In Florida, it eats mainly smaller birds (such as ground doves). The short-tailed hawk hunts from a soaring flight, often at the borders between wooded and open areas. A frequent maneuver is "kiting" – coming to a stop, the bird heads into the wind, with its wings held stationary. It typically attacks prey with a nearly vertical swoop, sometimes pausing and then continuing downward in a "stair-step" manner. Typical prey ranges from a New World warbler (Parulidae) to a bobwhite (Colinus sp.) in size. In Florida, icterids—namely the red-winged blackbird (Agelaius phoeniceus), the common grackle (Quiscalus quiscula), the boat-tailed grackle (Quiscalus major) and the eastern meadowlark (Sturnella magna)—make up the bulk of the prey. In one case, 95% of a single hawk's prey selection was found to consist of red-winged blackbirds. Hunting success is apparently relatively low. In one study, 30 hours of observation showed that only 12 of 107 hunting attempts (or around 11%) were successful. There are isolated records of short-tails preying on sharp-shinned hawks (Accipiter striatus) and American kestrels (Falco sparverius). Among tropical populations, they are several records of this species regularly taking frogs (especially tree frogs), lizards, snakes, large insects such as wasps and locusts. Such prey, which serves merely as alternate foods for Florida populations, apparently provides a much larger proportion of the diet in tropical populations. In all parts of the range, they occasionally supplement their diet with smallish mammals, principally small rodents such as mice, rats, and bats. Among the heaviest prey recorded are young common marmosets (Callithrix jacchus) and similar small monkeys; these do not seem to form an important prey item, however, and are only snatched when the opportunity presents itself. They are primarily an aerial predator, taking most prey by swooping down to trees or the ground. Rarely, they have been recorded still hunting from a perch.

The large stick nest is built in a tree, at a height ranging from 2.5 to 30 m (8.2 to 98.4 ft). In Florida, the bald cypress (Taxodium distichum) is a popular nesting tree of the short-tailed hawk. The nest is bulky, measuring 60–70 cm (24–28 in) wide and 30 cm (12 in) deep. Its 1–3 eggs per clutch are white, usually with dark spots and blotches. The nesting season is January through June in Florida and is possibly similar in the tropics. Incubation occurs over 34 days with no known details of their fledgling period. In Florida, American crows have been known to consume eggs of this species.

The short-tailed hawk is uncommon and local in most of its range. It is quite difficult to detect unless in flight, since it is often concealed while perched by dense canopy or with only the head showing (unlike most Buteo hawks which generally prefer prominent perches). Due to the fact that it is believed to be regularly overlooked in the field, no comprehensive population surveys have occurred for the species. However, due to its wide extent of occurrence, it is not considered threatened by the IUCN. Sightings of soaring short-tailed hawks are fairly frequent within their range. The Florida population is considered Critically Imperiled, and the Arizona population considered Imperiled by NatureServe.