A relatively large, distinctive, long-winged Pterodroma ("gadfly") petrel. The clearly defined black cap, separated from the dark mantle by a weak white collar, and the conspicuous white 'rump' (uppertail coverts) forming a broad 'U' shape, are unmistakable, even from substantial distances.
No other Pterodroma within the range of the Black-capped Petrel shows the fully white neck distinct from black cap, although at least one Bermuda Petrel photographed off Hatteras, North Carolina has shown a thin white collar (Howell 2012). The presumably extinct Jamaican Petrel (Pterodroma caribbaea), which formerly bred only on Jamaica (see Geographic Variation), was similar in structure but with mostly dark plumage, so this species would be unlikely to be confused with Black-capped, but several other related species could pose identification challenges. In nonbreeding periods, the distributions of several species may overlap with Black-capped: Bermuda Petrel (Pterodroma cahow) is smaller, less robust and usually lacks white hindneck and rump; additionally, Bermuda Petrel is smaller billed and has longer wings, coupled with a generally more buoyant flight, that make Bermuda less robust and stocky than Black-capped Petrel. Additionally, an obvious difference between Bermuda and Black-capped usually is the head pattern, with Bermuda Petrel showing a dark or black cap contiguous with dark gray back and nape meeting on the side of the breast in a straight line before meeting the mantle. However, at times, separating these species under field conditions may be difficult. Herald Petrel Pterodroma arminjoniana (also known as Trinidade Petrel) overlaps at least occasionally in the western North Atlantic, but this species is generally much darker, especially on the head and neck, as well as having dark axillaries and darker underwings; Fea's Petrel Pterodroma feae, and conceivably Zino's Petrel Pterodroma madeira, are both much paler on the mantle than Black-capped and on the tail, and show extensive dark gray and black underwings. The similar looking Greater Shearwater Puffinus gravis is structurally and behaviorally quite distinct, and also is larger, darker and less contrasting above, lacks the black edge to underwing, and has different gestalt and flight behavior.
As its name suggests, this species has a clearly defined cap, separated from the dark mantle by a weak white collar. The conspicuous white 'rump' (uppertail coverts) forms a broad 'U' shape. Distinctive black and white underwing pattern, comprising a broad black trailing edge and diagonal black bar across the secondary coverts. Throat and underparts snowy white. Darker upperparts intrude at the shoulder, forming a weak half collar. Sexes similar. In general, upperparts appear dark or blackish, with the exception of variable white or whitish patches on the uppertail coverts, nape and forehead. The white on the nape accentuates the brownish black cap; dark coloration extends from cap to eye, nape and towards upper breast, forming partial collar. Brownish grey mantle and upperwing. White rump and uppertail coverts. Dark brown tail. Entirely white underparts. White underwing with narrow black trailing edge, black tip, broad black edge between primaries and carpal joint. Band extends towards center of wing.
This species may seem variable in appearance, but at least one recent treatment of the species suggests that such variation may be more discrete than previously thought (Howell and Patteson 2008). "Black-faced" and "white-faced" individuals form discrete plumage and morphological clusters: "black-faced" birds tend to show dark or black cheeks and auricular areas, with larger and more obvious dark collar, more extensive dark cap, and darker plumage on the nape; "white-faced" birds are primarily opposite of these patterns, in showing substantially reduced dark coloration in these areas. Note, however, that this variation is not particularly well understood, nor is it completely discrete - intermediate plumages exist.
Howell and Patteson (2008) suggest that variation in Black-capped Petrels may reflect multiple cryptic species, as evidenced by different plumage characters and different molt sequence and timing. Their discussion is the most extensive and comprehensive treatment to date for this species, but even they suggest that additional information is needed to understand whether this variation is a functional of subpopulations, geographic variation, multiple cryptic species, molt timing, or some combination of these.
Black bill, occasionally with slight pinkish or grayish pink color visible at the base of maxilla and mandible (age related or environment related?). Dull pink legs; feet pink proximally but becoming black distally.
Mass: females range from 347-545 grams, with substantial clustering into three different classes of mass that relate to black-, white-, and intermediate-face color patterns; males of the same mass classes range from 349-591 grams. Wing chord: females range from 268-305 mm; males range from 279-317 mm. Exposed culmen: females range from 18.104.22.168 mm; males range from 30.1-34.7 mm. All data summarized from Howell and Patteson (2008).