I think that you can see the problem here - if you're zipping along at a low altitude, you are going to smash into a large number of birds and cause a lot of damage. As a result of this damage during the Vietnam conflict, as well as normal non-military flights worldwide, aircraft builders started to test their designs for resistance to bird strikes. In reality, they could shoot any bird at the test designs. One would guess that swans and pink flamingos could also be used, but this would anger many. Let's face it, when it comes to choosing a bird, the lowly chicken becomes the prime candidate.
Chickens are cheap and common, so they are ideal for the bird test.
Now before you start screaming about cruelty to animals, I should point out that the testers use dead birds. You know, carcasses. The birds were on their way to someone's dinner table, but instead they took a detour to become a projectile in the rooster booster. Yes, these are heroic chickens. Apparently, the majority of these chicken cannons work off of compressed air.