The Black-billed Magpie is an opportunistic omnivore, eating many types of insects, carrion, seeds, rodents, berries, nuts, eggs, and also garbage and food from pets that are fed outside. Chicks, however, are fed animal matter almost exclusively. Magpies can land on large mammals, such as moose or cattle, to pick at the ticks that often plague these animals. More typically however, they forage on the ground, usually walking, sometimes hopping, and sometimes scratching with their feet to turn over ground litter.
Black-billed Magpies are also known to make food caches in the ground, in scatter-hoarding fashion. To make a cache the bird pushes or hammers its bill into the ground (or snow), forming a small hole into which it deposits the food items it was holding in a small pouch under its tongue. It may, however, then move the food to another location, particularly if other magpies are in the vicinity, watcning. Cache robbing is fairly common so a magpie will often make several false caches before a real one. The final cache is covered with grass, leaves, or twigs. After this the bird cocks its head and stares at the cache, possibly to commit the site to memory. Such hoards are short-term; the food is usually recovered within several days, or the bird never returns. The bird relocates its caches by sight and also by smell; during cache robbing, smell is probably the primary cue.