When an airborn object is exposed to radar, it reflects the radio waves back to the source. The source then determines what the object is by analyzing patterns in the reflected radio waves. These patterns are called radar profile.
Every object has a radar profile, including rain, snow, birds, and insects. Most military radar stations have filtering hardware designed to ignore these smaller objects. Without the filtering, the radar screen would be full of blips detailing the locations of all the birds and insects within range.