Douglas A. Nelson Ecology, and Organismal Biology Copyright by

ii This dissertation uses an avian hybrid zone as a study system in which to explore potential causes for the configuration and movement of such zones. Stable hybrid zones are commonly considered to reflect a balance between selection and dispersal. Both exogenous and endogenous selection factors have been advanced to explain barriers to gene flow across narrow hybrid zones. Exogenous selection involves adaptation to local environments. In contrast, endogenous selection involves adaptation or coadaptation on the genomic level regardless of the external environment. The focal avian species of this dissertation, black-capped (Poecile atricapilla) and Carolina (Poecile carolinensis) chickadees, are permanent residents in Ohio. Currently, their distributions abut in an east-west hybrid zone in the northern aspect of the state. The black-capped chickadee resides north of the hybrid zone and the Carolina chickadee south. Based on historical data, the hybrid zone has move