It is that time of year that birds are starting to think about nesting and if you are at the car park at Flanders you could be mistaken in thinking that one of the birch trees on the edge of the Moss is full of birds nests. But what you are seeing are Witches Brooms. Not birds nests but birch disease. These masses of twigs are caused by a fungus called Taphrina betulina that enters into the tree and upsets the plant hormones (chemicals) that control the growth of the tree. The exact arrangement of twigs and buds in all trees are controlled by these chemicals, each tree has different chemicals which give the tree its distinctive arrangements of twigs and branches. This fungus messes with these chemicals and encourages a burst of twig growing. Why ? So that the fungus can then feed on the increase of plant material. The tree itself isn't actually harmed and can have these "birds nests " on it for years.
If the twig growth all goes in one direction then it can look a bit like a birch or witches broom and was probably used for sweeping where possible. The other theory on their formation is that they are formed when witches fly over the tree. In which case we have a witch flyway over the moss as there are so many.
So if you are in the car park before the leaves are back on the trees check out the witches brooms but keep an eye for low fliers overhead as well.
