Haha! First of all which sparrow are we talking about African or European?
Back long before RADAR there was this method of determining speed...
Clocking
Now I know everyone here understands relativity and speed so I will skip breaking out the chalkboard for now. However if little bird (clouds or whatever) is moving at a steady and constant pace but our friend increases rate of travel then we got a problem.
And back in a previous career we had to visually estimate a vehicle's speed within 5 mph before we could even flip the radar switch. Got retested every year just so we could play with the neat toys.
We never saw the other person's speedometer and won everytime.