PORTLAND, Ore. (AP) — Lab tests confirm that poison killed the crows seen falling from the Portland sky in late January.

The Audubon Society of Portland said in January that witnesses saw birds "falling from the sky" in Northeast Portland as a large flock traveled west toward downtown. Another dead crow was found near Portland State University.

"Literally, they're lying on the ground, seizing and gasping for air," said Bob Sallinger, conservation director for the Audubon Society of Portland.

Dead crow in Portland (Photo: Portland Audubon Society)

Portland Audubon said the deaths were consistent with exposure to toxic substances. The birds appeared to be in otherwise good physical condition.

On Wednesday, the Oregon State Veterinary Diagnostic Lab and Oregon Department of Fish and wildlife confirmed the birds "were fatally poisoned with a neurotoxin," according to Portland Audubon.

"They caused the crows to suffer a cruel and inhumane death and they put people, pets and non-target wildlife at real risk of secondary exposure," Portland Audubon said in a Facebook post.

The Audubon Society said the person(s) who poisoned the birds potentially broke at least two federal laws, the Migratory Bird Treaty Act (MBTA) and the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA).

Thousands of crows swirl through Portland skies on winter afternoons. A cacophony breaks out as the birds settle into trees in and near downtown. Sidewalks are often coated with bird droppings, as are vehicles parked under the wrong branch.

The Audubon Society is offering a $1,000 reward for information that leads to the cause of what happened.

The Associated Press contributed to this report.