A section of chromosome two in this species carries a large inversion, a stretch of DNA flipped end-for-end. Because inverted and non-inverted versions cannot recombine along most of their length, the inversion behaves as a single inherited unit covering many genes, including some affecting hormones and behaviour.
Birds carrying the inversion are white-striped; those without it are tan-striped. The two morphs differ consistently in behaviour: white-striped birds are more aggressive and sing more, tan-striped birds provision nestlings more. Pairing is strongly disassortative, with white-striped males almost always mating with tan-striped females and the reverse, so the effective mating categories are four: two morphs by two sexes.
The system is stable rather than transitional, and researchers describe the inversion as behaving like a second pair of sex chromosomes. It is the clearest vertebrate example of a behavioural polymorphism locked to a chromosomal rearrangement, and it persists because each morph does best when paired with the other.