Another description of Elfterdritches that I read, stems from the PA Dutch having alot of opposites in their culture. for instance: 7 sweets and 7 sours were to be served with dinner, Farmers were to have two farm dogs, for luck named Feuer and Wasser (Fire and Water) A Distlefink was supposed to be a bird of joy and happiness and the Elfderdritch was the opposite of the Distlefink. I have seen a two pictures of the mythical Elfderdritch. In both paintings they were birds with giant claws and oversize beaks with horrible shark like teeth.
Perhaps similar to Lewis Carroll's Jabberwock
"Beware the Jabberwock, my son!
The jaws that bite, the claws that catch!"