Feeding

• A class of ducks called "dabbling ducks" feed by sticking their beak or their entire head below the water's surface and snatching up plants or small fish and insects. Quick turns of the bill may result in a kind of vibration. Even a duck scrounging on land may "vibrate" its head to stir up insects or push aside vegetation.
  
  

Reproductive Displays

• Male mallard ducks will fling back their heads in post-breeding displays.mallard image by John Sandoy from Fotolia.com
  
  A male mallard duck can make an extravagant post-breeding display after mating with a female. In the so-called "bridling display," the male mallard flings his head far back and whistles. A particularly vigorous bridling display could be interpreted as a head vibration.
  
  

Shaking Off Water

• A dabbling duck or diving duck that fully submerges its head or plunges below the water's surface may vibrate its head to shake off droplets after reemerging.
  
  

Shivering

• A cold duck may shiver to warm itself, much as mammals do.duck image by Henryk Olszewski from Fotolia.com
  
  When exposed to particularly cold temperatures, ducks and other birds may shiver. In birds, shivering is usually confined to the pectoral and leg muscles, but pronounced shivering may cause a duck to vibrate its head.