Whether the swan will stay is an issue that has many possibilities. If the swans are new birds that you are introducing to the habitat, then both need to be kept in the cage. If either bird has not had a chance to assimilate to the new habitat (you or the feeding system, the parameters of the habitat, predators (wild and domestic), then they will wander and may not survive. If they are older birds, not bonded and you see fighting in the cage, then you are going to have a bigger issue once they get out, because the one placed on the pond will become the owner of the pond and may not let the other bird come out of the cage or stay in the habitat (pond for bathing or eating, or habitat for preening and grazing). If there is any sign of fighting, then you may need to find one of the birds a new home. There are no guarantees that the bird you keep will ever take another mate if these two have not bonded. If they have bonded, then both need to be released at the same time. Is the size of the pen the issue? If so, then you have a dilemma because both swans need to be kept in a pen for at least 1-2 weeks to assimilate to the pond. How long have they been in the pen? If you can keep them for at least 1-2 weeks, they should be okay to release onto the pond. If you have any questions or can provide us with more information, please do not hesitate to contact us. The Regal Swan