“…Hybrid zones can be relatively wide and dominated by morphological and genetic intergrades between the parental taxa (e.g., Sibley and Short 1964, Irwin et al 2009, Vallender et al 2009, Den Hartog et al 2010, or relatively narrow and dominated by individuals that are morphologically and genetically close to the parental taxa (e.g., Barrowclough et al 2005, Leafloor et al 2013. At the possible extreme, one study found little evidence for interbreeding between two sympatric and closely-related passerine species, and possible hybridization only can be speculated (Toews and Irwin 2008).…”