Variation in social behavior and plumage in the white-throated sparrow (Zonotrichia albicollis) is linked to an inversion polymorphism on chromosome 2. Here we report the results of our comparative cytogenetic mapping efforts and population genetics studies focused on the genomic characterization of this balanced chromosomal polymorphism. Comparative chromosome painting and cytogenetic mapping of 15 zebra finch BAC clones to the standard (ZAL2) and alternative (ZAL2 m ) arrangements revealed that this chromosome is orthologous to chicken chromosome 3, and that at a minimum, ZAL2 and ZAL2 m differ by a pair of included pericentric inversions that we estimate span at least 98 Mb. Population-based sequencing and genotyping of multiple loci demonstrated that ZAL2 m suppresses recombination in the heterokaryotype and is evolving as a rare nonrecombining autosomal segment of the genome. In addition, we estimate that the first inversion within the ZAL2 m arrangement originated 2.2 6 0.3 million years ago. Finally, while previously recognized as a genetic model for the evolution of social behavior, we found that the ZAL2/ZAL2 m polymorphism also shares genetic and phenotypic features with the mouse t complex and we further suggest that the ZAL2/ZAL2 m polymorphism is a heretofore unrecognized model for the early stages of sex chromosome evolution.