They are much lighter than airplanes with radar and heat signature similar to birds. Even the larger ones might look like several birds flying close in formation on radar.
Large flocks of birds tend to fly together right after sunset when these drones have been seen. So getting a visual maybe the only way to differentiate
“So getting a visual maybe the only way to differentiate”
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Amazing night vision binoculars (that can see birds) can be purchased for under $200. Imagine what the powerful military versions can see.