The Swift Fox is a nocturnal predator. It remains near a den during the day, sometimes basking in the sun on cold winter days. A social unit consists of a male, one or two females, and their offspring. After the young disperse, the parents remain together and are assumed to pair for life, but this has not been confirmed. The Swift Fox is the most den-dependent of all members of the family Canidae in North America. They dig their own dens rather than take over a den dug by another mammal. They prefer friable soil, but their dens may be in open rangeland or near roads or human habitation. The dens offer refuge from Coyotes, and temporary (escape?) dens may be dug for use in emergencies. The size of Swift Foxes' home ranges appear uncertain and probably varies with the productivity of the habitat.
Swift Foxes have lived more than 12 years in captivity. The normal life span in the wild undoubtedly is much shorter than this.
Diet varies seasonally, with mammals the most important component. Cottontail rabbits and jackrabbits are the most common prey in spring, whereas more kangaroo rats, pocket mice, deer mice, cotton rats, and ground squirrels are eaten in autumn. Large quantities of insects, especially grasshoppers, also are eaten, as are an occasional lizard or ground-nesting bird. Like other canids, Swift Foxes consume some grass. Finally, the fact that they consume large quantities of carrion makes them especially susceptible to poisoning.
Coyotes are the most important predators on swift foxes, although other animals (including badgers, bobcats, and possibly great-horned owls) take a few. Additionally, Swift Foxes commonly are killed on roads because they have a tendency to run down a road in front of a car rather than dart into the adjacent ditch. Finally, researchers have reported unexplained mortality of young Swift Fox in their dens.
Swift Foxes may form pairs and mate during the first breeding season after their birth. Breeding takes place in late December or early January, and gestation lasts about 50 days. Litters of from 3 to 6 young are born in a grass-lined burrow in March or April. The young are born blind and with their ears closed. Their eyes open at 10 to 15 days, and they are weaned at 42-49 days. The pups stay near the den until mid-June, after which they join their parents in hunting. By mid-July they are nearly full grown, but they remain in the family groups until August or early September.