chief reliance

See: mainstay
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Whitaker, Jr.: "Small birds, mice, snakes, snails, slugs, insects, and other small animal life are all legitimate prey of the chipmunk, but its chief reliance is on small seeds, berries, fruits, and nuts." My suspicion is that the chipmunk you observed has "learned" this behavior, and may have even become something of a specialist in stalking and killing small birds.