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These stubby little nomads are often first detected by their hard kip-kip call notes as they fly overhead in evergreen woods. The Red Crossbill in North America is quite variable, from small-billed birds that feed on spruce cones to large-billed ones that specialize on pines. Scientists have long puzzled over how to classify these different forms. Recently the Cassia Crossbill was given a full species status (previously considered type 9 Red Crossbill). Slight differences in call notes are generally enough to keep the different call types from mixing, and several kinds may occur in the same area without interbreeding.