The string may work against owls, herons, egrets and hawks, but will not protect the young swans from foxes, coyotes, bobcats or other predators that can dig under or crawl/climb into the pen from above. No matter the age of the swans, if they are held in a pen, the pen needs to be reinforced with chicken wire top to bottom so nothing can dig or climb. We have had experience with predators accessing topless pens and killing the birds inside, even in areas where the owners never had a problem or did not think predators were in the area. The Regal Swan