Confused chicken???

I have just witnessed one of my chickens appearing to 'mount' of the others....is this normal behavour, have I just never noticed it before or been around to see it happen?  I don't have a cockerell only 6 ladies 

I am a bit worried that they might be bored and starting to turn on each other.  During the winter they tend to have the run of the garden, but for the last few months I have kept them in their run.  It is more than big enough for them, but they have a crafty way of laying on the guilt rather thick 

Any thoughts gratefully received

Phew, I have never seen it before, must of just never been around.

Thank you  :happy: