Markham's Storm-Petrel is a large species of the genus Oceanodroma. Like other Oceanodroma, it is a relatively small seabird with long, pointed wings that are held bentat the carpal joint, and a deeply forked tail. It is entirely dark sooty brown, except for pale grayish brown upperwing coverts that form a distinct pale bar across the surface of the wing.
The boreal migrant Black Storm-Petrel (Oceanodroma melania) is sympatric seasonally with Markham's Storm-Petrel, and is very similar in size and appearance. However, Markham's has a shorter neck, squarer head, long bill, more deeply forked tail, a more extensive pale upperwing bar, and much shorter tarsi, which are best seen while foraging (Murphy 1936, Brooke 2004, Howell 2012). Markham's also prefers cooler water and has shallower wingbeats than Black. The "Chapman's" form of Leach's Storm-Petrel (O. leucorhoa chapmani) also may overlap with Markham's off of Ecuador, although chapmani generally has a more northern distribution. "Chapman's" is about two-thirds the size of markhami, and has shorter wings and a less deeply forked tail. Its flight is much more bounding, with deep wingbeats (although note that flight style can vary, as it is influenced also by wind conditions; Howell 2012).
Ashy, (O. homochroa), Matsudaira's (O. matsudairae), Tristram's (O. tristrami), and Swinhoe's Storm-Petrels (O. monorhis) are also entirely dark brown with a pale wing bar but are enitrely allopatric.
The following description is based on Murphy (1936), Blake (1977), Brooke (2004) and Howell (2012).
Adult: Sooty black aove, shading to blackish brown on underparts. Head, neck and mantle with plumbeous gloss in fresh plumage. Greater wing coverts and scapulars paler, more gray brown, forming a distinct diagonal bar across extended wing, which extends almost to the carpal joint, where it is broadest; this bar pecomes paler with wear. Distal uppertail coverts may be paler than back, creating slight contrast with blackish tail. Narrow whitish edges to subscapulars in fresh plumage. Underwing is blackish gray, and has a slight silvery sheen. Flight feathers almost black, though the bases of the primary shafts may be white. Upperparts often appear tricolored, with sooty brown back and lesser coverts, pale wing band, and blackish flight feathers. Underparts blackish brown.
Little information, but probably molt is initiated December-January and is completed in April-May. Second prebasic molt should be looked for from September-March (Howell 2012).
Tarsi and toes: black
Bare parts color data from Murphy (1936).
Total length: 210-240 mm. Wingspan ca 530 mm (Blake 1977).
Linear measurements: (n = 6 males, 5 females; Murphy 1936)
Wing length: range 163-181 mm; male mean 172.7 mm, female mean 169.8 mm
Tail length: range 87-97 mm; male mean 92.7 mm, female mean 89.4 mm
Culmen length: range 16.8-19 mm; male mean 18 mm, female mean 18.1 mm
Tarsus length: range 23-25 mm; male mean 23.9 mm, female mean 24.2 mm
Middle toe and claw: range 23-25 mm; male mean 25.6 mm, female mean 25.1 mm
Mass: mean 53.1 g (n = 40; Dunning 2008)