It’s a sad day at Caswell Cove: A swan that was returned there Wednesday after being sick has died.

The swan was returned to the Housatonic River beyond the Caswell Cove condominiums in Milford Wednesday after recovering from an ordeal at the local sewage plant.

Peter Reid brought the swan to Milford Wednesday from the Wildlife in Crisis nature center in Weston, and about a half dozen Caswell Cove residents turned out to see the swan that they considered part of the condominium family.

The swan was treated for respiratory issues at the Wildlife in Crisis nature center after swallowing sewage from an aeration tank at Milford’s wastewater treatment facility, which is next to the condominium complex.

Staff members from the wastewater treatment facility, working with local firefighters and animal control officers, rescued the swan from the aeration tank about two weeks ago. Then it was taken to the nature center in Weston to recover.

The swan was one of five juveniles that frequented the marina at Caswell Cove along with two adult swans. When the swan recovered, it was returned to the area.

On Wednesday, Reid, assistant director of Wildlife in Crisis in Western Connecticut, said the swan had been given two weeks of antibiotics and appeared healthy. The swan seemed perfectly at home when Reid gently placed it in the river behind the Caswell Cove condominiums Wednesday afternoon.

But the next morning, Caswell Cove Condominium Association President Dotti Bateman shared the sad news that the swan had been found dead in the morning by one of the residents.

“We are so heartbroken,” Bateman said.

She said it does not appear the swan was attacked but that it died in its sleep.