 The common starling is a highly gregarious species , especially in autumn and winter . Although flock size is highly variable , huge , noisy flocks - <unk> - may form near roosts . These dense concentrations of birds are thought to be a defence against attacks by birds of prey such as peregrine falcons or Eurasian <unk> . Flocks form a tight sphere @-@ like formation in flight , frequently expanding and contracting and changing shape , seemingly without any sort of leader . Each common starling changes its course and speed as a result of the movement of its closest neighbours . Very large roosts , exceptionally up to 1 @.@ 5 million birds , can form in city centres , woodlands or <unk> , causing problems with their droppings . These may accumulate up to 30 cm ( 12 in ) deep , killing trees by their concentration of chemicals . In smaller amounts , the droppings act as a fertiliser , and therefore woodland managers may try to move roosts from one area of a wood to another to benefit from the soil enhancement and avoid large toxic deposits . 
