Through this project, I can see that last year was the motley birch (half yellowed) 27 September, and this year September 20 - this year's autumn has earlier. but the gap is not so great. And what about your birches?
If you read the description of the nature of a sufficiently long poems, you can often find a mention of the leaves of scarlet, red, crimson. "In the garden in autumn, at the track, aspen claps her hands. That's why that week her palms turned red." R. Sef or "ginger-red forest" Pasternak, or "day soon blossom like a fiery maple" in Krapivina. And Pushkin, of course ... What determines the leaves become red or yellow? Are there any red ash or birch? A mountain ash yellow? Do you often meet red aspen leaves? Were you able to watch the tree that comes over every year in different ways?