I believe that coyotes can and will kill even adult deer (especially if there are two or more coyotes)--deer are faster, but just like any dog--the coyote will wear them down. They can be hard on the turkey population as well and that is something I really resent. As for the deer, overall, it might be possible for them to keep the herd healthier by keeping the numbers lower and if deer overpopulation is a problem in your area then perhaps they could help (perhaps). Another Problem however, just as wolves will prey on mature bulls after the rut, coyotes will also prey on post-rut bucks (who defenses and energy are low at the time). The good thing is that unlike wolves in the west, we can take a "pop" at coyotes whenever we see one and I say "shoot, when possible"