Do scarecrows actually scare away crows? Or anything else that’s evolved in the last billion years?

When the wind blows they do, if you have enough loose cloth.
That’s why they look so ragged.

Stationary devices intended to scare off birds generally work for a short period of time and then become much less effective as birds acclimate to their presence. After all, these devices are just “standing there” being quite predictable. Scare or Terror Eyes, a balloon with eyes, claim that it is “a totally realistic and scary predator!” I think most predators actually chase their prey (at least avian predators). As for the more realistic plastic decoys of owls I’ve seen in use, more often than not, I’ve also seen their intended targets (doves) sitting on the rail next to them. These devices may be more effective if placed out for short periods of time and moved around periodically.

Owls (Great Horned Owl Bubo virginianus) are the major predator of crows (Corvus brachyrhynchos :)) and you might think they would be pretty effective at scaring crows off. What the decoys do, however, is to induce mobbing behavior - crows go absolutely apeshit when they spy an owl (alive or plastic) during the day. I catch birds for a living and the only time I caught a crow in a net was to place an owl decoy with a “dead” crow decoy at its feet in front of a net. About 60 crows came within minutes of placement and they dove around screaming like banshees and avoiding the net. Except for one.

I never knew that owls ate crows but that mobbing does describe exactly the behaviour of a bunch of them I saw one time when I’d heard an almighty commotion high up in some trees once, and yes the subject of it all was an owl.

The owl was not a big one, I think it might have been a Barn owl, and I’d have thought it too small to take on a crow but they definately did not want it around.

casdave!, I think the Barn Owl’s (Tyto alba) diet is primarily composed of small rodents with an occasional small bird (crows are too big). Smaller birds will mob predators if discovered during the day and crows, attracted to the ruckus, are likely to join in the fun.