they remove every spec of water from the whole area and only leaving a small pond (quite likely to be only about 50cm in width) so that the animals have to come close enough for the magic to work
that would be my guess
If the birds can go at the speed of sound and are as sturdy as dragons, there is no reason why those birds can't just rip through living things like a bullet. I'll be surprised if those things would actually have a natural predator.
The lake is frozen so it will search for any source of water, even one as small as a pond or a puddle. Once all the birds are near that one spot, they will be an easy prey.