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These stubby little nomads are often first detected by their hard kip-kip callnotes as they fly overhead in evergreen woods. Red Crossbills in North America are 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. New research suggests that there may be as many as eight different full species of Red Crossbills on this continent. Slight differences in callnotes are apparently enough to keep them from mixing, and several kinds may occur in the same area without interbreeding.