Do PCBs cause eggshell thinning?

The evidence that PCBs are involved in eggshell thinning is examined. In experiments in which both PCBs and DDE were used, it was found that DDE, but not PCBs, caused eggshell thinning of mallard and American kestrel. For two other species, bobwhite and Japanese quail, no thinning was observed with either agent. Numerous other studies with DDE alone have demonstrated that this agent can cause eggshell thinning in many, but not all, species. It is concluded that DDE is the only compound that has caused significant eggshell thinning at environmentally realistic doses.