That deep dark hole you spy in a tree may look empty, but there is a very high chance that it will be home to some species of wild animal, if not a bird then maybe bats, or a squirrel, or even dormice. In breeding season, birds will create nests in which to lay their eggs.

Those that use holes as their nesting sites are known as cavity nesters. Nesting in holes is far safer than what some may refer to as the “traditional” cupped nest, and they provide virtually weatherproof homes for young chicks when hatched that are also able to retain heat well.

Dead or decaying trees are also usually packed with insects and grubs who aid the decomposition process, meaning an abundance of food right at your doorstep.

Half of all bird species are cavity nesters.

Cavity nesting only applies to birds who create or make their nests in holes ofexisting structures, rather than those who make independent structures such as the elaborate hanging pouches that some of the tit family, some orioles and weaver birds make for a nest.

Moving in

Half of all bird species are cavity nesters of some sort, be it one they have painstakingly carved out of live wood themselves, or perhaps a fungus has made a tree soft enough to remove the diseased parts, or just a dead and decaying tree, known as a snag when it is still upright, or a log when it has fallen.

There are two types: those who make their own hole, called primary excavators, and those who don’t make their own but use either natural or previously excavated cavities, called secondary cavity nesters.

Secondary nesters will use abandoned sites but will also sometimes aggressively oust a primary excavator. Secondary nesters will also readily use artificial nests such as bird boxes.

The woodpecker family is the most widely known primary excavator, but other species use their bills to make holes, like the nuthatches and a genus of bird called trogons, whose name comes from the Greek “to nibble”, which is how their cavities are made, just nibbling away at the bark, tiny chip by tiny chip.

These birds have stout hooked bills and they live in the Americas, from Arizona and down through the latitudes to Argentina. Owls, chickadees and parrots can also create their own nesting holes, but will prefer a ready-made one from decay if they can find one.

Owls are prime examples of secondary nesters – decaying trees are top-notch properties, and there are some owls who live in underground burrows, which they have either dug themselves or taken over from mammals like prairie dogs, or you can even find them in the ends of outlet pipes.

Primary excavators often choose softwood trees like aspen, or hardwood trees whose bark has been softened by fungal diseases. The spores of these diseases affix themselves to woodpecker bills, which then get spread to other trees, aiding other birds who lack the power tool of the woodpecker bill to make their own cavities.

Dig deeper

But it is the woodpecker who takes the prize for hole-drilling, and these birds are incredibly important to ecosystems, enabling all manner of cavity-dwelling species to use their abandoned creations.

It usually takes your average woodpecker two weeks to drill out a cavity, with both male and female birds doing the work, and those that make holes in living trees will either return to the same tree the following year to make a new one, or do an impressive amount of planning ahead and create several holes in one season and then use a new one each year thereafter. The red-cockaded woodpecker nests in living pines in the US, and can take up to two years to excavate its cavity.

Competition is so fierce for these old nesting sites that many birds will often freely and gladly use artificial birdhouses.

Whilst predators can of course enter a cavity and once inside there is no escape for the inhabitants, some bird species use a wide range of defence tactics that are often triumphant.

‍Woodpeckers usually create the cavity on the underside of a tree branch, in the hope that predators will overlook it, and some species have found ways to keep them out too. Red-breasted nuthatches smear the outsides of their nests with conifer sap, which acts as a sticky deterrent, and white-breasted ones will “sweep” their nest recess with foul-smelling insects to leave an uninviting pong emanating from within.

Birds rarely use the same cavity twice (especially if it is sticky and smelly), which leaves them for other species to use. Abandoned woodpecker holes make up the majority of new homes for secondary nesters who will simply construct a basic nest on top of the old one, maybe adding things like moss, animal fur or grasses if it has become a little threadbare.

Some birds are very aggressive secondary nesters, so even if you have been lucky enough to find your own patch, species like starlings or sparrows will fight you for them, and you will likely lose. ‍

Competition is so fierce for these old nesting sites that many birds will often freely and gladly use artificial birdhouses, so if you have the means and the space, please do consider putting them up.

Purple martins used to live in colonies in trees whose trunks sported several abandoned woodpecker holes, but aggressive species are so abundant in their territories they are now entirely dependent on artificial houses placed throughout forests by conservation groups.

The more opportunities birds have to find a home for their potential family, the better.

Of course, not all cavity-dwellers live in trees – kingfishers make their nests in the gentle curve of muddy riverbanks, bee-eaters and sand martins flit in and out of the dusty walls of quarries, many a wren will find peace and quiet within the crumbled mortar between blocks of masonry of long-ago built houses.

The gilded flicker is a type of woodpecker that lives in the southwestern states of America and northern Mexico who excavates a hole towards the top of a saguaro cactus. The cactus defends from losing too much water into this cavity by secreting sap which then hardens into a waterproof structure known as a saguaro boot.

Let sleeping logs lie

If you have trees on your property that are dying or dead, or you see fallen logs in a place that you think may look “untidy”, bear in mind these may be home to a whole host of species who need their space.

If possible, leave dead branches on living trees, and ask a qualified Tree Risk Assessor to help you figure out if it needs to come down. The more opportunities birds have to find a home for their potential family, the better.

Given that birds will nest anywhere they can, always be mindful of using machinery that has pipes, chimneys, vents, or any other aperture that could fit a bird or two.

A family of great tits has set up house during breeding season for two consecutive years in the open-ended gate posts across the road from my house; the owners are long gone and the house itself is a ruin, but a clutch of tiny birds will hopefully keep popping out the top each June making this forgotten home alive once more.