What could be stripping leaves from just one of my cherry trees?

Birds are one possibility, especially if the damage is uneven or concentrated on the younger leaves near the ends of branches. Pigeons can feed in one tree and leave the others alone, so the untouched trees don’t rule them out. Watching the tree for a short while may help confirm it.

If the marks are very neat, rounded cutouts rather than ragged chewing, leafcutter bees are another possibility. They remove tidy pieces of leaves and generally aren’t a reason to treat the tree. That pattern looks different from leaves that have been shredded or torn.

I’d check the damaged leaves closely before deciding. Look underneath them and along the affected branches for insects, fresh damage, or clean cut edges, then compare those clues with what you see on a less-damaged branch. No visible caterpillar or droppings is useful information, but it doesn’t identify the culprit by itself.