Black swan flying in formation with Whooper swan - what's going on here?! (Pictures)

At Old Moor today, a flock of Whooper swans took off and circled overhead while arranging themselves in a loose V-type formation (what a spectacular sight, by the way!).  However, here's the thing: amongst them was a black swan!  What is going on here?  Do swans of different species fly together normally, or is this odd?