Building castles in the air

The common bird's nest is a miracle of engineering and design, involving everything from spider's egg silk to feathers. Simon Hadlington reports

Of all the great architects of the animal kingdom - beavers with their dams, termites with labyrinthine mounds, wasps with papery nests - perhaps the most prominent are the birds. Each spring, millions of nests are constructed, using a vast array of materials, natural and man-made. Most are disposable - they are used for one season only - and yet they can be remarkably elaborate.

Of all the great architects of the animal kingdom - beavers with their dams, termites with labyrinthine mounds, wasps with papery nests - perhaps the most prominent are the birds. Each spring, millions of nests are constructed, using a vast array of materials, natural and man-made. Most are disposable - they are used for one season only - and yet they can be remarkably elaborate.

But despite their ubiquity, birds' nests have received surprisingly little attention from naturalists. Dr Mike Hansell, a biologist from the University of Glasgow, is trying to put that right. For many years, he has had an interest in the behaviour of animals that build structures, and in the early 1980s he turned his attention to birds' nests.

A search of the existing scientific literature revealed that birds' nests had not been studied in as much detail as might have been expected considering their range and number. Furthermore, there are few extensive collections of nests in museums around the world. "I feel very strongly that if you want to understand what birds are all about in terms of their natural history, you must have a collection of nests," says. Hansell,who has begun a collection of British birds' nests at the Hunterian Museum at the University of Glasgow.

Birds' nests appear to be incredibly complicated constructions. But on closer inspection they usually follow a relatively simple, if elegant, design. Hansell has identified four distinct components or "zones" of nests - some may include them all, others only one or two. The principal structure is the cup, which holds the nest together. Lining the inside might be a layer of insulation, while the outside might be clad in a thin skin of material, often for camouflage. Finally, if a nest is suspended - from a branch, say - it requires some sort of supporting mechanism.

One type of nest that the Glasgow researchers have studied closely is that of the long-tailed tit. In early spring, the males and females work together to build what Hansell describes as one of the most impressive-looking nests of any British bird. Typically, it is located in a gorse bush or bramble thicket, and is domed, giving an egg-shaped appearance with an entrance in the side.

Hansell has discovered that the long-tailed tit uses a Velcro-like substance as its main building material, which greatly simplifies the task of assembling the nest chamber. "The main part of the nest consists of a bag of a particular type of moss held together with a particular kind of spider silk," he says. "The silk comes from spiders' cocoons, the fluffy wrapping around spiders' eggs. The mosses that are used have small leaves. These become entangled in the loops of the silk - so there is the hook-and-loop principle of Velcro."

It has been known for a long time that spiders' silk is used in nest-building, but it had been assumed that the part used was the sticky silk from the web - rather than the eggs - whose tackiness was important for holding together the structure. "The bird begins by making the floor of the nest, then building up the walls with the moss-and-silk mixture," says Hansell. "What is wonderful about Velcro is that you just slap the two sides together and it sticks. So, from the birds' point of view, there is no especially complicated behaviour involved. While the structure does appear remarkably elaborate, it is not that difficult to assemble once the appropriate materials have been mixed."

Once the nest chamber has been built, the tit clads the exterior in a thin covering consisting of flakes of pale lichens and white flecks of spider silk. Often, pieces of polystyrene or fragments of newspaper are used. Finally, the inside of the chamber is insulated. Each long-tailed tit's nest is packed with about 1,500 feathers. "This in itself is interesting because the birds nest quite early and frankly there are not a lot of feathers to be found lying around," says Hansell.

One of the aspects of nest-building that has intrigued him is how birds manage to locate such large quantities of specialised construction materials. To attempt to understand how long-tailed tits found feathers, the Glasgow researchers planted piles of marked pigeon feathers in the vicinity of nesting birds near to Loch Lomond. At the end of the nesting season the nests were collected and the feathers removed and counted.

"From this work it turned out that the birds would fly only maybe 100 metres to find our feathers, and the feathers we planted made up less than 3 per cent of the total number in a nest," Hansell says. This seems to demonstrate that, despite an apparent absence of feathers, the tits have evolved ways of locating plentiful supplies. Often, large numbers of feathers from a given type of bird are found, suggesting that the tits locate a carcass, or a "plucking post" used by a sparrowhawk to strip its prey.

The relative simplicity of nest-building is an important evolutionary feature, Hansell believes. "If you look at a bird, you can hazard a good guess at what it feeds upon but you would have no real idea about what kind of nest it builds," he says. "A pointy beak will indicate that it is likely to eat insects, or a short, stubby beak that it is adapted for cracking seeds. A bird is anatomically adapted for feeding, but not for nest-building." So building a nest is all down to patterns of behaviour, rather than the possession of any physical adaptation. Because of this, there will be evolutionary pressure to ensure that building behaviour is as uncomplicated as possible.

Birds will tend to use a limited range of building materials, for example. "This is nicely illustrated by the blackcap warbler, whose nest is made entirely from the stems of grass," says Hansell. "The bird simply takes a dead grass stem and buckles it at points along its length to create a polygon. It stacks many of these up, one upon the other, with the distance between the buckling points getting greater - so creating larger polygons. The end result is a cup-shaped structure of extreme elegance and one that conveys a strong sense of 'design', and yet consists only of one building material and one building routine. This is often a feature of birds' nests: a beautifully designed nest can be made from very simple rules applied in a very simple way."

The Glasgow team has also been investigating whether there is a correlation between the size of a bird and the size of its nest. "You might think that a bird's nest would be more or less the same size as the bird," says Hansell. "In fact, we find a huge variation. A song thrush's nest, for example, does weigh about the same as the bird. But a humming bird's nest weighs only about half as much as the adult. At the other extreme is something like a magpie. Here the nest consists of a lot of large twigs lined with mud, often with a canopy over the top. This can weigh 20 times more than the bird."

Hansell says that the relative size of nest could be important in avoiding predation. "The humming bird is small, so it might make as small a nest as possible to help conceal it. There is no way that a magpie can hide its nest, so it builds a heavy, highly fortified structure to make it difficult for a crow to nip in and grab an egg." A white stork's nest, constructed from twigs, grass, rags and paper, can grow to more than 2 metres across and 3 metres deep.

For Hansell, the study of birds' nest-building behaviour is a crucial part of their natural history and one that should be given greater significance. "People often express wonderment at the ability of some birds to use tools. And while this is undoubtedly astonishing, I do feel that when you look closely at how a bird constructs its nest this is at least as awe-inspiring."

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