Hollows can form in places where a tree has begun decaying. The built up rotted ashes get washed out by wind and water, thus forming cavities known as hollows.

However, the most hollows form as a result of work carried out by woodpeckers (Picinae.) This group of birds brilliantly forges hollows in trees to create safe places to lay their eggs and raise chicks.

In Poland, you can find 10 different species of woodpeckers.

Only small birds, such as the Willow tit (Poecile montanus) and the Marsh tit (Poecile palustris,) have been known to actually pick out hollows in trees, though only in soft rotting wood. These birds use hollows to raise their chicks, but also as a place to sleep.

Forging a hollow is hard work that is done by both male and female birds. The time needed to complete a hollow can range from twelve to over forty days.

Forest Fun Fact

So you look at a hollow and spot a squirrel!

Although squirrels, such as the red squirrel (Sciurus vulgaris) often live in self-built nests that are located in the branches of tree crowns, they also often choose to live in hollows and bird’s nests instead. They always line their homes with moss and lichens, which keeps them nice and warm.