Birds as Builders of Homes

Even the sparrow finds a home,
and the swallow has her nest,
where she rears her brood besides thy altars,
Oh Lord of Hosts, my King and my God.
                            Psalm 84 v.3

Birds, far more than any other animal besides man, are notable for their tendency to build a home in which to raise their young, and in some cases to use all year round.

Birds are not the only animals to make a nest; fishes, mammals, insects and spiders also build nests, sometimes quite complex ones, for rearing young and for their own general protection. For the extent of this page we will define a nest as anything constructed by a bird or where a bird lays its eggs. This is the broadest possible definition and allows us to discuss the full extent of bird nesting habits.

 

Bird nests range from non-existent to extensive, multi-chambered apartments which can be fully weatherproof and may last for years or even decades. Nests come in a wide range of categories, the following groups cover most nests you will find. Some of these 12 categories are quite small, others are large containing a huge diversity:

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No NestFloatingEdibleMounds
Simple Cup-shapedEnclosedHanging
Holes in the GroundMud NestsHoles in Wood Not Nests

Introduction

Before building a nest the pair have to decide on where to build. This is called 'nest site selection' behaviour. Different species go about this in various ways, in many species both partners work together to decide on the site. Birds in this category include many gulls. In some species, though the pair work together the female takes a definite lead on the proceedings, i.e. Blackbirds, Turdus merula, and surprisingly the Red-necked Phalarope Phaleropus lobatus. This is surprising because once the site has been chosen the female lays her eggs and departs, leaving the male to do all the incubation. In other species such as the Dunnock, Prunella modularis, the female chooses the site and builds the nest. In contrast, with Blue tits, Parus caerulea, European Sparrows, Passer domesticus, and Wrens, Troglodytes troglodytes, it is the male who chooses the site and then tries to attract the female to it. The male Wren is a bit of a workaholic and builds several nets, normally 4 or 5, but up to 12. The female chooses one of these and the male uses another to roost in. Just to prove that variety is the spice of life, Scottish Crossbills, Loxia pinicola, show no definite patterns. In some couples the male leads, in others the female.

The second behavioural activity is material collection and building. These range from 'sideways throwing' - a simple single movement to get nesting material to the nest, this is limited to ground nesters only. 'Sideways building' is similar but involves more care in the placing of the material and results in a better constructed nest. Sideways throwing and sideways building are exhibited by many ducks, geese, gulls, petrels, pheasants swans and waders.

Physically carrying material to the nest site is the next step up and is carried out by all the remaining nest building birds. At the simplest it is shown by penguins carrying a stone in their bills few metres to the nest site. At its most complex it involves birds searching out for particular substances such as cobwebs and feathers to bring to the nest.

Once material is brought to the site it needs to be incorporated into the nest. For ground nesting species this can be as simple as just picking it up. For tree nesting species, it usually involves some degree of interweaving the individual items until they form some sort of matrix. This can be fairly straight forward in the platform nests of pigeons, but reaches great sophistication in the weavers where material is actually sown together, where a considerable degree of manipulatory skill is needed. Whatever the type of nest, watching a bird build one is a fascinating and rewarding experience.

Architecture but not Nests

Finally there are structures built by birds which do not really fit into any of the above categories, because they are not nests, these are the bowers of the various species of Bowerbird.

Male Bowerbirds build structures which though often involving great effort are not actually nests. These bowers, built by 14 of the 18 known species of Bowerbird are stages or advertisements. They are built to attract females which presumably are attracted to larger more ornate structures and which judge a male on his collection of treasures. The real nest is built by the female after she has been mated by her chosen male and she incubates the eggs and raises the young on her own while the male stays with his bower to try and attract more females. Bowerbirds build different sorts of bowers even within one species. Different populations build different designs and collect different 'treasures'. Some simple bowers consist of an avenue of twigs which the male bird walks up and down to display himself to the female. Some of these may have the sticks painted with yellow, brown or purple plant juices.

More complicated bowers involve towers of sticks and display arenas on which the male arranges his collection of treasures and around which he displays himself. Treasures include feathers, particularly blue ones, snail shells, beetle wings and heads, bones, flowers and anything else which takes the bird's fancy which may include man-made objects such as silver spoons, car keys, gun cartridges, tin mugs, buttons and other colourful scraps of material.

The most impressive bower is built by the Vogelkop Gardener Bowerbird (Amblyornis inornatus) from New Guinea which in some areas builds a huge open-fronted roofed hut up to 2.2 m tall and 2 or more metres across. This structure is built by a bird the size of a Song Thrush.

Australasian Bowerbirds are not the only birds to build structures that are not ever destined to be nests. Others include the Tooth-billed Cat Bird (Scenopoeetes dentirostris), also from Australia, and Jackson's Whydah (Euplectes jacksoni) from Kenya and Tanzania. Both of these make structures which are quite simple in comparison with some of the bowers described above. Cat birds line a 1-2 m arena with upside down fresh leaves which they cut from vegetation with their toothed bill. Jackson's Whydah is a leking species in which each male has a personal arena about 1 m across with a pseudo-nest. Mating normally takes place during inspection of the pseudo-nest by the female. Like all leking species the female builds the true nest later, incubates the eggs and raises the young on her own.

Recommended Reading

If you have found this page at all interesting you will want to read "Birds as Builders" by Peter Goodfellow ISBN 0 7153 7410 (Out of Print). Not only does it contain far more fascinating information than I was able to include here, being 160 pages long, but it is much better illustrated.

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No Nest

Fairy TernThe simplest nest of course is no nest at all. Quite a few birds do not build any nest at all, though they do make a choice of where to lay. Beyond this is a simple scrape in the ground. Nightjars (Caprimulgidae) do not make even a scrape. They lay their two eggs directly onto the ground. While Short-eared Owls, Asio flammeus, lay simply on some trampled vegetation. Many waders, such as Plovers, lay in a simple scrape on bare ground, relying on the cryptic colouration of their eggs to protect them from predation.

Some of the more unusual examples of no nest laying include the Potoos, Nyctibius spp., which lay their single egg on top of a broken off tree stump. The bird then sits on top of the stump with its head pointing to the sky. In this position it looks like an extension of the dead timber. It generally chooses stumps of similar diameter to itself. Even trickier are the 'nests' of Fairy Terns, Sterna nereis, another species which lays only one egg. The egg is laid on a branch on a rock face, generally on an area too small to support 2 eggs. Laying the egg in a position where it won't roll off is tricky and young birds often lose their first attempts before they get the hang of choosing a safe spot. Fairy Terns have two adaptations to help them survive this seemingly precarious nesting habit - The adults have evolved to be very careful when settling on and getting off the egg - both sexes incubate the egg. The second adaptation is that young Fairy Terns have disproportionally large feet and very sharp claws which help them hang on.

The master of no nest nesting, however, has to be the Emperor Penguin (Aptenodytes forsteri). These birds nest in the Antarctic during winter when the temperature can fall to less than -30 degrees C. They nest well inland and when the female has laid the single egg she leaves for the coast to feed. Meanwhile the male has immediately taken over care for the egg by moving it up onto the top of his feet. He has special folds of skin on his belly which enfold the egg keeping it safe against all the severity of the elements. Here he stays, a living nest with an egg on his feed for the next 60 days without a meal or much movement until his mate returns and the egg hatches. During this time he has maintained the internal temperature of the egg at 40 degrees C, as much as 80 degrees C higher than the surrounding ambient temperature.





Simple Nests

Simple nests are constructed of either plant material, normally sticks or stones. They are quite common and like no nest nesters all the birds that use them are non-passerines.

The best example of a simple stone nest is that of Adelie Penguins. The nest is basically a mound of stones which serves as protection against flooding.

Most simple nests, however, are made of sticks and twigs built into a simple platform, which though it may have a depression in the middle and be quite extensive does not really have an inside and outside.

A common example of a simple platform nest are those constructed by many doves and pigeons. Those, like that of the Ruddy Quail Dove, Geotrygon montana, are often a simple lattice with just enough twigs to support the egg. In these cases the egg can often be seen through the nest from beneath. Sometimes a flimsy lining of grass and rootlets may be added and in some cases the lining may be more complete. Another bird which builds a simple platform is the Hoatzin from S. America. Another well known example of a platform nest and one which is more substantial than a pigeon's is that of the European White Stork, Ciconia ciconia, which has for centuries nested on chimneys and specially erected platforms across Europe. More complicated than the previous nests, it consists of sticks interwoven then plastered to some extent with mud. A depression in the middle is often lined with grass or paper.

Mute Swans' (Cygnus olor)nests are an example of a different form of simple nest construction. No material is carried to the nest. Instead, once the site is chosen and the vegetation flattened, the eggs are laid and the female swan builds up the nest around her by dragging material within reach to her so that eventually it forms a rampart around her.

Other birds which build simple nests include many gulls and herons. The most spectacular nests in this category are those built by various eagles. Most build in trees and the nests of the Bald Eagle (Haliaeetus leucocephalus)in N. America can be so heavy that they damage the tree supporting them. The largest nests on record, however, are those of the Golden Eagle, Aquila chrysaetos, which tend to build a new nest on top of the previous year's nest, resulting in huge structures often containing more than a ton of material. Fortunately, Golden Eagles prefer rocky alpine crags as nest sites which can often support these massive structures more easily than a tree. Golden Eagles are known to adorn their nests with sprigs of green boughs throughout the nesting season, well after the young have hatched. Many of the foremost researchers believe that this is evidence of a rudimentary aesthetic sense.

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Cup-shaped Nests

The next step up from a simple platform nest is a cup-shaped nest. These are the most numerous form of nest and one that most people consider a typical nest. They are distinguishable because they have a definite inside as well as an outside and the inside is normally lined. They require more effort to make but convey more protection to the eggs and young birds.

Cup-shaped nests can be built in a variety of places, but normally they are built in trees. Often the simplest form is wedged into a 'Y'-shape division of a branch, but many birds bind or cement them directly to a bough.

Simple Cup NestThe smallest cup-shaped nests belong to the Humming birds and Woodstars which build perfectly shaped thimble sized nests of moss and cobwebs. Often the female bird flies during the whole construction, hovering here and there while building up the shape. Different birds' nests take different lengths of time to build. Some are completed in a day, others take 2-3 weeks to complete.

Cup-shaped nests are often built of a mixture of substances. Redwings, Turdus iliacus, build nests of leaves, grasses and fine twigs cemented together and then lined with moss and feathers. Not all cup-shaped nests have a soft lining though. The female Song Thrush, Turdus philomelos, lines her nest with mud, dung and rotten wood all of which is cemented together to form a very hard inner surface.

The other extreme to the delicate nest of the Humming birds are the large scraggly looking structures built by crows. The initial outside of the nest, built of twigs and grass cemented together with mud, is larger than the adult bird. Within this rough exterior a deep cup lined with moss and feathers is made. Crows' Corvus corone nests often look rough and ready, but inside they are warm and comfortable. Crows build very well and their nests last for several years, but unlike the closely related but larger Raven Corvus corax, they never reuse a nest. Other birds, like Kestrels Falco tinnunculus, are happy to use them once the crow has finished with them.

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Enclosed Nests

Enclosed nests are basically an extended cup nest where the basic cup has deeper walls and a roof, with an entrance hole. Domed or enclosed nests would seem to offer more protection for the eggs and young than a simple cup. In some cases they obviously do, and they also supply better protection from the elements, though there is not much scientific evidence to support the theory that they offer better protection from predators. As with all these categories of nests there is a gradient of complexity and quality among domed nests. The simplest have only a loose roof like those of the Chiffchaff, Phylloscopus collybita, or a separate roof constructed about the dome such as the nest of the Magpie, Pica pica. Wrens and Dipper build more solid domes.

Hammercop on NestMany domed nests are very intricate and solidly constructed. They may be small and beautiful such as the nests of the Long-tailed Tit, Aegithalos caudatus, which camouflages the outside with lichens and lines the inside with hundreds of thousands of feathers. The largest and most spectacular domed nests belong to the Hammerkop, Scopus umbretta. These birds build a huge dome of over 8000 sticks which can be 2 m high and nearly 2 m wide. This nest is easily strong enough to withstand a man walking across the top of it. They have a long entrance tunnel and are the avian equivalent of a fortress.

A more unusual example of a domed nest is the nest of the Rufous-breasted Castle Builder, Synallaxis crythrothorax, which creates a dumbbell-shaped nest with a tube connecting both chambers, only one of which is used to rear the young.

Hanging and Woven Nests

Hanging nests, particularly woven ones are perhaps the most admirable of bird architecture. They certainly look very beautiful and most require great skill on behalf of the bird to build.

The simplest hanging nests are cup-nests slung from an overhead bough by a few cobweb supports such as the nests of the Goldcrest, Regulus regulus, the Fire Crest, Regulus ignicapillus, and various White Eyes Zosteropidae.

An interesting variation is the hanging cup nest of the Hummingbird (Planalto Hermit), Phaethornis pretrei, which has only a single support cable for its nest. To help keep it stable it has a streamer of grass and cobwebs hanging down below the nest.

Other birds which build hanging nests include the Reed Warbler, Acrocephalus scirpaceus, the Fantailed Warbler, Cisticola juncidis, and the Rock Warbler, Origma rubricata, though this last species is not technically speaking a warbler.

Better known than these are the hanging, often on mass, nests of the Icteridae and Oriolidae. There are nearly 100 species of weaver birds renowned for their carefully woven hanging nests. These nests tend to be either hung from the tip of a branch or leaf, or suspended between two twigs. They are globular in shape with a single entrance hole. Apart form globular nests, weaver Birds also construct kidney-shaped nests and retort-shaped nests are basically globular nests with an entrance tunnel. Whatever their shape, nearly all weavers make their nests out of grass and the nests are truly woven with the bird moving from side to side, poking part of the strand of grass through the wall from the side and then pulling it completely through from the other.

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Holes in the Ground

Many birds nest on the ground with nests of varying degrees of complexity but far fewer nest within holes in the ground. Nesting in holes would seem to be a better strategy than nesting in the open. One survey of over 180,000 eggs revealed that where non-hole nesters averaged 50% rearing success, hole nesters averaged 70%. Two main sorts of holes can be distinguished; those which open onto a cliff edge or a vertical edge of a river bank, and those holes in relatively flat ground. A second plane of division is between those birds which dig their own holes and those which make use of other people's.

Commonly recognised birds which nest in cliff edge holes include House Martins (Delichon urbica)and Sand Martins (Riparia riparia)and the widely distributed European Kingfisher (Alcedo atthis). Other Kingfishers such as the Malachite Kingfisher, Alcedo cristata, several parrots in the S. American genus Conure and the Ground Woodpecker, (Geocoloptes obroceus) of S. Africa also nest in cliff edges. More unusual are D'Arnoud's Barbet, (Trachyphorus darnoudii), and the White-whiskered Soft-wing, (Malacoptila panamensis), which nest in holes dug into flat ground. These sorts of nests may all be lined to some extent and are generally dry well-protected homes.

Excavating your own holes, of course, is hard work and many birds are happy to take possession of someone else's efforts. Thus the Shelduck, (Tadorna tadorna), among several ducks, and the Manx Shearwater, prefer to nest in abandoned rabbit holes. Not as pleasant as this, Cinclodes sp. and Sharp-tailed Stream Creeper's (Lochmias nematura)nest in rodent burrows. In the latter case the burrows are often in the edges of open sewers giving the bird the nickname 'President of Filth' in Brazilian shanty towns.

Holes in Wood

Mud or earth, of course, is not the only place to make a hole and many birds nest in holes found or excavated in trees, cacti and even in termite nests.

Making a nest in a hole that already exists in a tree is not really an architectural feat as it involves little effort on the bird's behalf. Still, holes in trees, alive or dead, make excellent nest sites and numerous bird use them. Some, like the Blue tit, Parus caerulea, and several of its relatives, Redstarts (Phoenicurus phoenicurus), the common Starling, Sturnus vulgaris, and many of its relatives, as well as various Flycatchers use existing holes to nest in, making very little modification to the hole except to supply some lining as nest material. Others, like the many hole nesting Parrots and Nuthatches, will modify existing holes to varying degrees. Fewer birds excavate their own holes in trees, but of those that do the woodpeckers, with their impressive hammer-drill impersonations are by far the best known.

However, the prize for champion tree hole nester has to go to the various species of Hornbills, Bucerotiformes. These large birds, with the exception of the two species of African Ground Hornbill, all nest in hollows in trees. This is no simple matter. The Great Indian Hornbill is a large bird, nearly a metre from bill-tip to tail-end, which likes to nest between 20-45 metres up the tree trunk so can only nest in trees which have a diameter greater than 1-2 metres at this height. Trees this large are now rare in many forests putting serious pressure on the breeding capability of these birds.

Both males and females help excavate the hole which needs to be quite extensive to house the female and several chicks for some weeks. Once the hole is large enough to accommodate the female, she gets inside and helps the male wall up the entrance with a mixture of guano, woodchips and mud. The female will remain in the hole until the young are ready to fledge. Only a small slit will be left in the mud wall to allow the male to feed the female and her young. During this time she will not only raise the chicks in great security but also moult all her feathers in one go. Hornbills are long-lived birds and mate for life, so the male has a considerable vested interest in keeping the female well-fed.

Nesting in holes may be secure form many predators and much of the weather, but it has one drawback. The warm, humid conditions make ideal breeding conditions for various avian pests and nest parasites such as bird and feather lice, ticks an fleas. With a captive food supply in the young birds, parasite loads in nest holes build up rapidly. Hornbills never use the same hole twice and the need to escape these pests may have something to do with this.

Other favourite sites for nest holes are cacti, many of which grow as large as small trees, and termite nets. Each of these provides an interesting example of commensualism. Firstly, the Orange-fronted Parakeet (Cyanoramphus malherbi) of Central America nests almost exclusively in mounds built by Nasutitermes termites. Unlike other birds, mammals, etc, which nest in termite mounds, Orange-fronted Parakeets do not have the nest hole sealed off from the rest of the termite mound. Soldier termites can wander around the nest and workers remove the young birds' faeces and any parasites they can find. Obviously, this is good for the Parakeet, but what the termites get out of it, no-on knows for sure.

The second relationship is quite amazing. The Gila Woodpecker (Melanerpes uropygialis) nests in holes which it excavates in the famous Giant Saguaro cactus of N. America. In the same hole with it, nests the Elf Owl (Micrathene whitneyi). The Elf Owl is tolerated because it has the amazing habit of catching Western Blind snakes and bringing them alive back to the nest. The snakes are insectivores so benefit by having a cosy home and free food in the form of avian parasites. The woodpecker benefits because it gets a reduced parasite loading, thus improving the health of its young. The Owl gets not only a reduced parasite loading and a free nest site but also protection for its young while it is hunting at night when the woodpecker is roosting in the hole.

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Mound Nest

One of the strangest nesting habits is that followed by the Megapodes. On Saro in the Solomon Islands, come nesting time the already mated female Scrub Hens leave the forest where they have lived all year and come down to the beach to look for an area of sand known as a geu. In a geu the sand is heated form below by geothermal energy as well as from above by the sun. Into this sand the females dig a hole about 60 cm (2 ft) deep, and after testing the temperature of the sun with special heat sensors on their tongues (about 33 degrees C is best), they lay their eggs and fill the hole in. The females then return to the forest and expend no further effort on their offspring's behalf.

This and similar systems by other Megapodes such as the use of hot springs on Celebes and rotting tree stumps on other Solomon Islands involve no building effort at all. However, these facilities are not always available and in Australia and Papua New Guinea various Megapodes use the heat generated by composting organic matter to hatch their eggs. These incubator composts can be huge and require a large input of effort on the bird's behalf.

Malleefowl have the most complex nesting habits of all the Megapodes. A male will commence constructing an incubation mound months before the breeding season. He first scrapes a hole in the ground (Megapodes = big feed) about 0.5 m (1.5 ft) deep. On top of this he piles all the vegetation he can find, as well as nearby topsoil. The resulting mound will be about 5 m (16 ft) across and 1m (3 ft) high. Similar, but slightly less complex nesting rituals are shown by the Scrubfowl whose ancient nests may be 12 m (36 ft) across and 5 m (16 ft) high. Also similar are Brush Turkeys, Alectura lathami, whose nests are similar in size to a Malleefowl. Less evolved species such as the Moluccan Megapode, Eulipoa wallacei, simply dig a hole in the soil, lay the eggs and then cover them up. No further control is exercised.

(Floating) Aquatic Nests

Though a number of birds build their nests at the water's edge, several groups build them out in the water. Coots build nests which though surrounded by water have a foundation of vegetation, which reaches the ground below. Interestingly the Horned Coot, Fulica cornuta, which breeds on mountain lakes in the Andes where water weed is scarce, build a foundation of stones nearly to water level before building the actual nest. More adventurous are various grebes. Grebes build the nests in shallow water, and though they are often anchored at one or two points they are basically floating on the water. This is necessary because grebes which are primarily water birds are very clumsy on land and find life works better if they can swim right onto the nest.

Coot NestTwo other groups of birds build nests which are completely afloat. These are the Jacanas (Lily Trotters) which build extremely flimsy nests which often sink into the water while the bird is sitting. Fortunately, the eggs are waterproof so getting dunked regularly does them no harm. The others are the 3 species of Marsh Terns (eg, the Black Tern, Chilodonias niger) which build nests of broken reeds in water up to 120 cm (4 ft) deep. These nests are sometimes anchored to nearby vegetation.

Mud Nests

Mud is often a common resource and it makes sense that birds somewhere should have evolved to use it for nest building. Flamingos are well known and colourful birds which build very basic nests of mud. In the middle of the soda lakes of Africa, you can find both species building up piles of mud and faeces. These structures are basically a hollow mound with a depression in the centre. They are not built all at once, but some mud is built up above water level and as this dries more is put on top. They can be as much as 45 cm (1.5 ft) high. There is no lining in these nests except the mud.

Many birds build cup-shaped mud nests which rest on boughs of trees. These nests often have straw or grass mixed in with the mud making them stronger when dried, much like ancient bricks. Normally, these nets are lined with grasses, leaves, moss and feathers. Some examples of cup mud nests in trees are the Magpie Bird, Grallina cyanoleuca, and the Willie Wagtail, Rhipidura leucophrys, from Australia. Two other Australian birds, both communal breeders, the Apostle bird, Struthidae cinerea, and the White-winged Chough, Corcorax melanorhamphos, also build mud nests. In East Africa the Spotted Morning Warbler, Chichladusa guttata, also builds cup nests.

A number of birds build more unusual mud nests. In southern Europe the Rock Nuthatch, Sitta neumayer, builds a large mud nest on the sides of cliffs that looks like a rounded volcano on its side, i.e. if the cliff was level ground the volcano would be the right way up. In S. America the two species of Oven birds, Furnarius rufus and F. cristatus, build perhaps the most complicated mud nests of all. The nests are globular and often situated on tree stumps. They are about the size of a football. On one side is a domed entrance which leads to a passage which curves around the left hand side before going into the central chamber. The inner chamber is well lined and comfortable looking.

The best known mud nest builders are the Hirundines. Swallows and Martins all over the world labour during the Spring to build their hemispherical nests on the edges of cliffs and under the eaves of houses. The mud is collected in small pellets and moistened with saliva before being applied to the wall or existing nest. Swallow and Martins, in Europe at least, build only in the morning, spending the afternoon feeding, this means that each day's work gets a chance to dry out and become strong before new mud is added. If this did not happen the whole thing would collapse under its own weight. Like other mud nesting birds, the nest is normally lined with dried grass and feathers.

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Edible Nests

Perhaps the strangest nests of all are those built by the species of Swiftlet in the genus Collocalia. They nest in caves and build their nests of saliva. To do this they have enlarged salivary glands during the breeding season. To make them even more amazing, these Swiflets often nest in pitch dark caves. They are able to do this by using echolocation, similar to bats. Quite a few species use saliva in their nests to glue various materials together. Three species however, Collocalia fuciphaga, C. esculenta and C. maxima, produce nests made almost entirely, or entirely, out of saliva. These are the nests used by Chinese chefs to prepare bird nest soup, one of the more expensive and tasteless dishes in the world. The nests are collected from Niah caves and though collecting is controlled by law, it still results in a huge and unnecessary death of eggs and young birds.
















Book Reviews


Bird Nests and Construction Behaviour, by Mike Hansell

 

 

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