Crows are black birds about 15 to 18 inches long with the annoying habit of congregating in large groups at night. Grackles, which are slightly smaller with more iridescent feathers, and blackbirds, which are even smaller, can also exhibit this behaviour. It doesn't matter which species you are dealing with, however, because the techniques will still work. Methods listed are aimed at trees and shrubs, which are usual nesting sites for crows, but will work on locations such as rafters and railings.
Start scare tactics about 90 minutes before dark. Birds are easier to spook when they are flying and coming in to roost than once they have become established.
Act as soon as you notice the birds. Birds will scare easier from new roosting sites than established ones.
Continue scare tactics for at least a week. It will take this long before the methods are effective. If you quit before then, birds may return. If the birds have been roosting there for a while, it may take longer.
Repeat scare tactics in the morning just before daylight when birds first become active.
Switch or move scare devices every few days before birds become accustomed to them.
Stop any scare tactics once it gets dark. The longer you use a method, the quicker a bird will become used to it. If the birds have not arrived by dark, they are not coming.
Play alarm or bird distress calls over loudspeakers. Distress calls work best when they are of the species you are targeting. Calls should be played for about 10 to 15 seconds every minute.
Tie balloons to branches of trees or bushes where birds like to roost. Change the colours and position of the balloons frequently.
Set up a motion-activated laser "hazing" device. These devices shine a beam of light towards birds and are very effective when used against crows.
Place flashing or strobe lights in the trees.
Set off noise makers such as exploding shells, gas exploders or firecrackers. They do not need to be set off continuously, but rather at random intervals every few minutes.
Spray streams of water into the trees.
Tie long streams of Mylar ribbon or tinfoil to the branches.
Hang aluminium pans in the branches. If you can tie them so the pans will hit against each other, they will be more effective. Old CDs also work, but don't make as much noise.
Set up a scarecrow that looks like a natural predator like an owl, hawk, coyote or snake. Scarecrows are more effective if they move and are moved to different locations every day or two.
Thin out trees and shrubs to make them less desirable to birds. A more open area will make the birds feel more vulnerable to predators and cause them to seek out a new resting area. Combine visual and auditory techniques to make them more effective.
It may take several days before your scare tactics start to work.