Tree trimming - Burr Oak, and River Birch
Most trees should be pruned in mid-late winter. Insect and disease activity is not a threat at that time. River birch trees are pretty immune to most of these assaults and therefore can be pruned at other times of the year.
They "bleed" sap quite freely when cut in the spring. While this is usually not a problem for the tree, it can be disconcerting for the owner when sap drips on outdoor furniture, walkways and people.
The best time to trim River Birch is in the late summer - fall. This would be a good time. Never remove more than 1/3 of the crown. If your tree needs a serious haircut, try to stage the work over a year or two.
Burr oaks can be pruned in any month with a "r" in it.