While no doubt a violent ending for the victim, this is a part of the cycle of life in nature. While the victim gives up its life, it provides the sustenance for the hawk to survive the winter and reach its breeding grounds in the spring. In the winter consider adding a suet cake for the insect eaters. A small suet cage can be hung near or underneath the feeder. See the recipe for suet that is given elsewhere on the BAS Web site.
Homemade suet is generally superior to the cakes that can be bought in stores.
In my opinion, there is no sure-fire way to defeat squirrels that are determined to dine on your sunflower seeds. There are certain systems that are squirrel deterrents, but I have never had a lot of luck with them. Think about it. All they have to do all day long is to figure a strategy for defeating your system. In the end, they usually figure it out.