“Radars filter by Doppler not size. That eliminates birds and such.”
Depends what type of radar.
Weren't we talking about search radars? But I can't think of any "modern" radars used in search, acquisition, or tracking that don't use a Doppler filter.
The only type that I can think of that doesn't is an illuminating CW, which would only be pointed at a known target to begin with.