River Birch infestation
This looks like the larvae of dusky birch sawfly. (They are not caterpillars.) They like to congregate and consume leaves, but usually do not do significant damage to large or established trees. Since they are so gregarious, it is easy to mechanically (i.e. using gloves hands) pull them off. Drop them in a bucket of soapy water to drown. When small, the larvae can be sprayed with horticultural oil, but that is usually more much labor intensive.
Here is info about sawflies: https://extension.umd.edu/hgic/topics/sawflies-trees-and-shrubs
Be mindful that trees grow many more leaves than they actually need. Losing some is not a problem. Also, larvae like this (and caterpillars, too) are a big food source for birds.