(I have nothing against birds.)
Suppose you’re trying to keep flocks of small birds away from an old barn that sees weekly use and the doors get left open. Any suggestions as to how to do this? Without harming the birds.
Are there bird repellents? A fake hawk on a rotating mobile?
Propane cannons are effective.
Your neighbors might not like them. 
What kind of birds, what kind of problem?
Barn swallows that make the mud nests and splatter poop down the walls and all around? Or miscellaneous birds that get inside and can’t get out?
For the first, you almost have to put netting over the areas where they like to nest. For the second, cover the windows so the only bright area is the door they came in through.
Bird scarers can be made from CD disks on fishing line, spinning in the air. And you can get hawk or owl statues.
Hey, I’m giving a GQ answer in IMHO. Well, IMHO, you take the four and twenty blackbirds and bake them in a pie with garlic, salt and pepper, and a sprinkling of nuts. As to whether that’s better than propane cannons, it depends on whether you like food or excitement better.
I don’t think the owl statues are as effective as they once were. The birds have gotten wise. What you need is some sort of whirlygig, windsock, weathercock, gizmo that is in the shape of a flying hawk or falcon. It should cast a shadow on the ground that looks like the predator in flight. Since it would be constantly moving it would be more effective than the stationary statue.
Putting shiny bits of foil or cd’s on a line is supposed to work, too, but it might depend on the type of bird you’re trying to scare. Crows may like shiny things.
Forget the technical whirligigs. They won’t pay off long term.
Go low tech. Get a couple of cats and place some cat food out in the barn. Get yourself a small, independent colony of farm cats and they’ll keep the birds (and rodents) under control.
There’s a company called Bird-X that sells bird deterrents of all types.
Ceramic/plastic owls haven’t lost their effectiveness – but they were never 100% effective (nothing is).
It turns out that “scarecrow”-type devices are more effective if they move. The “Scare Eyes” and “Terror-Eyes” balloons are good for this.
http://www.bird-x.com/products/terror.html
Just note that these don’t work for all birds. I’ve found independent scientific testing of this kind of thing will deter starlings, blackbirds, robins, and jays from fruit orcghards, but they won’t do a thing for, say, barn owls.
The ancient Greeks and Romans had devices like these – I gave a paper on it at last year’s New England Classical Association!
When I had a barn and a pigeon problem, I’d just go out every evening and shoot them with a pellet gun. If I could get them before they saw me coming, it took a while before new pigeons found the place.
I once sold satellite dishes.
After one install the owner asked me if there wasn’t a way to keep the birds from sitting on the feedhorn and messing up the dish.
I had heard that you can keep birds away by using a snake replica.
I bought one and wrapped it around the feedhorn.
While checking my handiwork a couple of days later I noticed that the snake was gone.
I bought another and it to disappeared.
I stopped by the house and the owner said," You know somebody keeps putting rubber snakes on the dish "
I asked him what he did with them and he said he just threw them away.
So I can’t tell you if it works or not but its cheap and just might work.
Hm. Actually, noise might be a good option.
Hm. Could be part of a solution.
Ha!
There are places cats wouldn’t be able to get to.
Actually, barn owls or hawks would probably be welcome if they kept the other birds away.
I think you’re right, the first order of business is to patch holes and stuff, cover attractive areas.
It’s actually not my barn, it’s a place being used for a museum near where I live. I’ll pass along the suggestions! Thanks.
The local orchards hang metallic streamers in the trees to scare away birds. Unfortunately they break down after a while and need to be replaced.
Contact your local falconer’s group and offer them the barn for training. 
Methyl Anthranilate
I used it to keep birds out of a warehouse. It was a spray that smelled like grapes. Worked well. It was given to me to use, I have no idea where to get it.