Feeding tracks

A picture of feeding tracks made by diving ducks

Diving ducks go down to the lake bottom and filter the bottom mud
with their beaks.  The beak acts as a sieve, which gets rid of the
mud particles and retains midge larvae and other invertebrate animals
which live in the mud surface layer.  The feeding activity of the ducks
breakes the light coloured oxidated mud surface
 and exposes the dark underlying anoxic mud.

Photo: Árni Einarsson

For how long do the ducks stay underwater?