Their breeding habitat is coniferous or mixed forest areas throughout North America. In otherwise optimal conditions they also utilize other habitat, but at the southern margin of its range it can only persist in its favorite habitat. Northern birds migrate further south, arriving in their winter quarters between mid-September and November and leaving to breed from mid-March onwards, with almost all gone by the end of April or so. Many populations are permanent residents or altitudinal migrants, while in cold years birds may choose to stay in their winter range and breed there. For example, in the Sierra Nevada of eastern California, J. hymealis populations will migrate to winter ranges 5,000–7,000 feet (1,500–2,100 m) lower than their summer range. In winter, juncos are familiar in and around towns, and in many places are the most common birds at feeders. These birds forage on the ground. In winter, they often forage in flocks that may contain several subspecies. They mainly eat insects and seeds.
Grackles have a unique adaptation in the keel within their bill which allows them to crack and cut hard nuts or kernels. The keel projects downward from the horny palate and is sharper and more abrupt anterior. It extends below the level of the tomium and is used in a sawing motion to score open acorns or dried kernels. Large adductor muscle within their jaw compared to other icterids also makes this adaptation even more useful for opening hard seeds and acorns.
Along with some other species of grackles, the common grackle is known to practice “anting”, rubbing insects on its feathers possibly to apply liquids such as formic acid secreted by the insects.