Long after some of the world’s most notorious pesticides were banned, traces of them are still being found in the eggs of wild birds.
A decade-long study of bird eggs collected across Spain has revealed that birds of prey and other species are being exposed to a complex mixture of environmental contaminants, including chemicals that have been outlawed for decades as well as pesticides still used today.
Among the most frequently detected substances was a breakdown product of DDT, the pesticide that is infamous for its role in the decline of many bird populations during the 20th century. Although DDT was banned in Spain nearly 50 years ago, residues of its by-products were still being detected in eggs from several species.
The persistence of these chemicals highlights a challenge facing conservationists worldwide. Some pollutants remain in soils, water and food chains for decades, gradually making their way into animals at the top of the food web.
The study also identified a range of modern pesticides, including fungicides, pyrethroid insecticides and fipronil, a chemical that has attracted concern because of its effects on wildlife and insects. Researchers found evidence that these substances can be transferred from mother birds into their eggs, exposing developing chicks before they hatch.
Isabel Fernández, a CAD researcher and the study’s lead author, said: ‘These types of contaminants directly impact the reproduction of species like the bearded vulture, which lays only one or two eggs per year, making reproductive failure a significant concern. The presence of DDT, for example, is linked to a thinning of the eggshell, which makes it more susceptible to the entry of microorganisms, and moisture loss, thereby jeopardising the eggs.’
While the study did not examine whether the contaminants directly harmed embryos or reduced breeding success, previous research has linked several of the detected chemicals to reproductive problems, developmental abnormalities and other health effects in birds.
Bird eggs are often used by scientists as natural indicators of pollution because they provide a snapshot of the chemicals present in a mother’s body and the wider environment. By analysing them over time, researchers can track how contaminants move through ecosystems and assess whether regulations are having the desired effect.
The findings suggest that while some environmental victories have been achieved, the legacy of past pollution remains far from over. At the same time, newer chemicals are entering food webs, creating fresh questions about their long-term impact on wildlife and the health of ecosystems.
The full research can be read here
Photo: Nils Söderman
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