Claire M. Curry & Michael A. Patten, Current and Historical Extent of Phenotypic Variation in the Tufted and Black-crested Titmouse (Paridae) Hybrid Zone in the Southern Great Plains, The American Midland Naturalist 171(2), February 2014, עמ' 271-300 doi: 10.1674/0003-0031-171.2.271