I have not seen a Red-winged Blackbird on a bird feeder before, but evidently it is not unheard of. Here is what Wikipedia say about the diet of these birds:

The Red-winged Blackbird is omnivorous. It feeds primarily on plant materials, including seeds from weeds and waste grain such as corn and rice, but about a quarter of its diet consists of insects and other small animals, and considerably more so during breeding season.[23] It prefers insects, such as dragonflies, damselflies, butterflies, moths, and flies, but also consumes snails, frogs, eggs, carrion, worms, spiders, mollusks. The Red-winged Blackbird forages for insects by picking them from plants, or by catching them in flight.[13] In season, it eats blueberries, blackberries, and other fruit. These birds can be lured to backyard bird feeders by bread and seed mixtures and suet. In late summer and in autumn, the Red-winged Blackbird will feed in open fields, mixed with grackles, cowbirds, and starlings in flocks which can number in the thousands.

Red-winged Blackbird - On a Bird Feeder

Red-winged Blackbird - On a Bird Feeder