Black-shouldered kites are around 35 to 38 cm in length (13.8 to 15 in) and have a wingspan of between 80 and 95 cm (31.5 to 37.4 in) and an average weight of 291 grams (10.26 oz). Adults are a very pale grey with a white head and white underparts. The leading edge of the inner wing is black. When perched, this gives them their prominent black `shoulders`. They have red eyes, with a black 'comma' that extends behind the eyes. They have a squared tail and a streamlined aerodynamic body. The bill is short with a sharp, hooked tip to the upper mandible. Their nostrils and the cere (skin at the base of the bill) are bright yellow and the bill is black. The legs and feet are also yellow, and the feet have three toes facing forwards and one toe facing backwards. The sexes are similar, with females only just larger than males, although they can be up to 15% heavier. The juveniles? markings follow a similar pattern to adult birds, but they are washed with a rusty brown on the head and upper breast, and the back and wings are mottled buff or brown with prominent white tips. The young birds? eyes are brown. The black-shouldered kite is very similar to the related raptor species, the letter-winged kite (E. scriptus) but has the black mark above and behind the eye, a white rather than grey crown, and shows all-white underparts in flight except for the black patch at the shoulder and dark wingtips.
Black-shouldered kites live almost exclusively on mice, and have become a specialist predator of house mice, often following outbreaks of mouse plagues in rural areas. They take other suitably sized creatures when available, including grasshoppers, rats, small reptiles, birds, and even (very rarely) rabbits, but mice and other mouse-sized mammals account for over 90% of their diet.