What  caused  the  drastic  decline  of  the  Illinois greater prairie
   chicken population?

EXPERIMENT   Researchers observed that the population collapse of the
   greater  prairie chicken was mirrored in a reduction in fertility , as
   measured  by  the  hatching  rate  of eggs. In 1992, researchers began
   experimental   translocations  of  prairie  chickens  from  Minnesota,
   Kansas, and Nebraska in an attempt to increase genetic variation.

RESULTS   After translocation (blue arrow), the hatching rate rapidly
   improved, and the population rebounded.

CONCLUSION   The researchers concluded that lack of genetic variation
   had  started the Jasper County population of prairie chickens down the
   extinction vortex.