A 17-year-old male mute swan, one of two who have ruled over the lagoon at San Francisco's Palace of Fine Arts, died last week of apparent zinc and lead poisoning — which his mate was also being treated for in recent weeks.

The source of the poisoning remains a mystery, but on Thursday, Rec & Parks said that Blue Boy, the male of the pair, was "found unresponsive." Results of a necropsy are still pending.

The female swan of the pair, Blanche, was sickened in late January and relocated to the San Francisco Zoo for special care, and it was at that point that the lead and zinc were discovered. Blue Boy seemed to be healthy in early February when he was also relocated to the zoo as a precaution, as the department said at the time.

But then it seems to have been Blue Boy's turn to go first. He was actually the younger of the pair, believed to be about 17, while Blanche is 24 or 25.

"Veterinarians continue to investigate the cause of [Blue Boy's] death, as well as the source of the zinc & lead in both swans," the Rec & Parks department said on Twitter. "We are awaiting laboratory results for the necropsy & the soil in their habitat. We hope to have answers soon, so we can bring Blanche home to the Palace."

Swans have been part of the Palace of Fine Arts lagoon since the complex was first constructed for the 1915 Panama-Pacific International Exhibition. And at different times there have been more than two, as well as an assortment of other birds including ducks and a heron.

Blanche and Blue Boy have produced offspring in years past — as the resident "swan lady" Gayle Haggerty, explained to KQED last year, Blanche already had some health issues that prevented her from having any more babies, and some of the young cygnets were killed by coyotes in recent years.

Haggerty has been tending to the swans for 25 years, visiting them daily and managing their diet. She primarily gives them dandelion greens and swan food, but they occasionally get some other treats, including Cheez-Its for dessert — "They just go berserk for those," she said.

"Blanche is very sweet and very forgiving," Haggerty told the station. "If I have to handle her she will forgive me right away."

She explained that she told them apart partly by the color of the feet — Blue Boy was a black-footed variety of mute swan, so-called because they tend not to be very vocal. But she said she could pick them out from across the water.

"Blue Boy — he's the man of the lagoon," she said. "He doesn't like geese there, he doesn't like me. He’s a very large swan. And he's always on a patrol. He's always like, 'Who are you? Who let you in?'"

Hopefully Blanche pulls through to swim another day. Fingers crossed.