Congruence between morphological variation and altitudinal gradient across a hybrid zone between carrion and hooded crows

Abstract

Geographic variation of morphology and plumage colour was studied across a hybrid zone between allopatric populations of the carrion crow (Corvus corone corone) and the hooded crow (Corvus corone comix) in northwestern Italy. Principal component analysis on plumage colour scores and canonical variate analysis on 37 morphometric variables indicated that a clinal geographic variation in plumage colour and morphology existed across the hybrid zone. Morphology and colour showed congruent (P < 0.005) variation. Plumage colour variation was strictly correlated with altitude (P < 0.001) and the same held true for morphology (P < 0.01). Parental allopatric populations of the carrion crow and the hooded crow inhabit different habitats in northern Italy. The hybrid zone corresponds to a piedmont belt, through which a steep ecological gradient exists. Present evidence could suggest that carrion and hooded crows have parap‐atrically diverged under the effects of ecological selective gradients. However, congruence in variation of different character systems argues in favour of a ‘phylogenetic’ origin of this hybrid zone.