Ornithologists started observing something that seemed almost too bizarre to be true in the middle of the 1960s. The weight of the parent bird incubating the eggs caused the eggs laid by peregrine falcons on nesting cliffs in the eastern United States to collapse. The shells were just too thin to support. In areas where the falcons had historically flourished, populations were drastically declining and nesting efforts were failing at previously unheard-of rates. The mechanism was traced back to DDT, a pesticide that was sprayed on suburban communities and agricultural land with what now seems to be incredible confidence in its safety, after years of arduous research. In 1962, Rachel Carson released Silent Spring. In 1972, DDT was outlawed in the US. The peregrine falcon was classified as endangered.
The fact that this history is still incomplete is remarkable. Even if DDT was removed from the market more than 50 years ago, researchers still find amounts of its breakdown product, DDE, in the eggs of ospreys, bald eagles, peregrine falcons, and other apex bird predators that are concerning. When the sprayers stopped, the chemical continued. It was in the ground already. in the sediments of rivers. Small birds consumed the corpses of fish and invertebrates, which were subsequently consumed by larger birds. It has climbed the food chain and continues to do so.
Chemistry and physics work together to explain why DDT is so persistently persistent. The material resists the microbiological and photochemical processes that degrade the majority of chemicals in the environment because it binds firmly to organic matter in soil and sediment. DDT placed decades ago can continue to be chemically active for well over a century in aquatic systems where sediment layers build up over time and disturbance is minimal; some estimates reach as far as 150 years in cold, low-oxygen environments. The compound is still being released into waterways by old agricultural application zones, especially in the American Midwest and parts of Europe where aerial spraying was common, and the locations of former DDT manufacturing facilities, most notably a Superfund site close to Los Angeles that leached enormous amounts into the Pacific.
DDE destroys bird eggs by a specific and well-established biological mechanism. The chemical interferes with calcium metabolism during egg development when a female bird accumulates DDE in her fat tissue, which occurs when she eats fish or small mammals that contain the substance. In particular, it interferes with carbonic anhydrase, an enzyme required to transfer calcium carbonate into the eggshell gland. The end effect is an excessively thin shell. Not too thin—you can’t glance at one and see the issue right away. However, the shell is so thin that it cracks when an adult bird settles on the nest to incubate it. The egg fails to hatch. The nest is unsuccessful. It destroys a population when multiplied over a sufficient number of nesting pairs over a sufficient number of years.
Seeing this occur in species that have otherwise mostly recovered is very unsettling. Thru habitat preservation, captive breeding initiatives, and the removal of DDT from active use, peregrine falcons have recovered from extinction. In 1999, the species was taken off the US endangered species list. Bald eagles also made a significant comeback. However, the conservation initiatives have outpaced the resolution of the toxic legacy. The substance may still be traced back throughout the food chain to contaminated water sources and old sediment deposits, and researchers keeping an eye on these populations continue to find eggs with detectable DDE contamination and instances of shell thinning.
As contaminated sites are cleaned up and sediment disturbance progressively moves the old DDT deposits thru the system and into degradation pathways, the situation might get better. However, the timescale of that process is geological rather than human. As the world continues to introduce new persistent compounds into the environment, many of which won’t fully manifest their effects for decades, the lesson that lies beneath this story—that some chemical decisions made in the middle of the twentieth century are still playing out in the eggs of birds in the twenty-first—is worth holding onto.
