Pesticides and Birds
Citizen scientists spur investigation of possible link between pesticides and unhatched eggs

In 1999, data collected by participants for The Birdhouse Network showed an unusually high number of unhatched eggs among cavity-nesting birds. At least one egg failed to hatch in more than 20 percent of nests belonging to 7 of the 10 most common cavity-nesting birds, excluding House Sparrows. In our Birdhouse Network summary for that year we discussed possible causes, such as acid rain (Phillips, Summer 2000 Birdscope). Linda Farley, a representative from the American Bird Conservancy's (ABC) Birds and Pesticides Campaign, wrote to me asking why we hadn't considered pesticides as a possible culprit for unhatched eggs. We had, but because the database had not queried nestbox owners about pesticide use, we lacked the evidence to support such an argument.

American Ketstrel by Sam J. Norris

An Investigation Begins
While setting up the Barn Owl cam in Florida this past January, I was fortunate to meet Farley, who was working to document the lethal effects of fenthion, a mosquito control used only in Florida. With her encouragement, I have worked with pesticide experts, scientists at the Lab, and ABC to incorporate pesticide-related questions into the 2001 database for The Birdhouse Network. By answering these questions, participants will help us to gauge whether pesticides may indeed be contributing to high numbers of unhatched eggs.

Before learning of the Birds and Pesticides Campaign I was unaware that some 672 million birds are directly exposed to pesticides on farms alone and that nearly 10 percent of them die. Even more frightening, the most toxic pesticides to birds-diazinon and chlorpyrifos-are used mainly in home gardens. Approximately one billion pounds of pesticides are applied annually in the United States, with 4.4 billion applications in yards and gardens.

Birds are particularly susceptible to backyard pesticides, but the problem is not well documented because few scientists can confirm that a bird death is pesticide related. Testable specimens are difficult to obtain, because dead birds are often overlooked, eaten by scavengers, crushed by cars, washed away, or destroyed by bacterial decay. Evidence of pesticide poisoning is often non-existent.

Effects of Pesticides on Birds
Pesticides can kill birds both directly and indirectly. DDT, for instance, kills birds directly by poisoning their nervous systems. But it also reduces reproductive success by causing thin eggshells and reducing hormone levels necessary for egg laying. Were it not for local disappearances of highly visible birds such as Peregrine Falcons, Ospreys, Bald Eagles, and other top predators, this deadly pesticide might still plague North America. Although DDT is banned in the United States, it is still widely used in other countries and continues to adversely affect Neo-tropical migratory bird species, which maintain high levels of DDT in their fat. During times of stress, birds metabolize fat, releasing toxins into their blood, causing effects such as eggshell thinning. Currently, the U.S. Environmental
Protection Agency (EPA) regulates more than 800 pesticides, many of which have toxicity levels and ecological effects that are unknown because of " 'inert'. . . ingredients whose identity is protected by trade secrets" (Cox 1991. Journal of Pesticide Reform 11:2-4). One well-documented case is the pesticide diazinon, which according to the EPA "has caused widespread and repeated mortality of birds." Diazinon is an organophosphate insecticide commonly detected in urban and agricultural watersheds and air-monitoring samples. It has been shown to reduce the number of eggs a bird lays, decrease survival of eggs and nestlings, and increase the number of deformities in developing chicks. In laboratory tests, a single diazinon granule killed five House Sparrows (Cox 2000. Journal of Pesticide Reform 20:14-20). Although the EPA prohibited the use of diazinon on golf courses and sod farms in 1988, diazinon is still used frequently in the United States, with more than six million pounds applied annually. Homeowners and pest control companies use it to kill fleas, aphids, carpenter ants,
grubs, termites, cockroaches, and fire ants, accounting for 70 percent of all diazinon used.

What You Can Do
Although such statistics may be disheartening, we're happy to report that the EPA has ordered a gradual phase-out of diazinon in all household and home garden applications over the next three years. It's encouraging that people like Linda Farley are actively working to remove harmful pesticides from our environment. And citizens can do their part by decreasing the demand for harmful toxins, writing to local county and state representatives asking them to promote pesticide alternatives, and maintaining a safe and bird-friendly yard (see "Reduce Yard Pests the Nontoxic Way"). To our citizen scientists: thanks to your efforts, we've documented unusually high rates of unhatched eggs among our cavity nesters. There are probably multiple causes, but with your help we can establish whether there is a link between pesticide use and hatchability. Remember, one of the biggest obstacles in addressing pesticide-related problems is the difficulty of documenting them. Won't you help us by sending your data? For more information about The Birdhouse Network go to

Suggested citation: Phillips, Tina. Pesticides and Birds. Birdscope, newsletter of the Cornell Lab of Ornithology, Summer 2001.

For permission to reprint all or part of this article, please contact Miyoko Chu, Editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, New York. Phone (607) 254-2451. Email