We have six river birch trees in our yards. Each tree has it's own characteristics as far as leafing out and dropping leaves. One thing they all have in common is aphids. We've learned to treat the trees every year or so and that takes care of the aphids. When a tree has not been treated, the birds and ladybugs can't keep up with the aphids. It took a little while for us to figure out that the leaves not dropping was connected to the aphids. Maybe all that sticky residue the buggers leave behind.