Hopefully, the Michigan DNR did not have them killed. Michigan and other state and federal wildlife officials are labeling (falsely) Mute Swans as invasive, non native and aggressive so the public will buy into the killing program. The ulterior motive is to introduce the larger Trumpeter Swans so that they can be used for Trophy Waterfowl hunting purposes. States make more money off of these hunting permits so they kill the Mute Swans and take their habitats for Trumpeter Swans. This is happening not only in wild settings, but also in captive settings such as parks and other private and public ponds.

If the DNR is not responsible, then the smaller bird was actually another younger swan, probably a female which formed an alliance with the remaining swan from your swan family. They have now parted for the migration period to return in the spring and find another habitat to start their own family. The Regal Swan