Witnesses also saw crows suffering seizures on the ground, and the Oregon State Veterinary Diagnostic Lab and Oregon Department of Fish and Wildlife say it was because of a neurotoxin known as Avitrol, which is marketed as a bird-control solution.
The Audubon Society of Portland first reported the lab findings.
The group says the pesticide is highly toxic and should only be administered by a licensed applicator. It describes the poisonings as irresponsible and probably illegal.
It’s unknown how many birds died, but it was at least 10.
Thousands of crows swirl through the sky on winter afternoons and settle in or near downtown Portland. Though some Portlanders are fascinated by the ritual, others consider the noisy birds and their droppings a nuisance.