OP,
I keep a couple of birds. They're flighty with anything they don't trust, and it's hard to get them to trust anything. Even new food is something they'll run from.
You may not be a threat, or have been the last 5,000 times you road by. But they don't know that cause they ran away the last 5,000 times: Cause you could have been a threat.
If a bird can't get into the air it's done for if you turn out to be a threat. And taking off is a relatively slow and difficult process. Once it's in the air it's safe from ground animals. So it opts to fly to the nearest tree, wait for you to leave, and return to scavenging.