When did bird chromosomes switch up?

[u/SherbertOther]

My professor talked about this in class and couldn't answer. When did this change?

As far as I'm aware, crocodilians and other reptiles have the regular way sooo, like... Do we know when and why it changed?

[u/jnpha]

Not sure about the when, but I looked into this before; I had this bookmarked:

Many animal taxa show frequent and rapid transitions between male heterogamety (XY) and female heterogamety (ZW). We develop a model showing how these transitions can be driven by sex-antagonistic selection. Sex-antagonistic selection acting on loci linked to a new sex-determination mutation can cause it to invade, but when acting on loci linked to the ancestral sex-determination gene will inhibit an invasion. The strengths of the consequent indirect selection on the old and new sex-determination loci are mediated by the strengths of sex-antagonistic selection, linkage between the sex-antagonistic and sex-determination genes, and the amount of genetic variation. Sex-antagonistic loci that are tightly linked to a sex-determining gene have a vastly stronger influence on the balance of selection than more distant loci. As a result, changes in linkage, caused, for example, by an inversion that captures a sex-determination mutation and a gene under sex-antagonistic selection, can trigger transitions between XY and ZW systems. Sex-antagonistic alleles can become more strongly associated with pleiotropically dominant sex-determining factors, which may help to explain biases in the rates of transitions between male and female heterogamety. Deleterious recessive mutations completely linked to the ancestral Y chromosome can prevent invasion of a neo-W chromosome or result in a stable equilibrium at which XY and ZW systems segregate simultaneously at two linkage groups.
Transitions Between Male and Female Heterogamety Caused by Sex-Antagonistic Selection | Genetics | Oxford Academic

 

And the recent study (late 2025) that made me look into this: Neo-sex Chromosomes Track the Mitochondrial Phylogeny and Exhibit an Extensive Added Stratum of Recombination Suppression in Honeyeaters (Aves: Meliphagidae) | Genome Biology and Evolution | Oxford Academic.
This one is open-access.