The powerful insecticide DDT was widely used in the 1940s and 1950s to curb mosquito populations, until it was discovered that the chemical caused thinning of eggshells in some birds. This resulted in a rapid decline in bird populations.

Photograph by James P. Blair

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    1. The shell of this pelican egg is unusually thin, and has already developed hairline cracks. Why are thin eggshells a hazard for pelicans and other birds?

      Thin eggshells are much more likely to break, preventing the development and hatching of healthy chicks. Eggs with thin shells may be broken on purpose, by predators. They may also be broken accidentally, as a mother arranges herself on a nest, or the eggs are jostled by movement from wind or water.

    2. The powerful insecticide DDT (now outlawed in the U.S.) was linked to thinning eggshells in pelicans in the 1960s, when this photograph was taken. Most other birds affected by DDT were raptors. What are raptors?

      Raptors are birds of prey, which hunt other animals from the air using their excellent eyesight, strong talons, and sharp beak. Pelicans are predators, but are generally not considered raptors. Eagles (including bald eagles), osprey, kestrels, California condors, hawks, falcons, and owls are raptors that experienced thin eggshells when they were exposed to DDT.

      Chickens and quail, which are not raptors, did not exhibit thinner eggshells after exposure to DDT.

    3. Pelicans and raptors are near the top of their food chains. Why do you think they suffered more dramatic effects from DDT (such as thinning eggshells) than lower-level predators such as chickens and quail?

      The concentration of some chemicals, such as DDT, increases as it gets higher in the food chain. This process is called biomagnification.

      The concentration of DDT is relatively low in plants and primary consumers such as fish and insects. Secondary consumers, such as chickens, have higher levels of DDT. Tertiary consumers, such as eagles and pelicans, have the highest concentrations of DDT in their bodies.

  • Term Part of Speech Definition Encyclopedic Entry
    biomagnification Noun

    process in which the concentration of a substance increases as it passes up the food chain.

    concentration Noun

    measure of the amount of a substance or grouping in a specific place.

    DDT Noun

    (dichloro-diphenyl-trichloroethane) toxic chemical used as an insecticide but illegal for most uses in the U.S. since 1972.

    food chain Noun

    group of organisms linked in order of the food they eat, from producers to consumers, and from prey, predators, scavengers, and decomposers.

    Encyclopedic Entry: food chain
    hazard Noun

    danger or risk.

    nest Noun

    protected area built by birds to hatch their eggs and raise their young.

    pesticide Noun

    natural or manufactured substance used to kill organisms that threaten agriculture or are undesirable. Pesticides can be fungicides (which kill harmful fungi), insecticides (which kill harmful insects), herbicides (which kill harmful plants), or rodenticides (which kill harmful rodents.)

    predator Noun

    animal that hunts other animals for food.

    primary consumer Noun

    organism that eats plants or other autotrophs.

    raptor Noun

    bird of prey, or carnivorous bird.

    secondary consumer Noun

    organism that eats meat.

    tertiary consumer Noun

    carnivore that mostly eats other carnivores.