The concentration of DDT is highest in top consumers due to a phenomenon known as Biomagnification.

Biomagnification refers to an increase in the concentration of toxic chemicals at successive trophic levels. This happens because a toxic substance accumulated by an organism cannot be metabolised or excreted, and is thus passed on to the next higher trophic level. This phenomenon is well known for mercury and DDT.  High concentrations of DDT disturb calcium metabolism in birds, which causes thinning of eggshells and their premature breaking, eventually causing a decline in bird populations.