This is a difficult question for us to answer, because we are not sure the species of bird that you are referring to that was attacking the swans. If the birds were large raven/fish crow/hawk type birds, then their main goal was to get to the cygnets. If the bird was vulture like, then it would mean that one of the cygnets or parents might have been severely injured/sick and death was imminent which mean that the vultures were going to wait until their demise. If each of the birds were attacking both adult swans, then it could have been a territorial issue. You might try to contact Wendy Hermon, Swan Lifeline in Eton, England to see if she knows the area and might be able to tell you why the birds were attacking and the species. The Regal Swan