Painted Buntings (Passerina ciris) breed in a variety of habitat s across the southern United States; however, a 500-km gap divides the species into eastern and western populations with dramatically different molting schedules. By contrast, the closely related Indigo Bunting (P. cyanea) is syntopic with Painted Buntings, but its range includes the 500-km gap. To date, no well-supported hypothesis explains the gap in the range of Painted Buntings. We used MaxEnt to describe ecological niches of both species and performed comparative analyses of model results to evaluate niche similarity between the two Painted Bunting breeding populations and the range gap. All present-day niche models for both species predicted a single contiguous breeding range, which suggests that the gap in the Painted Bunting range is not bioclimatic in origin. Comparative analyses of the three different environments suggest little bioclimatic divergence. Distribution models during the Last Glacial Maximum suggest that Painted Buntings likely bred as far north as ∼28°N latitude, with two disjunct populations in what are now Florida and northern Mexico. Although alternatives exist, the most parsimonious explanation is that the Gulf of Mexico serves as a migratory divide and there are fitness costs to birds attempting to fly around or over the Gulf to reach their molting or wintering grounds. This was a primary factor contributing to the origin of the current allopatric breeding distribution. Historical distribution models imply that the species may not have filled the 500-km gap as their breeding range expanded northward; divergent molting schedules may reinforce the existing range disjunction.