Wildfowl and Wetland Trust, Arundel, West Sussex: It is beset with parasites, some the size of footballs and one engulfing an entire bough, but they cause no long-term harm

As the light began to fade, we loitered on the path, hoping to spot marsh harriers drifting in to roost in the reedbeds. I was soon distracted by a family party of long-tailed tits foraging in a stand of silver birches, their lollipop stick tails flitting like leaves blowing in a breeze.

As I followed the ripple of movement through the bare branches, I noticed that one mature tree was studded with witches’ broom galls. Each emerging from a single point on a branch, these dense clusters of twigs resembled scrappily constructed pigeon nests or, as the name suggests, traditional besom broom heads. They can be caused by insects, mites, fungi or bacteria, but the most common culprit affecting birches is Taphrina betulina, a fungal pathogen.