Three dozen juvenile herring gulls have turned up dead in Portland’s Deering Oaks park in less than a month, and city and state officials don’t yet know why.

Three of the ocean birds have been sent to the National Wildlife Disease Lab in Wisconsin for testing. Those results are expected to be returned in the next seven to 10 days, but Judy Camuso, a wildlife biologist for the Maine Department of Inland Fisheries & Wildlife, said initial necropsies on several birds have ruled out the possibility that the birds were poisoned.

Camuso said while it’s unusual to see this in urban Portland, bird die-offs like this are not uncommon.

“On nesting islands, this happens but people don’t see it,” Camuso said. “The reason we wanted to alert people in this instance is because it’s a populated place and people might notice the birds. We don’t view this as a crisis.”

Portland parks director Ethan Hipple said municipal park rangers have been out looking this week for additional birds but have not found any die-offs outside of Deering Oaks.

“This is unusual for us. Usually, anything more than one or two dead birds a week is out of the ordinary,” he said. “But it’s reassuring to know that there doesn’t appear to be any risk to other animals or to people.”

City and state officials said if anyone finds dead birds in Portland, they should contact wildlife officials at 592-0605, but should not handle the birds in any way.

Herring gulls live in habitats all across the country but are popular in ocean-bordering states along the East Coast. They often eat a marine diet of fish and invertebrates like crabs or mussels but will eat almost anything, Camuso said.

She said when there is a cluster of bird deaths like this, bacteria that can spread through bird-to-bird contact is often the culprit.

“We really won’t know until we hear back from the lab, but at this point we can say that we don’t believe any toxins were present,” Camuso said.

The pond at Deering Oaks, which borders State Street, is the only major source of fresh water on the Portland peninsula and is often home to a variety of birds – geese, ducks, gulls and others. On Thursday afternoon, hundreds of birds went in and out of the pond. None appeared impaired. A handful of park visitors said they hadn’t heard about birds dying and said they haven’t noticed any unusual behavior among the birds either.

Hipple said it was park rangers who first noticed an uptick in the number of dead birds and contacted state wildlife officials. That was in late August. Since then, he said, some park visitors have reported seeing birds that appeared sick or injured.

Another mysterious bird die-off occurred earlier this month in Boston’s Dorchester neighborhood, when nearly 50 birds fell from the sky, and no explanation could be found, according to a report from NECN. Twelve of those birds died and health officials are still trying to determine what happened.

“When I arrived, birds would fly, like from a house to a tree, they would flop in the tree and they would fall to the ground,” Alan Borgal of the Animal Rescue League of Boston told NECN. “The weaker ones were just falling right out the sky.”

Those birds were all grackles, black songbirds that travel in flocks.