You should report this to local ag office and ask where your local agency who tests these birds for free for public safety service..In our case cornell university tests and reports to CDC id there is an outbreak of west nile ect...we had a rash of birds die at our barn several years ago.. would walk in the barn and you would find a few dead every other day..It was West nile virus...They had us collect with gloves a bird or 2 in doulble ziploc back sanitize the outside of bag with a clorox solution and put inside 2nd bag and put in a freezer til we could get it to them.. Your local police should have those numbers as well//..They could have gotten into some kind of poison , but again public safety the canarey in the coal mine, if there is poison laying around somewhere little kids can get in it. The tests can tell for sure what is going on...you might even be seeing the beginning of a bad mereks outbreak year