How Humans Have Driven Bird Populations to the Brink of Extinction
From The Observatory
Source: Earth Food Life Project
From DDT and pesticide exposure to collapsing insect populations and habitat disruption, birds are warning us about the growing environmental pressures reshaping ecosystems worldwide.
This adapted excerpt is from Sophie A.H. Osborn’s Feather Trails: A Journey of Discovery Among Endangered Birds (2024, Chelsea Green Publishing). It is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License (CC BY-NC-SA 4.0) with permission from Chelsea Green Publishing. It was adapted and produced for the Observatory by Earth • Food • Life, a project of the Independent Media Institute.
“How Humans Have Driven Bird Populations to the Brink of Extinction” by Sophie A.H. Osborn is licensed by the Observatory under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License (CC BY-NC-SA 4.0).
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In 1963, Joseph Hickey—a professor of wildlife management at the University of Wisconsin—read Derek Ratcliffe’s recently published paper on the peregrine falcon’s status in Britain. The report highlighted the bird’s desertion of nesting territories and suggested that pesticides might be responsible for the falcon’s precipitous decline. As he absorbed Ratcliffe’s alarming analysis, Hickey recalled a rumor he had heard the previous year that not a single peregrine chick had fledged in the Northeastern United States.
Ratcliffe highlighted the bizarre egg breakages and disappearances that he and others had documented in peregrine nests since 1949 and speculated on the causes of this unprecedented ornithological phenomenon. One possible cause, he wrote, might be “defects of the egg.” “If, for some reason, Peregrine eggshells had recently become thinner,” he continued with visionary boldness, “an increase in breakage might be expected.” But he conceded that he had not measured the thicknesses of the broken eggshells, and given the normal thinning of shells during incubation, none of the eggs he’d examined seemed unusual.
Reading these speculations, Hickey noted that Ratcliffe’s hypothesis could be objectively tested. “I am told poultry science people... routinely measure shell thickness,” he wrote on Ratcliffe’s manuscript, recommending that the British scientist measure peregrine eggs collected before and after 1950 to compare them. Another of Ratcliffe’s colleagues, Desmond Nethersole-Thompson—an authority on birds of the Scottish Highlands—commented that eggshell thickness was related to eggshell weight, so Ratcliffe could evaluate shell thickness by weighing older eggs and comparing their weights to those of more recently laid eggs.
Although there were many possible explanations for thinner eggshells, in 1968, Hickey and his research partner Daniel Anderson provided scientific evidence that the synthetic insecticide dichlorodiphenyltrichloroethane (DDT)—long considered relatively harmless—was likely the cause. The researchers compared eggshell thickness in five herring gull colonies in the north-central U.S. and New England region. Variation in the shells’ thicknesses was closely related to the levels of dichlorodiphenyldichloroethylene (DDE), or the breakdown product of DDT, found in the egg remnants.
Over time, more studies added to a growing body of evidence suggesting that DDE wasn’t just correlated with eggshell thinning, but was actively causing it. When scientists fed DDE to mallards in a laboratory, the ducks subsequently laid thin-shelled eggs. Captive American kestrels that were fed DDE in different doses showed a related range of eggshell thicknesses. And British birdwatching chemist David Peakall showed conclusively that DDE was absent from eggs before the DDT era but present in eggs from 1947 onward. Furthermore, the levels of DDE in shell membranes and traces of the original egg contents were directly related to the thickness of the eggshells that enclosed them. Skeptics’ claims that eggshell thinning had occurred before DDT was widely used were finally laid to rest.
Despite growing scientific evidence that DDT was harmful to wildlife, powerful agrochemical and farming interests fought mightily to preserve the use of the wonder product that had protected millions of soldiers and civilians from insect-borne diseases in World War II, spared acres of crops from the ravages of insect pests, and promised to help eliminate diseases such as malaria. In doing so, influential parties—backed by the Department of Agriculture, which was responsible for the development and registration of pesticides until the Environmental Protection Agency was founded in 1970—faced a growing tide of public concern about the indiscriminate use of pesticides and their potential effects on wildlife and human health.
Often, the overarching concern of environmental poisons is their insidious, unintended effects. Countless birds either feed on insects year-round or provide insects to their fast-growing nestlings as a protein source during the breeding season. But the widespread use of neonics is increasingly believed to be a major contributor to the alarming disappearance of insects—a phenomenon serious enough to be dubbed an insect apocalypse—and is being documented by scientists in the U.S., Europe, and other parts of the world. The rest of us are taking notice, too. Car windshields used to be smeared with dead insects during night drives in our childhood summers, but windshields are now disturbingly clear.
The state of the world’s bird populations is widely viewed as an indicator of the health of our environment. And while the reasons for current bird declines are as varied as rays of sunlight deflected through a crystal prism, the dark-eyed, smoke-backed peregrine and its DDT-suffering compatriots taught us in the 1960s and beyond that pesticides are a threat that merits our utmost attention.
Fortunately, on August 20, 1999, amidst a gathering of more than 1,000 celebrants, the American Peregrine Falcon (Falco peregrinus subsp. anatum) was officially delisted, or removed, from the federal list of endangered species. The move followed thirty-five years of work by numerous organizations and thousands of people.
A variety of biological factors contributed to the peregrine’s successful recovery in North America. The falcon’s relatively small population made it easier to establish and reach manageable restoration goals. The peregrine’s docility and long relationship with humanity made breeding it in captivity relatively easy once successful techniques had been developed. The bird’s longevity and generally high survival rate enabled a greater return on captive breeding investments than in organisms with low survival rates or shorter lifespans. The tendency of peregrines to return to their natal areas allowed vacated regions to be targeted and repopulated. And the falcon’s flexibility in adopting atypical nest sites in areas where food was abundant helped bolster overall peregrine numbers. But the banning of DDT in 1972 was, indisputably, the most important factor—and the most important action taken—in recovering North America’s peregrines.
Perhaps the most fundamental takeaway from the peregrine’s conservation success story is that the primary threat to a declining species must be dramatically curtailed—if not eliminated—for sustainable population recovery to be achieved. Ultimately, every species in North America that experienced DDT-induced eggshell thinning, reproductive failure, and population decline recovered. In the meantime, it became clear that birds—when conspicuous, easily viewed, and intensively studied—could be successful indicators for detecting and monitoring the effects of contaminants and other environmental perturbations.
Conservation biologist Thomas Lovejoy, who coined the concept “biological diversity” after witnessing the Amazon’s ecological riches in 1965, said, “If you take care of birds, you take care of most of the environmental problems in the world.” Birds are visible, vocal sentinels that alert us to a warming climate, disappearing and deteriorating habitats, declining insect populations, hidden and not-so-hidden poisons, toxic air and water, excessive persecution, resource scarcities, invasions of foreign organisms, the proliferation of harmful trash, and even outbreaks of new diseases. We do well to listen and respond to the tales birds tell us through their declining numbers or their failure to thrive, since their status so often warns us of perils that threaten other organisms, including humankind.
