Faust, I like your theory, but I'm with the other posters that it doesn't hold water.
The most obvious problem is the fact that there would be evidence that a mutilation was done by a predator. For a start there would be marks on the
back of the cows where the Thunderbird picked them up. Then a bird that's able to lift up such big prey, won't just pick out certain body parts
("Autopsy reports show that removal of sexual, digestive tract, and sensory organs are most often the organs of choice. Soft tissue organs, such as
the reproductive organs, tongue, eyes, lips, ears, udders, and the rectal area are also occasionally removed") with "surgical precision"... And
there is no explanation for the absence of any body fluids.
This site:
Cattle Mutilations basically explains why your theory doesn't work.