Without seeing the wounds, it is hard to make a diagnosis, but, swans may have small lumps/bumps with cut like structures on the bottom of the joint. If these bumps get infected or are seeping, these wounds turn into what is called bumblefoot. There are several reasons for bumblefoot, i.e., injury to the feet by cuts or scrapes leading to infection and/or constant abrasion with hard substances such as rocky or concrete surfaces, etc.

If the bumps are not that pronounced, you may want to get the swans to an experienced avian veterinarian for antibiotics so that the wounds do not turn into bumblefoot. Once this condition occurs, there is no real cure and the joints will become so large that walking will be difficult. So, the preventative measure is to keep the swans on soft/grassy surfaces and provide antibiotic medication so that the wounds do not become infected. The Regal Swan