This time of year, those caterpillars would be fall webworms. In the northwest, there is one generation per year with the caterpillars appearing in late summer to fall. The caterpillars feed on leaves enclosed in the web (some may venture out as they get older - like teenagers). Despite their numbers, the webworms rarely cause enough injury to harm trees. Fall webworms will feed on a wide range of deciduous trees, including those you mentioned. 

Tent caterpillars feed in a similar manner - spinning silk tents in trees. However, tent caterpillars appear in the spring and their tents are are made in the crotch of tree branches whereas the fall webworm webs are usually made toward the end of branches.