The highly variable North American red crossbill complex has presented one of the most controversial problems in avian systematics. Although crossbills wander nomadically and breed all year, they have not become homogenized in bill and body size. This study suggests that this crossbill complex contains several distinctive sibling species that breed sympatrically and show ecological differences.
Evolutionary Differentiation in Morphology, Vocalizations, and Allozymes Among Nomadic Sibling Species in the North American Red Crossbill (Loxia curvirostra) Complex
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