This was great, Parker. I guessed that Issac was hearing the call of a loon. The cry is unmistakable, once you've heard it. Nice take on the prompt. I really enjoyed it. Oh, and I got a big laugh out of the following paragraph :
Isaac had met a few other shifters in his varied life – all of them powerful, almost larger-than-life individuals – and predators. Those were the real shifters. There was a huge silver bear in Florida, an incredibly handsome and cocky wolf in Minnesota, and a sleek goshawk in Los Angeles, of all places. All way out of his league. And there was that mountain lioness in Colorado, who stalked him along the edge of a bone-chilling mountain lake until he had shifted to his avian form, preparing to fly. Only when Isaac heard her human gasp did he understood he'd been trailed by a fellow shifter. At least she'd apologized.