In the book, Silent Spring, what is the connection between the demise of the elm trees and the disappearance of birds in North America?

In Chapter Eight, we learn that birds are disappearing from communities that once enjoyed seeing multitudes of migrating birds. Insecticides use to treat and prevent Dutch elm disease and fire ants has killed birds by the thousands. Spraying began in the 1950's and immediately resulted in the death of robins and other birds that lived and nested in the elms. The birds ate their usual diet of earthworms. The earthworms, however, dine on the mulch of elm leaves that fall in the autumn, subsequently, ingesting the DDT used against the elm disease that would not wash off of the leaves. The DDT collected in the earthworms in high concentrations, so that when the robins ate the worms, the birds were killed.