@Secuono,
This may sound different, but I rather enjoy figuring out who killed the birds. A bird who-dunnit, I suppose you could say.
It sounds like you have more than one predator. It sounds like a cougar, because of the small cow, etc. But bobcats and wolverines, if they are around, cannot be ruled out either.
Regarding the mass deaths, could be a clan of weasels/mink/etc. They will "over kill" and return for food. They will essentially kill everything in sight, and often leave bodies and body parts all over. Weasel/mink deaths suggest rodent control needs to be addressed.
Owls will pick off a bird at night. Typically one or two birds. Owls will take and eat the head and leave the rest of the body.
Cats will take an entire animal and leave little to no sign, and you will typically not every find the remains.
Racoons and opossums will reach in, pull through, and rip chicken wire. They love to rip out the birds bellies and eat the contents. You will often find a carcass under something days to weeks later.
Eagles, will either carry off the food (they hunt during the day) or if no one is around, they will eat the bird where they caught it and leave one big mess along with a stripped clean carcass. (If they are feeding young, the carcass goes.
Dogs leave feathers and blood and there is evidence of a chase and usually a body to be found.
Any more "clues" for this arm-chair bird-predator detective to consider?