“Radar actually picks up on lots of things in the atmosphere, like bugs, sharp temperature changes, and even birds,” KARE Meteorologist Ben Dery said.
https://www.king5.com/mobile/article/news/verify/weather-verify/yes-birds-can-appear-on-weather-radar/536-c29f29bd-3df5-46e6-a07b-1fe11f1fdd9a