Lots of it! I noticed my MPG had dropped a lot and power was down. Decided it was time for a tune up and replaced plugs and when I got to the air filter, this is what I found. 80% of it was clogged. Bird or mouse, I don't know, but they sure liked seeds!
That is exactly what just happened to my 2000 VFR. I took it out of storage this year and the bike ran terribly. Idle was all over the place, power was down, and a huge drop in MPG. Replaced with a K&G and the bike runs great again. Its amazing how those little bastered can get in the airbox