I can find no verifiable evidence to show frozen birds are used in these tests. Yes, the testers do purchase frozen birds, but thaw them before use.
I have looked through the engineering archives of the USAF and NASA and found nothing.
I would appreciate any evidence for the intentional use of frozen birds in bird strike tests for military or commercial aircraft testing.
Here is a typical link to see that explains only non-frozen birds are used.
BAM! The science of bird and aircraft collisions - National Research Council Canada