Maintenance of a hybrid zone: The role of female mate choice

Hybrid zones between different plumage morphs are common in birds. These zones can be maintained by (1) divergent selection pressures on either side of the zone or (2) some restriction to mating between the forms that limits gene flow from one side to the other. In eastern Australia, there is a distinct hybrid zone between two plumage forms of the Australian Magpie (Cracticus tibicen), with black-backed birds in the north, white-backed birds in the south, and both forms plus intermediates in a zone ∼100 km wide. On the basis of social groups, there is no evidence of assortative mating in the hybrid zone. However, extrapair fertilizations (EPF) occur in other Australian Magpie populations and, thus, may also occur in the hybrid zone. We examined evidence of EPFs in the hybrid zone to test for either (1) positive assortative mating or (2) preference for the brighter plumage form. Although there were significant levels of EPFs of ∼30%, there was no evidence for positive assortative mating or preference for the brighter white-backed males. Other explanations for the current distribution of the hybrid zone and its maintenance will need to be investigated.