Wilson's Storm-Petrel is a small, dark brown storm-petrel of the genus Oceanites, with a broad, white band across the uppertail coverts, and pale gray brown greater coverts. This species has long tarsi with diagnostic yellow webbing in the feet. The toes often project beyond the square or slightly notched tail in flight, which is fluttery and gliding.
Elliot's Storm-Petrel (Oceanites gracilis), is very similar to Wilson's Storm-Petrel in shape and coloration,but generally is smaller, and has a characteristic large white belly patch. Elliot's also has whitish tips to the greater and median underwing coverts, although these may be difficult to see; furthermore, the 'Fuegian' subspecies of Wilson's (chilensis), which is the population that is sympatric with Elliot's, also may show a similar pattern. On Elliot's, the amount of white in the rump is reduced. This may best be seen on birds sitting on the water. Their smaller size also contributes to a flight style that is slightly more fluttery than that of Wilson's (Howell 2012).
Wedge-rumped Storm-Petrel (Oceanodroma tethys) has longer-armed, narrower, and more crooked wings. It flies in a more typical Oceanodroma fashion, with strong deep wingbeats in an erratic, bounding manner. The rump patch covers the entire uppertail coverts, and is very large and conspicuous. Note that feeding Wedge-rumped patters on water similarly to Wilson's, but the tarsi of Wedge-rumped are not as long, and this species lacks yellow webbing between toes. Wedge-rumped Storm-Petrel is smaller overall smaller than Wilson's, but overlap in size the 'Fuegian' subspecies of Wilson's (Onley and Scofield 2007, Howell 2012).
The Pacific subspecies of Band-rumped Storm-Petrel (Oceandroma castro bangsi) also exhibits the fast, strong, bouyant and bounding flight with deep wingbeats typical of Oceanodroma. Band-rumped is larger than Wilson's, with longer wings, a slightly longer tail, a squarer head, and a larger bill (Onley and Scofield 2007, Howell 2012).
From above, Black-bellied (Fregetta tropica) and White-bellied (F. grallaria) storm-petrels show a brown back and the compact, square-tailed appearance of Oceanitidae, but are larger with longer and broader wings. Their pale ulnar band is less distinct and narrower than on Wilson's (Howell 2012).
The newly described Pincoya Storm-Petrel (or "Puerto Montt Storm-Petrel") Oceanites pincoyae) is very similar to Wilson's, but has a white ventral area, which connects to the rump patch (unlike Elliot's). There is a prominent white wing bar across the underwing coverts, and the ulnar bar on this taxa is rather extensive (Harrison et al. 2013).
The following description is based on Murphy (1936), Blake (1977), and Howell (2012), and refers to nominate oceanicus; see also Geographic Variation.
Adult: Sexes similar. Overall sooty brown or blackish (browner when worn); may appear blacker at a distance. Slightly paler on throat and belly. Grayish gloss on dorsal surface inconspicuous and present in fresh plumage. Flight feathers blackish, with upperwings contrasting pale gray (when fresh) to pale brownish (when worn). Upperwing panel mostly restricted to greater coverts and does not reach the bend in the wing; may show a broad but diffuse, reflective pale band on greater and median coverts. Uppertail coverts and lower flanks white, forming conspicuous band across the rump which shows from below like a bulge of cotton wool. Tip of tail square or slightly rounded. White basal shafts and inner webs of outer rectrices not usually visible at sea.
Juvenile: May show whitish feathers in lores, and narrow whitish borders on the feathers of the belly (Witherby 1924).
The following summary of molt in Wilson's Storm-Petrel is based on Howell (2012).
Adult and second prebasic wing molt relatively rapid in North Atlantic, beginning mid -April to early June, and finishing mid-August to late September before southbound migration. Tail molt mostly September to October. Complete preformative molt indicated by a second wave of birds that begin primary molt; in mid-July to early September, finishing late October to december or later. This molt may be suspended on some transequatorial migrants. Preformative tail molt mainly October to January. Similar pattern of prebasic molt in May to August/September and presumed prebasic wing molt starting in August also occurs in North Pacific migrants off California and Baja California. Some presumed nonbreeders that may not migrate to the northern hemisphere start primary molt in March or earlier.
Iris: dark brown
Tarsi and toes: Tarsi and toes black, except for unique yellow webbing between toes.
Bare part color data from Murphy (1936) and Blake (1977).
Total length: 171-184 mm; wingspan 36-41 cm.
oceanicus (n = 97, sexes combined; Murphy 1936)
Wing length: mean 144.9 mm, range 136-155.5 mm
Tail length: mean 63.7 mm, range 56.5-73 mm
Culmen length: mean 12.6 mm, range 11.1-13.2 mm
Tarsus length: mean 34.6 mm, range 31.3-36.8 mm
Middle toe and claw: mean 27.5 mm, range 25.3-30 mm
chilensis: (n = 33 males, 18 females; Murphy 1936)
Wing length: male mean 136.2 mm, female mean 139.4 mm, range 130-146 mm
Tail ;ength: male mean 58.4 mm, female mean 60.5 mm, range 55-63 mm
Culmen length: male mean 11.6 mm, female mean 11 mm, range 10-12 mm
Tarsus length: male mean 34.5 mm, female mean 34.9 mm, range 33-36 mm
Middle toe and claw: male mean 27.2 mm, female mean 26.8 mm, range 24-29 mm
Mass: oceanicus (Crozet Island), mean 32.0 ± 3 g (range 27.0-39.0, n = 31, sexes combined; Dunning 2008); chilensis (?; sample from Chile), mean 28.9 g (range 25-25 g, n = 9, sexes combined; Dunning 2008)