Given that a bird’s nest is something so commonplace, it is strange that we don’t really understand how or why they are constructed. With recent research, however, we are beginning to understand what causes such variation, and how nests function.
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Where do birds build their nests?
Birds build their nests in many different places. Terns and plovers, for instance, scrape a rudimentary hollow into the ground, relying on the markings of their eggs for camouflage. Kingfishers and sand martins excavate burrows in sandy banks, laying their eggs safely out of sight. Warblers weave grass and plant material into neat cups, deep in the vegetation.
Many nests are discretely hidden from predators; others are glaringly obvious. The vast platforms of sticks built by ospreys in the treetops, for instance, are hard to miss, particularly as they are re-used and added to over the years. Grebes build conspicuous floating nests in large ponds and gravel pits (their eggs are incubated half-submerged in the water, yet somehow still hatch), while house martins plaster their familiar, semi-circular cups under roofing eaves by the countless muddy mouthful.
Nest structure is as variable as location, and the finch family in particular illustrates just how diverse a domicile can be. The larger members, such as the hawfinch and bullfinch, rely on loose aggregations of sticks fashioned into an open cup lined with a few roots or lichen, while smaller goldfinches and chaffinches build neat cups of moss and lichen lined with hair and feathers. Linnets opt for a simpler structure, weaving grass cups lined with roots.
But even similar-sized birds use contrasting materials when building nests. Great tits build a moss base and line the cup with hair. If a nest is mainly moss and leaves, then a pied flycatcher is at home. Tree sparrow nests are almost all feathers, while the elusive nuthatch builds its nest almost entirely using flakes of bark.
It is often possible to identify a bird species from a nest, yet construction is not a fixed behaviour. Birds are adaptable and build homes that vary considerably in size, shape and – in particular – composition. Some of this variability depends on what materials are on offer. Pied flycatchers across Britain rely on leaves and moss to build the bulk of their nests, but are not fussy foragers – they utilise whatever leaves are most common in their neck of the woods. Similarly, blue tits often line their nests with feathers, but will readily use wool if sheep are grazing nearby.
Why do birds build nests?
Irrespective of variation in form and content, all bird nests are designed for one thing: egg incubation – and, in many species, chick rearing. A nest enables an incubating bird to maintain the correct temperature and level of humidity for her developing eggs, as well as providing protection from the elements and predators.
If a blackbird is successful, their nest will comfortably hold four fully grown fledglings later this spring, each the same weight as an adult, and the dwelling may be used at least twice this year. Likewise, if she has a good year, a blue tit will raise her single brood of an eye-popping 10 youngsters.
What are nests made of?
So, why do birds vary so much in their nest-building behaviour? And why use such different materials? Does all this effort really keep the eggs and chicks warm, safe, and secure – and, if so, how?
Nest building and egg formation require lots of energy – both the blackbird and blue tit have to carry more than twice their own bodyweight to build their nests. The clutch of 10 eggs laid by the blue tit weighs as much as she does. Thus, anything that can minimise heat loss from the nest will help the adult to conserve its energy stores. The incubating adults will remain in good condition and be able to successfully forage to feed their impending hatchlings.
Nonetheless, a nest is not a simple egg-container. The walls can be complex in structure and comprise a surprising range of plant and animal-derived materials that vary according to the prevailing climate. In the north, where springs are colder, blackbird and tit nests are more effectively insulated than in the warmer south. Yet, despite this variation, research has shown that a range of small garden bird species build nests that provide, on average, similar levels of insulation.
Insulation seems to relate to the proportion of moss and hair in the nest walls. Birds appear to have found a range of solutions to the heat-retention problem – with varying degrees of success. Research in my lab has shown that chaffinches, for example, line their nest cups with a bounty of feathers and hair. Despite being only 6–10cm thick (about a third of the wall thickness), this layer provides 90 per cent of the total insulation for the whole nest wall. By contrast, the same thickness of the grassy blanket lining a blackbird’s nest only offers 40 per cent.
While the blue tit is incubating her eggs within her snug little nestbox, the poor old blackbird is taking a battering from the prevailing weather. When it rains, she sits tight over her clutch, the water running off her back and into the nest walls, preventing the eggs from becoming wet and chilled.
The component materials of a nest matter greatly when it comes to rain. Though water drains freely through the nest wall, some of it will be absorbed. Moss in particular sucks up water like a sponge, and a moss-heavy nest will take a considerable amount of time to dry out. In contrast, nests that don’t use moss dry out very quickly. The ‘classic’ stick nest of a hawfinch, for instance, is very free-draining and dries out in no time. Blackcaps seem to create the same effect by simply sticking to using grass in their nest construction.
A degree of moisture is important, though. Eggs need to be kept at a suitable level of humidity during incubation, to ensure they don’t lose too much water through their pores. Nest walls are rather leaky, which allows all-important oxygen to enter and carbon dioxide to exit. It seems that they can also retain just enough water vapour to raise the humidity of the nest to the necessary levels – rather like a fluffy duvet.
Nests also have to be engineered to be structurally sound – after all, it is imperative that they can support the weight of an incubating bird, plus its entire brood of nestlings. The larger the bird species, the heavier its reliance on woody materials for structural integrity.
How do birds build nests?
How these materials are used also varies. The stick nest of the bullfinch is built towards the end of a tree’s branches, with the strongest twigs deployed at the base. The heavier hawfinch, on the other hand, nests in the crook of a branch next to a tree trunk, ensuring maximum support from below. It then places the strongest twigs around the sides of the nest, where support is most needed. The ‘twiggy’ nests of both species may appear flimsy, but they are surprisingly hard to dismantle.
How the song thrush builds its nest
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- A female song thrush selects a site, often a triple branch fork, on which she constructs a base platform of twigs. This is then filled with moss.
- She constructs a cup from grass stems, tucking and then twisting them into each other. Finer grasses are woven into the rim for strength.
- She adds moss and a few leaves to the outer cup for camouflage, before lining the nest with wood pulp and mud, using her body to smooth it down.
What don’t we know about bird nests?
Though research is allowing us to learn more about how bird nests work, we are unlikely to run out of questions. Why are some materials only used in the nest cup and others in the nest walls? How do birds decide on what materials to use and where to place them? There are also many species whose nesting habits have yet to be studied.
Do birds reuse their nests?
Sadly many birds will not reuse their nests. Come autumn, these dwellings will be abandoned, and will eventually break up and decay as winter sets in. If the birds survive until next spring, they will have to begin the process again – as will their offspring, who miraculously build their nests without ever being taught how to do it. Perhaps that’s the greatest mystery of them all.