In the early 1960s, a group of bird enthusiasts noticed a very worrying trend; the number of birds of prey, like Golden Eagles and Peregrine Falcons, was falling across the UK. They spoke to a scientist called Derek Ratcliffe and he arranged for a national survey of Peregrines. Once all the data had been collected and analysed, Derek’s conclusion was shocking, “The species was in the middle of a headlong crash almost countrywide".
Scientists then began to investigate what was causing this crash in the UK Peregrine Falcon population. There could have been a few reasons. Many birds of prey are shot or poisoned by people who don’t like their eating habits. If you look after pigeons or game birds, you’re not likely to be friendly towards the birds that eat them. Other factors could have been habitat loss or not having enough prey to hunt (which can also be tied to habitat loss). Today we would also consider climate change as a potential factor, but in the 1960s this was not yet a mainstream concern.
However, none of these factors seemed to be the cause. Further investigation showed that although the Peregrine Falcons were nesting and laying eggs, very few chicks were successfully hatching out. Derek Ratcliffe hypothesised that something was affecting the strength of the eggshells, making them thinner. This was then causing them to be crushed under the weight of the adult falcons, preventing chicks from developing and hatching. But what was causing it and why was it only affecting predators at the very top of the food chain?
DDT
DDT (DichloroDiphenylTrichloroethane) is a pesticide. It’s very effective at killing insects and, when it started being used in the 1940s, it really helped control mosquitoes and other pests. It was promoted as being a safe chemical, there’s even a video of someone eating porridge that’s had DDT sprinkled into it! However, no one really knew what the short or long term effects of the chemical were on either wildlife or humans.
DDT was widely used in the UK from the 1940s onwards. When the use of DDT is graphed against the decline in Peregrine Falcon numbers, the increase in pesticide use is inversely proportional to the decrease in falcon numbers. Scientists hypothesised that the DDT was bioaccumulating in the smaller birds that the falcons preyed on, and the chemical was then biomagnified at the top of the food chain, in the Peregrine Falcons. The scientists believed that that biomagnification of DDT was causing eggshell thinning.
However, correlation is not always causation. For example, the amount of crude oil imported into the US from Norway each year has a very close correlation with the number of drivers killed in collisions with trains in the US each year. This does not mean that there is any connection between the two.
In order to be sure that the correlation between DDT and eggshell thickness was a direct causation, and not simply a correlation, scientists devised a research project to test their hypothesis. They measured the amount of DDT in Peregrine Falcon eggs that had been laid in different years. Their results showed that eggs laid before 1947 did not contain DDT and were of a natural thickness. Eggs laid after 1947 however, did contain DDT and were thinner than expected. There did not appear to be any other variables affecting the results.
DDT was banned in 1986 in the UK and in 1972 in the USA. Peregrine Falcon numbers have been recovering in both places ever since.