I get that the changing weather might change their habitual latitudes but there was and is always some "boundary" between the two, no? So there was always a boundary but it moves north or south depending on warming or cooling climates (these birds have been around for millions of years). How did they only mate now?
A lot of the differences between species is due to behavior issues, not actual physical difficulty. It's likely that both species of jay mate at different times or display different mating signals. They've been separated for something like 75 million years which leaves plenty of time for their behaviors to change.
That sounds very, very wrong on the biological timescale.
(They have been renamed to "Canada Jay," but that's a hilarious story for another day)