Around this time, toxicologist David Peakall was measuring DDE levels in the eggs of peregrine falcons and California condors and finding that increased levels corresponded with thinner shells.
Naquel momento, o toxicólogo David Peakall estaba medindo os niveis de DDE nos ovos de falcón peregrino e de cóndor de California e atopou que un aumento nos niveis do produto correspondíase cun maior adelgazamento da casca dos ovos.