I'm tired of the little devils pooing all over my car, but the wife feeds all the other interesting and more rare birds so a scarer probably isnt going to be a good idea.
The only bird scarer I've seen is from when I was a kid, it wasnt silent, it filled up a chamber from a gas bottle then ignited it, bang, probabyl not want you want while your watching Tv though.
Motorola make cheap and powerfull tweeters, Ksn range I think, one of those coupled to a 555 would make a supersonic racket, not sure what frequency birds dont like though, for insects its 20 - 30khz.
If you build one put a photocell on so it shuts off at night, bats are getting endangered at least in this country.