Plant Diseases/river birch
River birch tree has numerous spider webs around many ends of the branches and they appear to choke off and kill the leaves/branches that are surrounded by the webs.
I am not aware of spiders alone being an issue on trees. They may be there looking for an underlying insect problem. Carefully inspect a "webbed" branch tip for insects and get any unknowns ID'd be carry them to your local nursery/garden center. Don't overlook the possibility of fall webworm on the birch. These pesky critters can defoliate trees, however established/otherwise healthy trees usually recover with no overall significant impact on the health- thanks to other predators.