In my last post I looked at wall wildlife in terms of the plants and lichens that have adapted to grow on them. This time I’ll introduce some of the birds and animals that make use of walls. This may be as refuges, nesting holes, feeding grounds or perches. It would be easy to think of walls as dead and devoid of nature. But an incredibly diverse range of organisms make use of them. This makes them remarkable manmade mini nature reserves, especially if the base is left unmown with plenty of rough grass allowed to grow. Birds and beasts of all sizes can even use them as highways to travel safely and secretly through the countryside without touching the ground.
A vast array of invertebrates live or spend time on walls. Because invertebrates are generally small, whether a wall is dry stone or mortared is less of a determining factor to whether they can make use of one or not. Even a mortared wall will often have enough differences in texture to provide a hiding place, hunting ground or egg-laying spot.
A number of spiders build their webs around small cavities in a wall. The funnel weavers, three species in the Amaurobius genus, are particularly common on walls. They often make a tubular entrance to the web to hide from potential prey. Woodlice, millipedes, earwigs, harvestmen and solitary bees will all also take advantage of any tiny holes in a wall to hide from predators, hunt other invertebrates or lay their eggs. Some invertebrates, such as slugs and snails. feed on the lichens, mosses and plants growing on walls instead. Groups of snails sometimes also overwinter together in wall cavities. Where ivy grows on a wall, pollinators can find food well into the winter as a result of its late flowers.
Butterflies often take advantage of sheltered wall faces, and the heat they generate, to bask in the sun. Whereas most butterflies are easy to spot on a wall, many of our moth species have more cryptic markings. Some have patterns that blend in with stones and rocks. Walls are often a useful alternative, allowing them to rest safely during the day. This camouflage even helped demonstrate natural selection in action in the 19th century. As industrial cities were blackened with soot, darker peppered moths were better camouflaged on soot-coated walls and trees than lighter ones. They were consequently more likely to live to pass on their genes. They rapidly outnumbered lighter individuals for a time until the government passed the Clean Air Act in 1956.
A few species of butterfly and moth also overwinter as adults in the UK. These include the peacock, brimstone and small tortoiseshell butterflies and the herald moth. Going into a dormant state for the colder months, they sometimes use wall holes or thick ivy coverings to do so.
Amphibians and Reptiles
All those invertebrates inevitably attract the larger creatures that eat them, including reptiles and amphibians. The UK’s two commonest lizards are the common, or viviparous, lizard and the slow worm, often mistaken for a snake but actually a legless lizard. Both eat a variety of invertebrates, many of them found on or around walls. Spiders, flies, caterpillars and a host of other insects, plus snails, slugs and worms are all prey items for the two species. A wall’s many holes are also useful hiding places for lizards and slow worms to take cover themselves when avoiding larger predators. In addition to being productive hunting grounds, walls are often also great basking spots. Because reptiles are cold-blooded, they need to spend a certain amount of time basking in the sun to keep themselves warm. South-facing walls are perfect for this.
Frogs and toads also take advantage of walls for both food and shelter, although both species need water to reproduce. They lay hundreds of eggs in either clumps (frogs) or strings (toads) of spawn. These hatch into tadpoles after a few weeks. Once frogs and toads are in adult form, they often spend most of their time across a variety of habitats. Frogs are less tolerant of drier habitats than toads, but both ultimately prefer moist, damp places. Damp, dark wall holes are therefore perfect spaces for them.
As well as being rich in many of their prey species, such as slugs, snails and other invertebrates, they are also useful for shelter. This is especially true in hot weather if no water bodies are nearby. Frogs and toads both hibernate in the winter. They sometimes use wall cavities, or even the piles of rubble from a collapsed wall, to do so.
A variety of bird species utilise walls for a range of purposes, feeding being one of the most obvious ones. Where ivy has gained a foothold, blackbirds and thrushes feast on the fruits in late winter and early spring. And of course, just as for reptiles and amphibians, a wall’s invertebrate populations are an obvious draw. Smaller birds such as wrens and blue tits will pick off tiny organisms including spiders, millipedes and flies. Larger birds, such as corvids and raptors, will hunt reptiles, small mammals and small birds feeding or sheltering on a wall.
Walls are also good vantage points for birds such as stonechats and raptors searching for food in the surrounding area. Waders such as redshanks, curlews and oystercatchers use them to look out for danger. Song thrushes sometimes use them as snail anvils. Raptors also use walls as plucking posts where there are no suitable trees in the area. They are good places to look for prey remains and pellets from regurgitated meals. Birds probably also use linear walls to help navigate.
Birds use walls during the breeding season as well. Even before pairing up, males use walls as song and display perches. These territorial displays are aimed at deterring rival males from the area, as well as announcing their attractiveness to potential mates. Once breeding proper begins, a number of species use wall cavities for nesting. Wrens and starlings in particular nest in walls. In fact, in spring it seems every wall in Shetland is riddled with baby starlings. Robins, blue tits, great tits, spotted flycatchers and little owls amongst others are also sometime wall nesters.
In upland areas wheatears commonly nest in dry stone boundary walls, sheep fanks or old sheiling walls. And incredibly, in some places, including the island of Mousa in Shetland, the tiny storm petrel nests in drystone wall cavities, as well as their usual burrows or spaces under boulders. On Mousa, this even includes the walls of the island’s Iron Age broch.
Last, but not least, many of our smaller mammals find shelter and feeding opportunities in walls. Voles, mice and shrews make nests inside any cavities, lining them with grass, fur and leaves. Shrews use walls as well to hunt for their invertebrate prey, including insects, slugs, snails and worms. Stoats and weasels both commonly nest in walls. This pair of tiny predators make a number of nests within their territory. They often take over the homes of the small mammals they have killed. They also use walls as cover to hunt their prey. Hedgehogs, meanwhile, take shelter in any rough grass at the base, as well as hunting any slugs and snails around. Where there are no suitable trees, caves or buildings around, bats will sometimes roost in any higher wall slits. Like birds, they may also use walls as navigational aids.
To some of our larger mammals, walls can be more of an impediment to movement, of course. Most deer, as well as foxes, can easily jump all but the highest walls. But they are more of a barrier to rabbits, hares and badgers. Because of this, the Drystone Walling Association of Great Britain recommends putting in holes or ‘smoots’ to allow animals to pass through. This is especially useful if there is evidence of badgers in the neighbourhood as they can otherwise easily dig underneath, causing unwanted damage to a wall. Damaged walls are not only no use to the humans creating them in the first place, but they also lose a lot of their attractiveness to wildlife. This means that although some degradation in the form of soil production and holes are fine, walls need to be carefully maintained to ensure their overall structural integrity.
Wildlife and Walls
Just as plants and lichens have adapted over thousands of years to take advantage of the opportunities that walls offer, so many of our birds and beasts have too. Complicated food webs can be seen in action on walls across the UK. Whether it is a snail feeding on the numerous mosses covering a wall, a wren hunting spiders from its nooks and crannies or a weasel on the prowl for that same wren, predators large and small find food here. And many of these species also rear their young, shelter from adverse weather, hibernate or travel across the countryside in or on them too. From their earliest use by our first farmers 3,000 years ago, to their continued presence across huge areas of our uplands in particular, walls and wildlife have formed a remarkable partnership.
Anyone interested in the history of the UK’s dry stone walls or in supporting their preservation can find information from the Dry Stone Wall Association’s website. They also run training courses at all levels for anyone wanting to learn how to build walls, either for themselves or professionally.
Ireland has an equivalent organisation, found here.
The Wildlife Trusts have a short guide to building your own mini dry stone wall to help wildlife here.