Its the soft egg shells.
And the lies of Rachel Carson may finally be put to rest.
Which might have been a side effect of a bird population suddenly starving for insects, not the chemical effects of the DDT on the egg shell. Insects are where birds get most of their dietary calcium.
Kind of hard to be the birds, but there could be other ameliorating measures like having special bird feeding in widespread DDT treated areas to compensate for the loss of insects.