Someone will probably post that it is a bad idea... they usually do with this sort of thing. It is often entertaining, and occasionally enlightening, to hear what they have to say, so let's hope for it...
I've done it with no ill effect that I could determine. Try a little and see what happens. My only concern is that bird seed will be pricey compared to mill scratch. If you have access to it in quantity for some reason, then lucky you!
Scratch is kinda overblown. It should be kept simple. It should consist of whatever cracked grain is available at reasonable cost. Often this means second grade grain and is usually some combination of corn and wheat. It should be fed sparingly and not as the main diet.
Check with your area grain and feed mills and see what they offer. It is usually cheaper to get it this way. I get a 50# bag for $7-8, depending on what's available. If I could buy and store it in ton-fractional lots, it would be even less.
Note: it has gone up since the last time I got some.
