I just read the article on songbirds in the November, 2019 Scientific American. The article explains that songbirds have an extra chromosome, called GRC (germ-line restricted chromosome) that other bird species don't have, and there's a hypothesis that this may have been partially or wholly responsible for the diversity of this group of species.
The following line caught my attention:
Once the GRC originated in the last common ancestor of songbirds, members of that ancestral species that carried the GRC could produce fertile offsping only with mates that also had the GRC.
But when the mutation that created the GRC occurred, who did that bird mate with in the first place?
My belief is that most genetic changes are relatively minor, so they don't immediately result in incompatible mating. Speciation occurs as genetic changes accumulate over time. But the above quote implies that an entirely new chromosome is too drastic to allow interbreeding at all.
Wouldn't there need to be a whole bunch of birds who all had this extra chromosome at the same time to allow it to spread through the population? That seems unlikely to occur.
Am I just reading the line too literally? Could early birds with the extra chromosome mate with other birds, until the new chromosome accumulated too many changes?