52 million years ago... but still, because this means that even though nowadays the Southern Hemisphere is much richer in avifauna than the Northern, there was a time when tropical birds did live in the Northern hemisphere. As an example to show how much richer the avifauna is in the Southern Hemisphere than in the Northern one, in my country of birth [which is a teeny, tiny thing, smaller than Washington State] there are more species of birds than in all of Europe!
https://www.sciencedaily.com/releases/2 ... 113347.htm