Two moose calves were found dead in Silverthorne last month, and a toxicologist says they were poisoned by yew trees.

Colorado Parks and Wildlife confirmed that necropsy results showed that the two calves consumed parts of a yew plant before dying in a yard in the Eagle’s Nest neighborhood in Silverthorne. Yew plants are highly poisonous to humans, wildlife, horses, cattle, sheep, goats and dogs because of the alkaloid taxine.

Jake Kay, wildlife district manager for Parks and Wildlife, was called to Silverthorne about the calves, and he said initially, he was not entirely sure how the calves had died. They were thin, but that is common for wildlife toward the end of winter — when fat reserves are low and greenery has not completely grown back yet.

Kay said it looked as if they had just dropped dead from where they had been standing.

“They were in a little shady spot under some trees and there was a fair bit of snow,” he said. “There were moose tracks walking in, but it didn’t appear the moose were running or anything like that. There were some scavenger dog tracks, like domestic dogs, but there were no signs of mountain lions, no bears — nothing that would indicate predation. It kind of looked like these two moose just sat down and died.”

Because of the circumstances, Kay said that he performed a necropsy in the field and found no trauma or bullet holes. Their organs also looked normal, as well.

“There was really nothing abnormal that we found,” Kay said. “In addition, their stomachs were full of food that formed fecal pellets. Sometimes when they’re sick, they’ll have diarrhea or if they’re really sick, they can’t even move. They won’t have food in their stomachs because they’re not out and about eating. They also didn’t have any ticks or anything like that — no parasites on the outside, as well.”

At this point, Kay said that he suspected toxicity and consulted a wildlife pathologist, who suspected yew plants. After looking into what yew leaves looked like, he went to see what he could find.

“I went back through the stomach contents of the moose and found what to me looked like the yew images,” he said.

Stomach samples from each calf were sent to a lab, where it was determined to be yew.

According to Colorado State University’s guide to poisonous plants, the highest concentration of taxine in yew plants is generally found in the leaves in winter time. Adult cattle and horses have been fatally poisoned with as little as 8-16 ounces of yew leaves.

Yews are evergreen shrubs or small trees with glossy, rigid, dark green leaves up to 2 inches long with pointed ends, and they are closely spaced on the branches. Western yew and American yew are two indigenous species. English yew and Japanese yew are not indigenous but are commonly cultivated species in North America and are sometimes used in landscaping.

A Facebook post detailing the event grew popular online, garnering over 800 reactions and over 100 comments within the first day of being posted. The post includes a photo of a deceased moose calf.

Kay added that residents who use yew bushes or trees in their landscaping should be mindful of access to wildlife.