It could be Birch aphids. I have seen these leggier than normal aphids clustered on leaves and yes, they quickly turn yellow.

Part of the reason for this is not just the direct feeding activities of the insects but because the tree detects this attack and starts to manufacture toxic chemicals to repel the insects. This takes a lot of energy and resource from the tree and it will abort some older leaves, not before withdrawing the vital substances from these leaves first and then aborting them. Interestingly, any birch trees downwind detect the chemical signal from the tree under attack and start manufacturing also!