Mate preference: a possible causal mechanism for a moving hybrid zone

The study of patterns and underlying mechanisms within hybrid zones may provide insight into speciation. Black-capped (Poecile atricapillus; BC) and Carolina (P. carolinensis; CA) chickadees hybridize in an east-west band in the U.S.A. from New Jersey to Kansas. Within the past century, the Ohio portion of this hybrid zone and the CA range to the south have been moving northward while the BC range has retracted. We examined mate preference in females of both species as one possible causal mechanism for this shift. To be conservative about the nomenclature and results, the samples are referred to as either 'BC-like' or 'CA-like' due to the observed genetic introgression in the study individuals. Given a choice within an aviary setting, in the aggregate, BC-like and CA-like females that had not observed the direct social interactions between a dyad of a BC-like male and a CA-like male preferred to associate with the BC-like male. In nature, both species form within-sex dominance hierarchies. In the aviary, CA-like males dominated BC-like males unless a CA-like male was substantially smaller. Once females of both types had observed the physical interaction of a particular heterospecific dyad, they associated preferentially with the dominant male, regardless of species type. Thus, the effect of CA male intrasexual dominance on female mate preference may be contributing to the northward movement of the hybrid zone. (C) 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.