Our pond is fed by a salmon spawning creek, so the vulture could very well have been after a dead fish. This is pretty early for any fish to spawn, although I do believe it is a 'pink year' (humpies) and perhaps they are earlier than the other salmon. Or any of a number of small mammals, there is a large woodlot behind the pond home to all sorts of critters (squirrel, racoon, possum, weasel, etc.)
