Egg volume, chick growth and survival across a carrion/hooded crow hybrid zone

Abstract

The carrion and the hooded crow (Corpus corone corone and Corvus c. comix) are usually considered as morphs of a single species. They have parapatric distributions with narrow belts of overlap where fertile hybrids are produced. This three‐year study analyzed the variation of egg volume, and of chick growth and survival among allopatric parental populations and phenotypically parental and hybrid sub‐populations in a hybrid zone. Hybrid females laid eggs significantly smaller than those of parental females, living both in areas of sympatry and allppatry. No differences in chick growth were observed between pairs of different composition in the hybrid zone. Nestlings in the hybrid zone were significantly lighter than those in allopatric areas at all ages, and grew to a lower asymptotic weight. Tarsus grew faster and to a higher asymptote in the carrion crow area than in the hooded crow and in the hybrid areas. Chick survival was significantly influenced by female but not by male phenotype. Chicks in hybrid female nests had a comparatively low survival rate. The results indicated that, overall, crows in the hybrid zone are less fit than crows in allopatric areas. In particular, some hybrid females appear to be inferior to parental ones in their laying performance and their chicks are less surviving. The coincidence of the hybrid zone with an ecotone suggests that environmental factors could be effective in determining the observed pattern of variation in the breeding parameters considered.

Key words:

• Corvus c. corone
• Corvus corone comix
• Hybrids
• Egg volume
• Growth and survival