We will be receiving two female sister swans soon who will be 5 months old. We have a two acre lake that is completely fenced in, in an urban area, so there are no predators. Let's assume they are in full mature feather by the time we get them, will it be necessary to provide them with a pen? Or since predators were eliminated years ago, can we just release them on the lake? (We will, of course, feed them daily.) They have an island and a peninsula (with a separate fence and gate and hence restricted access) on which to nest.