This site contains all info about Common buzzard.
Buzzards do not normally form flocks, but several may be seen together on migration or in good habitat. The Victorian writer on Dartmoor, William Crossing, noted he had on occasions seen flocks of 15 or more at some places.  Though a rare occurrence, as many as 20 buzzards can be spotted in one field area, approximately 30 metres (98 ft) apart, so cannot be classed as a flock in the general sense, consisting of birds without a mate or territory. They are fiercely territorial, and, though rare, fights do break out if one strays onto another pair's territory, but dominant displays of aggression will normally see off the interloper. Pairs mate for life. To attract a mate (or impress his existing mate) the male performs a ritual aerial display before the beginning of spring. This spectacular display is known as 'the roller coaster'. He will rise high up in the sky, to turn and plummet downward, in a spiral, twisting and turning as he comes down. He then rises immediately upward to repeat the exercise.