DDE was present in sufficient concentrations to account for significant eggshell thinning in 1946 in Great Britain and as early as 1948 in California. Later, he would apply similar methods to California condor eggshell fragments as evidence that this species was extremely sensitive to DDE.
The presence of DDE was sufficient to cause noticeable eggshell thinning in 1946 in the UK and by 1948 in California. Subsequently, he utilized a comparable technique on California condor eggshell fragments to demonstrate that this species is highly susceptible to DDE.