Several years ago the hawks were taking baby ostriches. The fences were tall enough that I could string fish line back and forth across the top of the pen and then tied bright ribbons on it. They tried one more time, a lot of the fish line was in a big tangle. No hawk was present and no babies were missing. I replaced the line and tied more ribbons on it and it never happened again. Depending on their situation this may or may not be a solution for the song birds.
Hawks primarily hunt prey from the air. So if a feeder is under a branch our something (thin enough so a squirrel can't climb on it) it's unseen by a flying hawk. If a hawk is sitting in your yard (which is uncommon) the only humane answer is to take the feeder down for about two days - the hawk will move on but feeder birds will return.
It may alleviate some worries that hawks only catch songbirds very occaisionaly. Birds are quick and alert and fly away. Mostly hawks catch mice, rats and squirrels
Owen Yost (Landscape Architect)
Wild Bird Center Denton