In winter, when birch trees have lost their leaves we often see birds' nests, waiting for spring to come again and bring back the birds - but is that really what we are seeing? We can wait in vain to see birds flying to and fro from the tree once the warmer weather has come, because what we are seeing are actually dense bunches of short shoots with small leaves caused by the fungus Taphrina Betulina. It is commonly known as Witches' Broom because if all the extra shoots grow in the same direction it can take the shape of a traditional broomstick.

The fungus enters the tree and stimulates extra growth in the place of one shoot, and then feeds on this extra growth. It does not seriously harm the tree, and because of this the Witches' Broom is classed as a gall. Plant galls are part of a plant that have been caused by a parasitic attack within the plant cells, causing abnormal growth within the tissues.

It used to be believed that witches had flown over the tree to make them appear. More often, the shoots grow as a cluster of growth, with no fixed direction and it resembles a bird's nest instead. These can be found wherever birch trees are found.

Fascinating Facts

• Witches' Broom generally appears only on birches in Britain, but in different parts of the world can appear on elms, pines or other kinds of trees; the growths can be safely removed, although they might reappear elsewhere on the tree.

• Mature birches can support up to around 100 growths.

• Traditional (and witches') broomsticks are made from birch twigs.