Racers are slender, quick-moving snakes. When alarmed, or on the defensive, many of these snakes rapidly vibrate the tip of their tail, and if they are in dry weeds or leaves, it produces a buzzing sound suggestive of a rattlesnake. A startled and cornered adult may begin violent, undulatory movement, and may strike and bite. Although they are non-venomous, the slashing bite and recurved teeth may cause bleeding.
The racer's scientific name, Coluber constrictor, is an error: it is based on the Latin word for "constrict", but racers don't constrict. They do, however, sometimes use a loop of their body to hold their prey before swallowing it.
Adults of most kinds of racers are more or less uniformly coloured on the upper part of the body. The mature racer has very smooth, shiny scales, and the normal colouration is very dark on the back, with variations ranging from black, green, bluish green, pale blue or grey, to olive brown, yellowy brown or reddish brown. The newly hatched are blotched or spotted with greys, browns and reds.
The average adult length ranges from 90 to 190 cm (35 to 75 in.). Racers can live more than 10 years in the wild.
Racers inhabit open, bushy, or lightly wooded areas. They prey mainly on small rodents, frogs, lizards and insects.
Three of the nine subspecies of racer usually recognized in the United States occur also in Canada: the blue racer (Coluber constrictor foxi), which once occurred in extreme southwestern Ontario, but now is restricted to a small population on Pelee Island; the eastern yellowbelly racer (Coluber constrictor flaviventris) in southern Saskatchewan; and the western yellowbelly racer (Coluber constrictor mormon) in southern British Columbia. Some authorities do not consider the blue racer to be a valid subspecies, but only a variant of the eastern yellowbelly racer. The western yellowbelly racer is sometimes regarded as a full species, Coluber mormon, distinct from the subspecies of Coluber constrictor.