Yes. I have had a ground unit radar operator call out a “small aircraft” in a restricted zone repeatedly that we passed over (without seeing it) repeatedly, only for it to disappear and reappear again in the same general area. It was on the third or fourth pass I noticed two large birds flying wingtip to wingtip exactly where they were saying, and disappeared when the split apart.

So the size of a bird? Maybe not. The size of TWO birds? Yes. Annoyingly so.