The state has been unable to determine the cause of death of more than 200 red-winged blackbirds in Cumberland County last month, the Department of Environmental Protection said.
Its Division of Fish and Wildlife completed a review of necropsy, toxicology and histopathology tests of birds collected from two recent die-offs in rural Stow Creek Township, but the results were inconclusive.
About 200 birds died suddenly Nov. 22. About two dozen died Nov. 3.
"We cannot rule out some sort of pesticide poisoning because of the highly localized nature of the mortalities," said DEP spokesman Larry Hajna.
He said the state ruled out pesticides commonly known to be toxic to wildlife and determined the deaths were not likely caused by compounds reported in wheat seed planted in the area or by infectious disease.
Necropsies performed at the Department of Agriculture’s Animal Health Diagnostic Laboratory found internal bleeding and trauma from the birds hitting the ground but no obvious evidence of chemical poisoning, he said.
Toxicology tests looked for a large array of pesticides, including those used to control nuisance birds, and did not detect any, he said.
Histopathology tests, which look for changes in tissues, found no evidence of cellular changes as the result of disease or chronic exposure to a toxin, Hajna said.
Wheat seed from a nearby farmer’s field was tested at the University of Pennsylvania and no chemical compounds were detected. But the farmer provided information to DEP’s Bureau of Pesticides that the seed was treated with the fungicides difenoconazole, Mefenoxam and Sedaxane and the insecticide Imidacloprid, according to DEP.
Those compounds are not considered to be very toxic to birds, said Hajna, so likely did not cause the deaths.
It is legal and no permit is required for farmers or other landowners to poison blackbirds, cowbirds, crows, grackles and magpies if they damage crops or livestock feed, cause a health hazard or structural damage, or to protect an endangered or threatened species.
Those birds are exempted from federal protections for other migratory birds because of their potential to do damage and because their population numbers are robust, according to the federal government.
Up to 5,000 red-winged blackbirds that fell from the sky Dec. 31, 2010, in Beebe, Arkansas, had no disease or toxins in their systems, according to a Jan. 6, 2011, report in the Wisconsin News.
Instead, they had likely been frightened by New Year’s Eve fireworks into flying at night, and being poor flyers in the dark, they flew into homes, cars and power lines.
New Jersey Audubon’s Vice President for Stewardship John Cecil has said his organization, which works to protect birds in the state, doesn’t see farmers’ use of poisoned seed to kill blackbirds as a big issue.
Habitat loss is of much greater concern to the organization, he said, and often blackbirds and invasive species such as European starlings outcompete native birds such as bluebirds, causing native species to decline.
In 2012, a die-off of 200 to 300 blackbirds in Millville was traced back to a farmer’s legal use of an avicide. In that case, dead birds fell into a suburban neighborhood, causing widespread alarm.