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

Geographic variation of morphology and plumage colour was studied across a hybrid zone between allopatric populations of the carrion crow (Corvus coro ne corone) and the hooded crow (Corvus corone cornix) in northwestern Italy . Principal component analysis on plumage colour scores and canonical varia te 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 colo ur variation was strictly correlated with altitude (P < 0.001) and the same held true for morphology (P < 0.01). Parental allopatric populations of th e carrion crow and the hooded crow inhabit different habitats in northern I taly. The hybrid zone corresponds to a piedmont belt, through which a steep ecological gradient exists. Present evidence could suggest that carrion an d hooded crows have parapatrically diverged under the effects of ecological selective gradients. However, congruence in variation of different charact er systems argues in favour of a 'phylogenetic' origin of this hybrid zone.