FLYING a state-of-the-art drone over a home to spy on its owners’ taste in decor is usually frowned on. Fabrizio Sergio and his team, however, had a good excuse. The homes in question were nests belonging to birds of prey called black kites. Their decoration style? Tatty white plastic bags. In some cases, lots of them, making the nests as conspicuous as a reindeer on your front lawn or a plastic Santa climbing onto the roof.
Sergio, at the Doñana Biological Station in Seville, Spain, wanted to answer a question that had puzzled him and other biologists for decades – why would a bird decorate its nest with conspicuous tat? This kind of flamboyant adornment would be likely to attract predators to vulnerable eggs and chicks, harming a bird’s chances of passing on its genes – hardly a trait that would be favoured by natural selection.
It might seem odd to talk about nests in terms of natural selection: after all, they aren’t part of a bird’s body, which is the obvious product of its genes. But 35 years ago, biologist Richard Dawkins suggested that a gene’s influence could reach beyond the body that houses it and manipulate its environment – a phenomenon he called the extended phenotype. Structures built by animals, such as beaver dams and birds’ nests, are a good example.
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Extended phenotypes should help, not hinder, reproduction. Take the eye-catching work of the male bowerbird, which builds grass tunnels and litters them with stones, glass and plastic to court mates. Males with the most pleasing bowers mate with more females. But bowers never contain eggs and can be abandoned if their gaudiness catches the attention of predatory eyes.
More discreet nest adornments are also known to have reproductive benefits. Some bird species and even cigarette butts to repel parasites, while others tuck feathers or .
But what about the kites? These birds have a long history of adding human-made objects to their nests, including smalls pilfered from washing lines, a habit noted by Shakespeare in The Winter’s Tale. “Everywhere you study them, it catches your attention that in those nests you find all this rubbish and strange materials,” Sergio says. The showy displays favoured by black kites are hard to explain, particularly because both sexes are involved in nest building. To find out more, Sergio and his team turned to a dense population containing 500 breeding pairs in the Doñana National Park.
This group is exceptionally well-studied: the team knew the age, body size, condition and migratory history of many individuals. This meant they could link the birds’ decorating behaviour to aspects of their biology such as their survival and reproduction, and their place in the social hierarchy, which reflects a bird’s ability to defend its territory from competitors. These hierarchies establish which are the strongest birds, to prevent them wasting energy engaging in conflicts they are unlikely to win.
Doñana’s breeding pairs each have a nest in a tree that they will fiercely defend, along with a small surrounding territory. Territories closest to the communal hunting ground are highly coveted and occupied by the strongest, most dominant birds. Weaker, lower-status kites occupy poorer territories, and some find themselves nestless. “Their only chance to get a territory is to usurp it,” says Sergio.
Curiously, the team found that the most decorated nests suffered the fewest attacks from nestless birds. These belonged to individuals in their physical prime, ones that had the best territories and were the most dominant. This suggested that the decorations were a signal of the physical strength and social status of the resident, and an “honest” one at that – it wasn’t that weaker birds were gaming the system by adding plastic too. And these signals were heeded by other kites.
Those with highly decorated nests also raised more young, possibly because their homes deterred attacks and food theft – a clear reproductive benefit of decoration.
What stops birds cheating the system? To find out, the team added extra white plastic to the nests of low-status birds to see how the others would react. These nests experienced an upsurge in intrusions from other kites that the residents found hard to repel, revealing a social control to enforce honesty. “There are individuals that are constantly checking a subset of nests,” says Sergio. “And when they see a change in the decoration, they come to check that you are not lying.”
Intriguingly, the birds had strong decor taste preferences, opting for white plastic over the green or transparent stuff the team left out for them. What’s more, weaker, low-status birds quickly removed any extra white plastic the team placed in their nests. “They had very clear ideas of what they wanted in the nest in terms of plastic,” says Sergio. “I wouldn’t say ‘conscious’, but surely it’s a decision that they take.”
Cool decorations
Why the preference for white? Sergio wondered whether it helped the kites resolve a trade-off between keeping the nest sufficiently shaded from the sun to keep it cool, and making it visible to flying birds. White would certainly stand out against green canopy. “It was very compatible with a message towards the sky,” says Sergio.
Last summer, he and his team published the results of the study in which they flew a camera drone over experimental nests in Doñana, to see how the plastic affected visibility.
Human volunteers shown the images were , even from great heights. So the plastic clearly made the nests conspicuous – probably more so to the razor-sharp eyes of a roaming kite.
Biologists had previously seen a handful of examples of species’ extended phenotypes being used as a signal to other individuals, but these were almost always related to getting a mate. The kind of status-signalling nest decorations displayed by the kites was new and suggested that nests were more complex than generally believed. “Up until about 10 years ago, people thought nests were boring, and not worthy of much attention,” says Mark Mainwaring, who studies nests at Lancaster University, UK. Sergio’s kite study in particular has sparked renewed interest in the field, and ecologists now think nests have a number of functions beyond safely cradling a brood, says Mainwaring.
And it seems these birds aren’t alone. In a slightly revolting twist on the technique, burrowing owls decorate their nests with clods of cow or horse dung that are neither too fresh nor too old. Finding out why makes for icky research. “It’s not a popular task for students when you tell them they’ve got to go out and collect garbage cans full of manure and then go and scatter it around nests,” says behavioural ecologist Courtney Conway at the University of Arizona.
But it is not just the choice of decorations that is odd. If ever a bird ought to keep its home under wraps, it’s this one. Burrowing owls nest underground, some in holes they dig for themselves, others in abandoned animal burrows. These usually have only one entrance and no emergency exit to flee hungry coyotes and badgers, looking for tasty eggs or nestlings.
Instead of being discreet, the owls decorate their nests with abandon – strewing the entrance with manure and other items such as moss, grass, plastic, paper and aluminium foil. Males stand guard, motionless, puffing themselves up to full height if they spot a potential threat. “They appear to be trying to stand out,” says Conway.
For many years, ecologists assumed the manure masked the scent of the burrow and so made it harder for predators to find. But in 2004, a team led by showed that the manure offered little camouflage. Instead, . This made it one of the few examples of wild birds using tools.
Yet it may not be the whole story. Conway and his team had also shown that the owls’ manure decorating behaviour trapped insects. But this didn’t explain why some individuals were garlanding their burrows with other materials. Burrows can be in short supply and attract the attention of single males looking for somewhere to nest. Resident males attack and usually see off any intruders. Could the owls, like black kites, be using a signal to avoid conflict?
To find out, Conway’s team scattered manure around unoccupied test burrows at the start of the breeding season, and discovered that owls visited these less often than test burrows without manure. They then placed a stuffed owl near real occupied nests to see how their owners would react. Not only did the males vigorously attack the stuffed intruder, they immediately fortified their burrows with more manure, .
Seeing how owls behaved when manure was removed proved difficult: the owls replaced it as fast as the researchers could take it away. “They work overtime to thwart our experiments,” says Conway. “That told us there was a function to having that manure around the entrance to the burrow – it wasn’t just that they were poor housekeepers.”
Intriguingly, about half the burrows the team studied were also decorated with other materials, which the owls were just as quick to replace if removed. And given a choice, owls didn’t specifically seek out manure, instead favouring materials that were closest to hand. Conway can’t rule out the possibility that other materials somehow attract insects, although he thinks the signalling hypothesis is the most plausible one.
“The owls replaced the manure as fast as the researchers could take it away”
Another bird, the diminutive rock sparrow, seems to have a similar signalling motivation behind its decorating behaviour. Many birds line their nests with feathers, possibly to keep eggs at the right temperature. But Matteo Griggio at the University of Padova in Italy and his team noticed that rock sparrows stacked feathers on the rim of their nest like hunting trophies.
When the team added extra feathers, these altered nests experienced fewer intrusions, and were defended more vigorously by their occupants. This suggests that the decorations, like those of the black kites, are a signal to deter competitors.
What’s more, resident females laid more eggs than those in unaltered nests, , and were less likely to abandon it. Female rock sparrows are the nest decorators, so Griggio’s team speculates that it is a way for a female to signal her quality to a male, persuading him to stick around after mating and invest more effort in raising their brood. This would make it a rare example of sexual selection favouring an ornament on females – the ornament being outsourced to the nest.
The closer ecologists look, the more they are realising that nests aren’t just a safe home for offspring, but can also work as an extension of the body – like a puffed-up chest to see off rivals, or a head crest that attracts a mate.
If only they could be aware of it, birds might preen themselves to know that science proves their taste in decor to be a sign of great sophistication.
This article appeared in print under the headline “Nest in show”
