Between the 1940’s and early 1970’s, DDT, an industrial insecticide, was used extensively throughout the United States to control crop pests and to help eradicate malaria.
Scientists had concerns about the possible hazards of DDT, but public opinion only began to shift after Rachel Carson published her famous book Silent Spring. Rachel Carson surmised that DDT use in Nassua County, New York was endangering human health and poisoning wildlife.
Rachel Carson stated in Silent Spring, “It is ironic to think that man might determine his own future by something so seemingly trivial as the choice of an insect spray.” Concurrently, a toxicologist, David Peakall, found a correlation between concentrations of measured DDE, a denatured form of DDT, in peregrine falcon eggs, and the thicknesses of the eggshells. When there was a higher concentration of DDE, the eggshell was thinner.
At the same time, data about declining bald eagle populations was widely publicized. While DDT is toxic in small concentrations to a wide range of organisms, the reproductive rate of large birds fell drastically due to eggshell thinning and cracking during nesting. DDT affected large birds of prey disproportionately because it ‘bioconcentrates’ in fatty tissue at higher trophic levels.
Finally, in 1972, the EPA banned most uses of DDT. Some bird populations, like the osprey, began to recover quickly after the ban, reaching pre-DDT populations within a decade, but other populations like the peregrine falcon never naturally rebounded in the East and have been reared in captivity to increase local populations.
If it were not for the 1972 DDT ban, we would not have as many large birds of prey in the Arboretum today. A nesting pair of bald eagles fishes on the Cannon River nearly year round, unless the river freezes over, in which case they will migrate to the ice-free Mississippi.
Ospreys have become regular migratory visitors, flying overhead in the spring and fall, and sometimes seen along the Cannon River in summer. The sharp-shinned hawk can be seen in the Arb until late October. The Arb is full of wonderful large birds—get out this weekend and see them!