Talk to any sales clerk at a backyard bird supply store long enough and you're likely to hear the story about the new customer who wanted to start feeding the birds in their backyard; so they bought a feeder and some seed, and went home happy.
However about two weeks later the same customer called back quite upset. Whenever there was a large number of finches and other small birds gathered on the feeder, a hawk would suddenly swoop through, grab one, and fly away.
Many thanks to James in Florence, AL who contributed this superb image of a Cooper's Hawk, a well known "feeder swooping" species, to the Wingscapes BirdCam Photo Gallery. Indeed, James notes that whenever this hawk appears, all the other birds leave.