Hm.. I see a lot of dead birds here. But there's a feral cat that is killing them. They are very obviously ripped apart..
Maybe some disease? Or honestly, it cold be the kids and their airsofts. I don't think that the plastic balls they shoot, would cause a bleeding or obvious wound. I would get them on video doing it and turn it into local law enforcement. Gun usage and misuse is a sore subject for me, as I love responsible gun ownership and shooting. I might even go so far as to say that they were shooting at me.. heck, if they are going all nuts with them anyway, who's to say a stray ball didn't hit your property?