Insects are not active outside in the winter, but deer are. This looks like deer browse. Yew are a favorite food of deer, unfortunately.
You may want to apply a deer repellant (Liquid Fence is effective) if you do not have a great deal of deer pressure. If you have a big, or growing, deer population, you may have to replace the yews.
Thanks Ellen. The shrubs are in a fully fenced yard in Logan Circle, so deer would not be able to get them. Do you have any other thoughts? Could it be birds?
Deer can jump a 6 foot fence from a standstill. Hopefully yours is higher.
That's not typical bird feeding. They feed on insects and berries primarily, not leaves.