This would seem to be aggressive behavior toward the other, especially if they are of the same gender and it is nesting season. However, the discolored fluid is what is of most concern. This could be some type of poisoning or could mean that the swan passed this fluid which could indicate internal bleeding, infection or even lead poisoning. The swan needs to be immediately captured and taken to a knowledgeable avian (bird)/waterfowl veterinarian. A blood chemistry analysis should be conducted to see if the swan is ill or hurt. If this is some kind of poisoning such as lead, then chelation followed by supportive care is required. If the swan is ill, the other swan will chase her from the area so that the illness does not spread. This is how nature prevents disease. If the swan is not eating, it is important to get fluids via an i.v. in her to prevent dehydration and to begin tube feeding with a nutritional source such as Exact or Care, followed by B-12 shot and an antibiotic.

The major issue is to capture her and get medical attention ASAP. Throw a towel or blanket over her. Wrap her in the blanket and gently control her legs (to prevent her nails from clawing you and escaping) and control her wings ( so she does not flail and hurt you or her). Please let us know how this situation progresses. The Regal Swan