Sexual dimorphism in color and pattern of contour feathers is rare in juvenile songbirds. We describe how captive-bred juvenile males of Scottish Crossbill (Loxia scotica) and nominate Red Crossbill (L. curvirostra curvirostra) can be differentiated from females prior to prebasic molt by an unstreaked patch on the males' upper breast. There may be a functional relationship between sexual dimorphism and the formation of pair bonds or breeding while the birds are still in juvenile plumage. Sexually dimorphic Red Crossbills and Bearded Tits (Panurus biarmicus) are known to form pair bonds, and even breed successfully, while still in juvenile plumage.
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