I’m no farmer and haven’t dealt with crows per se. But I’ll offer my two cents based on Cecil’s column.
In the city of Austin, grackles are the biggest bird pests. And I’ve seen people trying all kinds of ways to scare off grackles. No, nobody’s used the classic scarecrow, but Seton Hospital has loudspeakers that periodically play the sound of a screeching hawk. And several shopping centers put plastic owls in strategic places.
Now, the big question: do the hawk sounds or the plastic owls scare away grackles? Cecil put it well: Yes and no. Yes, they scare grackles away for a little while. But grackles aren’t stupid- in a few days or a few weeks, they figure out that there aren’t any real hawks or owls around, and they come back.
Indeed, at Seton Hospital, I’ve seen grackles perched nonchalantly on poop-covered speakers, and at the Arboretum shopping center, I’ve seen grackles perched on poop-covered plastic owls.
Crows are smart birds, at least as smart as grackles. You may be able to fool them once, but they’ll figure out the trick pretty quickly, and you won’t fool them twice.
At my company we had been having problems with a small group of Canadian geese flying in, making nests, and generally pooping everywhere in front of our building. The pooping is especially unwelcome when it is at our main entrance for visitors.
Then, about two months ago, a brilliant solution was found. Two large flat pieces of black hard sheet (not sure of the material) were cut into the shapes of slightly-cartoonish wolves (one with tongue lolling out, and another howling up) and propped up with metal stands on the bottom. The silhouettes from afar look like some kind of large black canines.
After setting them up in front of our building and moving them about once a day, the poop problem completely went away. Not a single sign of geese in front of our building. Away from the front of the building (namely, the employee parking lot), though, the geese still wander about and poop on stuff.
In some areas of Nepal they take old audio cassettes and use them to great effect. Tying one end to a stick and then stringing out the tape across the field to another stick. The tape, because it’s flat, makes an interesting (possibly scary to birds) sound whenever there is the slightest wind which keeps the birds away. The effect lasts long enough for the rice to germinate and sprout which is obviously the most important part of growing food.
OTOH, we had some geese who decided that our collie was okay and would let her gently herd them out of our yard. She’d then sit, alert for any coyotes (she hated coyotes), while the birds ate and rested. You could see how some animals got domesticated.
I looked at this in my first book. There’s plenty of anecdotal evidence, mostly in favor of the effectiveness of plastic owls, but the best solution of all seems to be moving predator-shaped “scarecrows”. Michael Conover (once of Utah State University in Logan) wrote papers about the effectiveness of motorized predators. Prof. Ronald J. Prokopy of UMass Amherst wrote about the only paper I’ve seen that put numbers on the effect. He found that the use of inflatable balloons with owl-like eyes, especially ones with lenticular-type eyes that appeared to move, were devastatingly effective in keeping many varieties of birds away from fruit trees. Without the ballons, fruit was ravaged at a ate of about 12.2 %. After he put up the ballons, damage was down to 1.5 percent within six yards of the balloons. After he placed ballons every twelve yards throughout the orchard, damage was down to 0.4 - 0.9%. Prokopy found these most useful againmst crows, starlings, bluejays, and possinly blackbirds.
I believe a very similar thing was used in ancient Rome, where so-called oscillate – faces made of bark or ceramic – were placed in trees. And not just any trees – olive trees, pine trees (with their pine nuts) and grape arbors, all with fruit attacked by birds. Furthermore, some of the ceramic oscillate had punched-out eyes that could appear to move, and were sometimes equipped with ceramic “clappers” so that the oscillate would make noise. (I wrote these up in my article “The Scarecrow of Os” :
Wilk, Stephen R. “The Scarecrow of Os: The Function of Antefixes, Oscilla, and Suspended Masks in the Roman Garden.” Classical World 107.3 (2014): 383-392.
I note that starlings, in particular, are the most devastating in olives crops, and thatvProkopy found Scare-Eye balloons most useful against them.
Interesting that the MCG (Melbourne Cricket Ground) had an ongoing issue with Seagulls on the ground, particularly at night (Australian) football matches. Large crowds and lights equal food.
A trial of using actual live wedge tailed eagles perched high on top of the grandstands with a handler had some impact but late last year they did the wires trick. Running wires from one grandstand to another and it seems to have worked,
On my airfield in Wiesbaden we had some bird cannons that worked quite effectively. They were spaced along the length of the runway, and once activated fired off in series. It was fun to take new folks out near our radar, have the tower fire the cannons, and shout “the Germans are invading!”