Sea ducks

[lukeh]

I was at Hartlepool Headland yesterday and noticed a flock of about 40 Eiders, all of which were male. I also recall seeing a small Common Scoter raft on many visits to the Headland over the winter, but all of them seemed to be female. Is this a co-incidence or is there a reason these sea ducks are in single-sex flocks?
Luke