BARNSTABLE — A baby Great Horned Owl is fighting for its life at the Birdsey Cape Wildlife Center after its nestling and an adult owl died from ingesting what seemed to be rat poison. “Sadly, one (tainted) mouse brought to the nest and shared with the family could sicken them all,” said Zak Mertz, executive director of the Cummaquid center that is run by New England Wildlife Centers. The surviving owlet was given a shot of vitamin K and fluids. Mertz said the owl, which was rescued Wednesday afternoon, seemed to be doing well during a medical exam Friday morning. "The owl was very perky, bright and alert, and he was eating," Mertz said. "In our world, if they're eating, that's a very good sign."