If there were some viral/bacterial infection, they probably would have all come down with the illness at one time, so you would have had swans, ducks, chickens, etc., all showing signs of an illness within days or a week of each other.

Because these episodes were spread out, we would suggest the possibility of two occurrences. Louisiana had two major severe weather issues, one in January and one in February. The severe inclement weather, going from hot to cold could have cause the birds to get chilled or the next possibility which seems to be the most plausible:

Because you had a severe wind/rain event, there is a possibility that run-off (insecticides, pesticides, animal waste, roadways) from perimeter areas washed into your area, thus poisoning your swans and duck. With the duck being close to the bayou/ intercoastal waterway, there is even a possibility that salt water got into the pond area or some toxin was washed into an area that only the duck and swans came into contact, and/or they fed on something such as fish larvae, crustaceans, etc., that might not have normally been ingested.

The only true way of knowing what is occurring is to get an immediate necropsy (animal autopsy) conducted. Should another bird die, you need to get the carcass to an animal diagnostic facility or veterinarian so that toxicology and other testing can be conducted. We hope that this information is of benefit. The Regal Swan