What kind of bird seed is it? a mixture? My birds love black oil sunflower seeds and I buy 40 lbs at a time and it goes quickly. They will eat 3 lbs in about a day. I dont know but perhaps the hawk is nearby and you cant see him/her. One time in my backyard a hawk swooped at a bird who was heading for feeder but it did not stop the birds from coming back later.

I just wondering.... do you have squirrels? If so, I'm surprised you still have full feeders unless you have a baffle to keep squirrels off. I had to move my feeders from trees to middle of yard away from trees and use a baffle.