Yes, swans will call out for the other swans. If a wild predator, raccoon, coyote, etc., did not take the family, then you have to look at your state and federal wildlife officials for the killing of the family.
Michigan and other states have perpetuated the largest hoax on the taxpayer stating that the Mute Swans are an invasive species (they are not-they are actually a sentinel species alerting to heavy metals in the environment as well as harmful microorganisms), they are non-native (actual fossils showing the presence of an ancestor to the Mute Swans have been found throughout North America), they are detrimental to other waterfowl/wildlife (actual scientific studies show that they increase the biodiversity of a habitat), that they are aggressive (science shows they are no more defensive than any other wildlife in defending their habitats or families).
So, why the deception? State and federal wildlife officials are killing the Mute Swans to open their habitats for the introduction of the larger Trumpeter Swans which are going to be used for Trophy Waterfowl hunting. There is a lot of money riding on increasing wildlife budgets through these more expensive "trophy hunting permits".
There should be an uproar by Michigan residents who are seeing decreasing budgets throughout the state, problems with water quality, yet, state and federal wildlife officials are worried about killing an entire species to open hunting for a very limited number of residents (hunters).
Maybe, the state and federal wildlife officials do not want the Mute Swans around to alert them to the problems with the water in the state. Again, many countries view the Mute Swan as a Sentinel species that alerts to the problems in the wetland and water habitats. If one Mute Swan dies, a team of biologists are immediately dispatched to the area to test the water and other environmental markers to see what is occurring.
The only time that a family would intentionally leave the other parent would be if the remaining parent was sick or injured and fear that a disease or predator could follow the family would cause the separation.
More than likely, a volunteer or someone paid by the state/federal wildlife officials or actual government officials entered your water habitat and killed the family if the family does not reappear.
A Michigan DNR official actually admitted at the 2014 International Swan Symposium that this program has been active since the 1990's, is currently ongoing and expected to continue until 2030 until all Mute Swans are killed in the state, and even at the great cost to taxpayers, this is an unsustainable program.
Additionally, both Michigan DNR officials present at the symposium admitted that they had no scientific research showing that the Mute Swans were more aggressive, harmed the environment or any other reason that they provide to killing the Mute Swans. One Michigan DNR official stated that they just wanted a native swan. Again, fossil research shows that the Mute Swan is native to North America and the only reason that the Michigan DNR officials want the Trumpeter Swan (that they consider Native as well as the Tundra Swan) is for hunting purposes. They cannot hunt Tundra Swans, as they had done for so many years and it is smaller than the Mute Swans. Hunters wanted a larger swan so the Trumpeter Swan, being the largest of all waterfowl is the goal for hunting organizations and state/federal wildlife agencies.
The only way that this atrocity is going to stop is for Michigan and other taxpayers to create such an uproar on wasted taxpayer monies and increase the awareness of the this despicable program. You and your neighbors, and taxpayers throughout the state need to question your state and local representatives, demand that they show how much is being spent on the program (they will either ignore you or state that the money is coming from hunting), yet, you the taxpayer "own" the upkeep of wetlands, water habitats and wildlife through taxpayer monies and should have a say in what is being done.
If these so-concerned wildlife officials are really concerned about the habitat, maybe they should be reassigned to the EPA department in your state to test and check on water quality. The money saved from the unsustainable Mute Swan killing program could be better served for testing and correcting water quality and other environmental programs. We hope that this information is of benefit to you. The Regal Swan