Nooks and Crannies

I find delving into the nooks and crannies of trees an irresistible curiosity.   That’s probably because, over the years, I’ve found all kinds of unexpected and very specialised beasts lurking in the gloom, many to my great surprise.

Tree holes and cavities come in many shapes and sizes, from totally hollow or fissured trees to the gaps in or behind peeling bark.  The creation of cavities is not simple process and often takes many years to form.  Most are initially the result in some form of mechanical or natural damage such as lightning strikes, gale force winds, animal behaviour or human activity.

After the bark is damaged, the underlying wood is open to attack and destruction by fungi.  This is often aided by insects, especially beetle larvae that spread the fungus as they bore into the timber. As the structure of the wood is broken down the damaged area is enlarged and rot-hole cavities are formed.

Some of these holes are in sheltered areas and remain dry while others, if exposed to driving rain, may remain damp or even collect and hold water.  This provides a wide range of niches for wildlife.

Let’s explore the wet holes first.  These tiny aquaria contain vast numbers of very specialised animals, ranging from the microscopic, slipper-shaped Paramecium and the asteroid-shaped Cyclops (remember them from your old school biology lessons?) to the three different mosquitoes which breed exclusively in such places.

Certain hover and drone flies also have their larval stage in these inhospitable stagnant pools containing rotting leaves.  They overcome the lack of oxygen in the water by breathing air through a snorkel-like tube which connects them to the surface.  These so-called “rat-tailed maggots” do very well in these watery crannies because of the abundant plant food, and the fact that would-be predators like beetles and dragonfly larvae do not use this habitat.

Even in times of drought these pools rarely dry out altogether, and as long as the decaying leaf mould is damp these animals’ eggs can survive, lying dormant until spring.

Permanently moist holes contain another group of creatures – including many slugs or snails – that need this environment to prevent them from becoming desiccated during the day.  Woodlice are also found and are very vulnerable to dehydration: to maintain body fluid levels they can take in water by drawing it up through their rear ends!

Dry holes will provide a very important opportunity for other animals and birds.  More than 20 species of British bird prefer to next in tree holes or cavities, including the blue tit, little owl, starling, kestrel and nuthatch.


Those instant hole makers, the woodpeckers, do an incredibly important job when hacking out their nest chamber.  After using its newly-excavated hole for one season, it’s left for other species to use in subsequent years.  Starlings in the countryside often use them, but as the years roll on and the tree narrows the openings by forming a callous around the hole, it only admits progressively smaller species like the great tit and blue tit.

Many mammal species use these holes as safe houses; red squirrels often use the, as do woodmice.  Of the county’s bats, the noctule is the species most often found in woodpecker holes, probably because they draw attention to themselves by their noisy chattering.

The nuthatch has a ploy to overcome odd-shaped or oversized entrances; it simply bungs up the hole with mud until it’s exactly the right size.  You can do a similar job to enhance any less desirable unused natural hole by using a wattle and daub technique with twigs as a framework, and a mixture of mud and moss thrown on.

In dry situations this can least a number of years, but a weak mix of sand cement and wood shavings makes a more durable solution.  Remember to consider carefully the hole size and the cavity’s usefulness for other wildlife before any project like this.

I’ve used this technique quite a lot now, because I feel bird and bat boxes look so out of place in a “natural” woodland or wilder garden settings.  Success may be a little slow to start with, but you will be surprised, after a bit of fine tuning in site selection, how productive and aesthetically pleasing this alternative is.

Barn Owl

I remember the highlight for me was encouraging a barn owl to next in an old hollow ash tree.  The problem was that the tree resembled a leafy chimney with a large opening at ground level and another halfway up.  By blocking the lower hole with clay and stones and part-filling the cavity with twigs and leaf litter, the tree was occupied within a year.  The barn owls have now successfully reared 12 youngsters over the last five years with odd breaks when tawny owls took up residence.  A goosander has been hanging around the area this year, so watch this space.

As already mentioned, rot-holes take a long time to develop, and as a result are usually found on mature, dead or dying trees.  In this age of increasing liability and drive for tidiness, “dangerous” or unsightly trees are increasingly being removed, in the main, for perfectly sensible reasons.

However, I would say that consideration should always be given to making the tree or its surroundings safer first, rather than removal being the only option.  One north Northumberland estate has gone to considerable expense and inconvenience retaining and making safe several important cavernous trees used by bats and owls.  What’s more important is that this action has greatly extended the overall wildlife potential of these old giants for many years to come.

I have heard recently that other intriguing techniques have been used to enhance this threatened habitat.  Wind-felled trees have been reduced in length and re-erected after cavities have been formed with chainsaw and drill.  Rotten wood, leaf mould, sawdust and even an odd dead rabbit have been thrown into the holes to get the natural processes started.  They even used explosives to blow off large limbs to allow fungi to get a quicker hold.  That’s lateral conservation thinking for you!