I’ve always been intrigued by what is hidden inside the secret pockets, holes and crevices in trees. I remember finding a young hatchling inside a tree hole when I was a child and being curious of it. Naively thinking that the bird had been abandoned, I started finding worms to feed it with. My parents soon told me not to interfere because the adult birds were probably keeping their distance -giving the chick a chance to fledge and fend for itself. Oops. Since then, my naivety may have lessened, but my curiosity has not so here is some information about the unique assemblages of wildlife found within tree holes.

Small holes and cavities provide habitat for wood-boring insects and fungi, but over time can turn into large hollows which offer shelter, breeding nest holes and access to a larder which would otherwise be out of reach. They are formed in many ways and take different lengths of time to become suitable for the many different inhabitants. Some holes are created by woodpeckers which go on to be secondarily used and adjusted by other species including wood-decaying fungi but also birds, mammals and insects. Others are caused by strong winds that break off branches to leave deep cracks, rot holes and snags – the latter are excellent roost sites for tree-dwelling bat species like barbastelles.

Great spotted woodpeckers are what we call primary excavators and create 5-6cm diameter circular holes in soft or decaying wood. They will use their bill to ‘drill’ through the outer bark and into the wood of mainly birch and oak trees. During the bird’s residency, the wood continues to decay, meaning their temporary home is gradually expanding and once it either fulfils its reproductive purpose or deemed unfit (too big with too much faeces inside), the birds will move elsewhere. They are, however, only the first in a long list of inhabitants throughout the tree’s life. A plethora of other bird species including nuthatches will occupy the empty hole. The empty dwellings of Great spotted woodpeckers are also among the favourite roosting sites for Noctule bats.

Rot holes are created when fungi gets into tree wounds when branches are broken off. These are ideal habitats for brown long-eared bats. Rot can also create hollows deep inside trees which is a preferred roosting habitat for Barbastella bats. When large limbs of mature trees are removed in high winds, the socket rot will eventually hollow out a hole suitable for Tawney owls and, if lowdown enough, even otters.

Invertebrates also make use of these spaces. The larvae of the cobweb beetle, for instance, will live in the small gaps behind old bark, and although we tend to think of honey bee’s living in the classic white hive that bee keepers have designed, in the wild, feral colonies build their nest inside hollow trees. In fact, there is a live, feral western honey-bees nest in the traditional orchard at Awnells farm in Herefordshire. The elegant hexagonal patterned structure is situated on the inside ‘walls’ of the tree about 1m from the ground. The entrance to the hive is through an oblong slit which is approximately 20cm tall and 4cm across (pictured). Honeybees line the nest walls with a mixture of antimicrobial plant resins and wax – a substance termed propolis – which is the cause for the darker colouration you can see in the image. This ‘propolis envelope’ provides some protection against parasites and disease and is an example of social immunity.

This is just a small snippet of the life that goes on inside tree cavities and there are so many other species that rely on hollows for shelter, breeding and accessing food. It is therefore important to leave old (or dying trees) as standing deadwood because these are essentially natures version of a block of flats.  

Ruth Moss

Wildlife Officer for Herefordshire