There is no way to determine the gender of a swan from a photo. First, we have seen males swans with small sized knobs as female swans with large knobs. So, even though the larger size knob usually means a male, it is not always the case.

The only guaranteed method of determining gender is through DNA Sequencing in which a licensed veterinarian takes a feather sample with DNA material and submits it to a licensed DNA lab. So, photos are pretty unreliable.

There are several issues that we think is occurring with your swan and the other two females.

First, you are introducing an interloper into their habitat and they have not and will probably not accept your swan because it is not only a threat to the existing pair, but also all the habitat resources, i.e., nesting, shelter, food, etc.

Second, we think that your swan is actually a female. This would be a major reason that the swan is not laying eggs, but is also being chased by the other 2 swans. A male will not hurt a female swan and will certainly not be chased by the females. He would have a field day trying to figure out if one or both would be a compatible mate. The females may not like it, but in nature if a male came available, one or both of the females would usually accept the male. Again, this is an individual attribute of the females to pair up, but since these swans have been paired for some time, have existed in a captive setting and hold dominion over their captive setting, they may or may not accept another swan.

Finally, if you are ever going to introduce another swan into a captive habitat with existing swans, a pen (enclosed top to bottom so predators cannot access the swan, 1/2 in water with 1/2 on land with a zero entrance, with a feeder inside) must be built and the swan kept inside for approximately 2 weeks to monitor for any defensive behavior. If any such behavior is seen, it will provide you with the information you require to determine if the swan can safely be introduced so that no swan gets injured or possibly killed from territorialism. The Regal Swan