In 1962, Rachel Carson’s Silent Spring, highlighted the harmful effects of DDT and other pesticides on birds. The hardest hit species included birds of prey: Peregrine Falcons, Sharp-shinned Hawks, Cooper’s Hawks, Eurasian Sparrowhawks, Osprey, Bald Eagles, and White-tailed Eagles. Brown Pelicans and herons also faced severe losses. Many people heeded Carson’s words and the scientific data backing her up, and DDT, the most notorious pesticide, was banned in most of the West by 1972. Today, many of the affected birds have largely recovered, but with other species in serious decline, it’s important that we don’t forget the lessons learned.
What does DDT do to birds?
DDT is an organochlorine pesticide. It kills insects by disrupting the nervous system—unfortunately, it is also directly and indirectly toxic to birds. To understand how DDT kills birds, we need to understand how the chemical behaves:
- DDT persists in the environment—it doesn’t break down readily and is detectable in soil, water and animal tissues for a long time.
- DDT is fat soluble: once ingested by an animal it remains stored in body fat.
- As smaller animals are eaten by predators, DDT moves up the food chain, becoming more concentrated: if a robin eats earthworms with DDT in their tissues, the DDT insecticide in the worms ends up stored in the robin’s fat. If the robin is killed and eaten by a hawk, the accumulated DDT joins the DDT already stored in the hawk’s fat.
- In times of food shortage, birds use their fat stores, releasing all of the accumulated DDT at once, which can result in lethal pesticide poisoning.
- DDT and related insecticides interfere with calcium metabolism in birds, resulting in abnormal eggshells. Eggs are thin-shelled and may not allow exchange of air for the developing embryo. Eggs break and embryos die during incubation. This is a particular problem for birds of prey, which tend to accumulate high levels of DDT.
- DDT can have a direct effect on breeding, causing some birds to lay fewer eggs.
Many people believe that DDT insecticide poisoning no longer threatens birds because DDT is banned. This isn’t true:
- DDT is still used in many developing countries, and chemical companies have continued to produce it and provide it. Birds in countries where DDT is used are being affected, and migratory birds pick up DDT in the south and bring it back to northern regions and northern birds of prey.
- Use of insecticides that contain and/or may break down to DDT, such as dicofol continued after DDT insecticide itself was banned.
- The continued illegal use of DDT in North America cannot be ruled out.
Other pesticides and herbicides kill birds too
Many pesticides and herbicides kill birds directly or indirectly: organophosphates, carbamates, and herbicides have all been implicated in bird kills and shown to cause declines in bird populations:
- Minute quantities of some chemicals are lethal to birds. For example, one granule of carbofuran will kill a House Sparrow.
- Invertebrates and other animals that have been killed by a chemical are often eaten by foraging and scavenging birds, which suffer pesticide poisoning in turn.
- Widespread use of pesticides kills multitudes of insects that birds eat—starvation results.
- Spraying of herbicides destroys plant habitat of birds and their prey.
- In birds, exposure to pesticides can cause loss of appetite, lower egg production, feminization of embryos, lack of parental care of nestlings, changes in activity levels, and increased susceptibility to predation.
It’s likely that herbicide and insecticide poisoning and the indirect effects of these toxic chemicals are still contributing significantly to the decline of bird species everywhere.
Related content:
Carbofuran - Another Bird-Killing Pesticide Banned
Sources:
"DDT and Birds." Ehrlich, Paul R, David S. Dobkin, and Darryl Wheye. Stanford University: 1988.
Firefly Encyclopedia of Birds. Perrins, Christopher ed. Buffalo: Firefly Books, 2003
"Pesticides and Birds: From DDT to Today’s Poisons." Cox, Caroline. Ecological Agriculture Projects, McGill University.
