HYBRID ZONE OF CARRION CROWS AND HOODED CROWS

AIsTP,CT.--The Carrion Crow (Corvus corone corone) and the Hooded Crow (C. corone cornix) have parapatric ranges in Europe with narrow areas of hybridization. Reproductive biology of populations of Carrion and Hooded crows and hybrids is described for four areas in northern Italy. Nonrandom mating was observed in a hybrid zone. Homogamic pairs were observed more frequently than expected by chance. Data on clutch size and reproductive success were analyzed for Carrion Crows and Hooded Crows in allopatry, and for parental and hybrid phenotypes in a hybrid zone. Clutch size did not vary across the hybrid zone. Pairs with Hooded Crow females in the hybrid zone produced significantly more chicks than those with hybrid females. The number of chicks recorded from Carrion Crow females was higher than for hybrid females, but not significantly. Significant variation was observed for the effect of the male's phenotype on reproductive success due to the comparatively high reproductive success of Carrion Crows in the area of allopatry. No significant difference in the reproductive success was observed in the hybrid zone between pairs containing only parental phenotypes and pairs containing at least one hybrid. Carrion Crows in allopatry produced more fledglings than Carrion Crows in the hybrid zone, whereas Hooded Crows in allopatry had a reproductive success very similar to that of Hooded Crows in the hybrid zone. Our results do not support the dynamic-equilibrium model but are compatible with the bounded-hybrid-superiority model. The hybrid zone studied coincides with an "ecotone" between the alpine valleys and the intensively cultivated plain. We conclude that the main- tenance of parapatric distributions of the two morphs is due mainly to ecological factors. The hybrid zone could be considered the narrow belt in which environmental features are such that hybrids are not less fit than parental individuals, its narrowness being caused by the steepness of the environmental gradient crossing it. The nonrandom mating observed could result from heterogeneous phenotype dispersion which, in turn, reflects differential habitat selection in the hybrid zone. Received 22 April 1991, accepted 10 January 1992.