show a distinct white collar and large area of white in the caudal area, a product of mostly white uppertail coverts and white bases of rectrices. Some Black-capped Petrels show reduced white in the nape and uppertail area, but these birds still show a dark cap and lack the dusky gray cowl typical of Bermuda Petrel, which has much less white in the uppertail than Black-capped (or lacks it altogether). Experienced observers note that the more slender body and wings of Bermuda Petrel distinguish that species from Black-capped, even when seen at moderate distances at sea, and that the shape of Bermuda Petrel is closer to that of Fea's Petrel (Pterodroma feae), which shows a mostly slaty underwing. Any of the North Atlantic gadfly petrels can show a carpal-ulnar 'M' pattern in the upperwing surface, an appearance that is partly a product of plumage, partly of lighting. Although Bonin Petrel (Pterodroma hypoleuca) never has been been recorded in the Atlantic basin, and would not be a bird expected to occur there, its plumage in many instances is very close to that of Bermuda Petrel, so much so that reports of Bermuda Petrel away from known areas of occurrence should be scrutinized with great care. Howell (2012) and Wingate et al. (1998) include more extensive information on plumage variation in this and similar species.
Adult: In fresh plumage uniformly dark gray above, with the exception of the bases of uppertail coverts, which are narrowly whitish; the extent of this white is variable, but most adults show a noticeable arc of white in these feathers, the maximum being about 25 cm. In some adults, the greater coverts, rump feathers, and outer primaries appear slightly or noticeably darker, almost blackish, thus forming an "M" pattern on the upperparts of the flying bird, as in other gadfly petrel species. Rarely, molting adults show a narrow white hindcollar (Howell 2012), similar to Black-capped Petrel (Pterodroma hasitata). Adults typically have crowns darker than the back, and the gray of the back continues onto the sides of the breast, creating a "cowled" appearance, unlike Black-capped Petrel (Wingate et al. 1998). Plumage surrounding the bill is narrowly whitish. Plumage surrounding the eye is darker than the back, closer to the very dark gray of the crown (or darker), and in some individuals, there is a narrow whitish supercilium that segregates the dark eye patch from the dark crown. Plumage tones in molting adults (and in petrels of all ages with worn plumage) tend toward shades of brown rather than gray.
Ventrally, adults are white on the chin, throat, breast, belly, undertail coverts, and most of the underwing coverts, with the medium grayish "cowl" slightly segregating the throat from the breast but never meeting mid-breast to form a complete breast band. The gray rectrices are narrowly discernable from below, as the white undertail coverts are rather long. From below, the dark gray remiges contrast starkly with the mostly white greater underwing coverts. From below, the leading edge of the wing (lesser and median underwing coverts) are blackish and also contrast strongly with the greater coverts here, though in most individuals there are dusky gray subapical marks, often in the shape of a fingerprint, in the greater primary coverts and also sometimes in the greater secondary coverts of the underwing. Axillars and humeral coverts are white, so that the black bar of the leading edge of the wing appears to angle in toward the belly and end before it reaches the body.
Juveniles and subadults are similar in plumage to adults, but some freshly plumaged juveniles show much more contrast above, with grayish tones paler than in adults and the "M" pattern above more pronounced; the "cowl" on the sides of the breast also may be paler than in adults (Howell 2012).
No published information. Because juveniles fledge in late May and early June, their molt should be expected in late winter through spring of their second calendar year, probably March through May (inferred from molt schedule in congeners that are not transequatorial migrants). Adults would appear to molt after they finish feeding young in spring, from ca late April through early July, and it is at this time of year that Bermuda Petrels in heavy molt of remiges and rectrices have been photographed off North Carolina, presumably adults or subadults. Over several plumage cycles, the number of which is not known, the molt schedule of young birds would synchronize with that of adults, as in other medium-sized tubenoses.
Bermuda Petrel has a black bill and dark eyes throughout life. The feet, like those of many medium-sized gadfly petrels, are bicolored, being blackish distally, dull pinkish (or orange pink or fleshy pink) proximally, with tarsi a similar dull pale pink.
Wing chord: given by Murphy and Mowbray (1951) as 260-262 mm (n = 3; mean 260.7 mm). The authors' measurements of nine specimens in the United States National Museum of Natural History (USNM), American Museum of Natural History (AMNH), and Bermuda Aquarium, Museum & Zoo (BAMZ) indicate wing chord for Bermuda Petrel is in the range of 245-265 mm (mean 254 mm) (Wingate et al. 1998).
These measurements compare to wing chord ranges of 258-282 mm (n = 40; mean 268 mm) in the slightly larger "Desertas Petrel" (Pterodroma feae deserta) and 241-254 mm (n = 13; mean 247 mm) in the slightly smaller Zino's Petrel (Pterodroma madeira) (Zino and Zino 1986). Jesus et al. (2009), using larger samples (of adult birds exclusively) and recognizing petrels nesting at Bugio (Desertas) and Fogo (Cape Verde Islands) as distinct species, give wing chord of "Desertas Petrel" as 226-283 mm (n = 74, mean 270.7 mm) and wing chord of Fea's Petrel (Pterodroma feae feae) as 263-279 mm (n = 28; mean 270.2 mm).
Howell and Patteson (2008) indicate wing chord of the larger Black-capped Petrel (Pterodroma hasitata) as 268-317 mm (n = 54), with females significantly smaller than males. Murphy (1951) gives wing chord of Black-capped as 287-305 mm (n = 7; mean 293 mm).
Total length: 35-38 cm by Howell (2012).
Data taken by Jeremy Madeiros from nesting birds consistently show the significant difference in mass and bill length between the sexes (greater in adult males than adult females), which, as he indicates (personal communication), "aids in sexing the birds outside the normal period (I generally sex the birds using external cloacal examination within three weeks of egg-laying, which is over 90% accurate)". His measurements are as follows:
Bill length (exposed culmen): range of both sexes 27.3-31.2 mm; mean = 29.32 mm (n = 35). Females 27.3-29.6 mm; mean = 28.3 mm (n = 14; males 29.1-31.2 mm; mean = 30.009 mm (n = 21).
Howell and Patteson (2008) give culmen measurement of the larger Black-capped Petrel as follows: exposed culmen: range of both sexes 28.8-36.5 mm (n = 50); females 28.8-34.0 mm (n = 19), males 301.-36.5 mm (n = 31).
Jesus et al. (2009) give exposed culmen measurements for "Desertas Petrel" of 27.9-32.0 (n = 65; mean 29.6 mm) and for Fea's Petrel of 26.0-29.8 mm (n = 28; mean 27.9 mm).
Mass: females, range 258-413 g; mean 324.55 g (n = 65); males , range 278-454 g; mean 363.56 g (n = 78). Howell and Patteson (2008) indicate the mass of Black-capped Petrel has a range of 347-591 g (n = 57), with males significantly heavier than females.