Quiz - How well do you know birds?

Whilst sifting through my many photographs today from a recent birding trip I came across this odd photo....What on earth did I see to warrant a photo?

I could say Mike took this one.... or the other birds were out of focus but I think I took it and it is of what it is ....a birds backside.

How well do you know your birds? what species is this?

Comments welcome with any guesses you wish to make. I will let you know the answer in my next bog after I have finished sifting through the 800 photo's taken in one day.