Boston’s birdwatchers are mourning the loss of two swans that frequented the Charles River Esplanade, victims of avian influenza that leave behind five healthy chicks.

The city first became aware of the birds’ illness on June 27, when the Animal Care and Control division got several calls reporting the swans’ potential illness.

In conjunction with the fire department, workers captured the birds and took them in for evaluation. They were deemed too ill to be saved, and were eventually put down.

“We were able to capture and transport the adult swans to the city's animal care facility. Unfortunately, the birds were quite ill, exhibiting symptoms consistent with Avian Influenza and were humanely euthanized,” said Liz Sullivan, director of external affairs for the parks department.

Animal control recovered their offspring two days later. The cygnets were taken to the Cape Wildlife Center in Barnstable, where each tested negative for avian flu. They were all in good condition, apart from one with a foot injury, and Sullivan expects them to be healthy enough to be released before the season is out.

“We work with several wildlife rehabbers in instances where we recover baby orphaned animals, or animals who have an injury from which they would likely be able to recover and be released,” she said. “In this instance, Cape Wildlife was both the best fit and first available rehabber to accept these cygnets and we are very grateful for the work that they will do to ensure that the birds have the best chance for survival in the wild.”

This year has seen an outbreak of avian flu across both wild and domestic populations, with the US Department of Agriculture reporting over 40 million confirmed bird infections.

While the virus can spread to humans, the Centers for Disease Control have deemed the current strain to be a low risk to the human public.

The tragedy won’t spell the end of the Esplanade’s swan population. The park’s greenery and clean waterways have attracted a different pair of swans for the last two years running, so it seems likely another family will make its home there in the years to come. The Esplanade also boasts a substantial population of unpaired swans.

Mike Nichols, executive director of the Esplanade Association, urged visitors to exercise restraint when enjoying the park’s waterfowl. The Esplanade’s swans have grown to be a significant attraction, but being followed everywhere by curious humans isn’t great for their well-being.

“The swans have prompted a fair amount of human interest with their babies, in addition to the dozen other swans that have spent time on the Esplanade,” said Nichols.

“We’ve struggled with that, it’s fair to say that we’ve had some regrets about that attention. It hasn’t always been the healthiest, from kayakers getting too close to a large group of people that have made the swans’ every movement the product of daily surveillance. All of that in the aggregate has had a difficult impact on the swan’s ability to live a happy life on the esplanade.”