There is an old English-language saying ‘birds of a feather flock together‘.  But today in the Rhondda I briefly witnessed a small flock of black coloured birds, I presume were crows, flying amongst a flock of pigeons.  It looked like a ‘V’ flying within a larger ‘V’.  I noticed the contrast in colour of the two types of birds.  Were the crows attacking the pigeons, or vice versa?  I’ve no idea, as I’ve never noticed anything like this previously.

Another theory : two flocks were flying and collided.  These Rhondda birds perhaps need an air-traffic controller!  Some interesting research here from Cambridge University on mixed bird flocks :

The researchers discovered that birds prefer to fly close to members of their own species, and that the larger and more dominant rooks take the lead by flying near the front of flocks.

Birds fly as a collective ‘flock’ for a number of reasons : safety in numbers, to conserve energy as there is less wind resistance.