What is a duck to do when the lake ice refreezes? If they can’t find open water, at least they might find some weak spots in the ice where a few pokes of the bill can open up a channel to water below.
Mallards “fishing” through small holes in the ice for the algae lying below. Frankly, it looked slimy and completely unappetizing.
Seems to be a successful strategy. When one of a pair finds a good hole, they took turns sampling the slime.
And there are plenty more holes to try…
Back Yard Biology 
It is not a sight I am used to and feel quite sad for the poor Mallards but it is all about survival. Amelia
They do have the option of flying to open water, but in this case, I guess they made use of a new opportunity.