The Common Grackle has a long dark bill, pale yellowish eyes and a long tail. Its feathers appear black with purple, green or blue iridescence on the head, and primarily bronze shine in the body plumage. The adult female is smaller and usually less iridescent; her tail in particular is shorter, and unlike the males, does not keel in flight and is brown with no purple or blue gloss. The juvenile is brown with dark brown eyes.
The common grackle forages on the ground, in shallow water or in shrubs; it will steal food from other birds. It is omnivorous, eating insects, minnows, frogs, eggs, berries, seeds, grain and even small birds and mice. Grackles at outdoors eating areas often wait eagerly until someone drops some food. They will rush forward and try to grab it, often snatching food out of the beak of another bird. Grackles prefer to eat from the ground at birdfeeders, making scattered seed an excellent choice of food for them. In shopping centers, grackles can be regularly seen foraging for bugs, especially after a lawn trimming.
Along with some other species of grackles, the common grackle is known to practice anting i.e. rubbing insects on its feathers to apply liquids such as formic acid secreted by the insects.
In the breeding season, males tip their heads back and fluff up feathers to display and keep other males away. This same behavior is used as a defensive posture to attempt to intimidate predators. Male common grackles are less aggressive toward one another, and more cooperative and social, than the larger Boat-tailed Grackle species.