The feathered variety I mean!

I'm trying to identify some birds which have recently descended on the lake where I walk round every day.

They are around Mallard size, one, (obviously?) the male is bigger. It has a Black head and it seems mostly White underparts. The smaller ones (females?) have Brown heads and a lot less White.

When on the water they don't hold their heads up like Mallard but seem to hold their heads tucked down close to their body and they seem to keep a low profile on the water. They dive for food and are obviously finding plenty. It looks like they are catching fish, the lake was recently re-stocked with small fish but the birds keep well away from people and paddle to the opposite side of the lake if anyone walks round. I can't be sure that it's fish they've caught as they're usually too far away to be sure.

They don't associate with other birds, they don't care for the gulls and I've seen gulls diving at them if they've found food which makes them dive underwater to get away from the gull.

They keep so far away I can't see what the beak is like or see any other distinguishing marks. They all stick together and the numbers vary. There were nine the other day, seven yesterday but only three today. If there's any ice around, they don't come.

They look a bit like Scaups. I'll try to take some pics but the weekend usually has lots of disturbance round the lake so I might not get a pic until Monday.