Should we feed waterfowl when the lakes are frozen over?

Hi All.

I live near the gravel pit lakes at Rickmansworth and have noticed that the birds seem quite keen to find out if I have food - even Moor Hens (normally fairly shy in my experience) have taken to following me around a little when I'm close to the lake (which is mostly frozen over).  My question is:

Should one feed Coots, Grebes, and Moor Hens (ignoring the ducks and swans for this question) during cold times like this, or can they generally cope?  If it's a good idea, what is suitable food for these creatures?

Thanks,

James.