More than likely you have two male mutes. During mating season, this type of behavior occurs and they are seen as rivals. If you had no other swans (especially females) on the pond, they would probably bond and pal around for the rest of their lives. With a female swan (regardless of species), this rivalry is going to be present during nesting season.

Black swans also produce two clutches of eggs per year. Normally, black swans and mute swans will not naturally mate. We have seen humans artificially encourage mating between the two species, but normally, this does not occur. The best thing that can be done is to separate the two species so that they are not able to pal around during the mute swan nesting season which is usually March-April.

Black swans are usually the most aggressive species of swans and will take on Mute swans, waterfowl, etc., to chase the co-existing wildlife off the pond. If the black swans become to aggressive, two things are going to occur.

First, the mute swans may fight back and since they are much larger, they could significantly injure or even kill the much smaller black swan.

Second, if the mutes or black swans become too scared of each other and they are chased on land and not allowed in the water, they are going to suffer dehydration (especially in warm summer months), lose weight from not being allowed to eat and finally, be susceptible to predator attack because they cannot enter the water to flee from the predator. If they sit on the bank or go further onto land to escape the chasing/aggressive swan, they may be attacked by a predator because they are too far away from the water. Predators may include wildlife, domesticated pets, cats and dogs and even humans. The Regal Swan