Among life’s small, durable joys is the discovery of a bird’s nest: perhaps in the shape of a bowl, woven from grasses and stems with a skill that seems inexplicable. How could a creature with such a small brain, equipped only with beak, feet, and wings, create something so exquisite, so far beyond my own capacities?
At some point in my life, I came to think that nest-building was a purely instinctive behavior rather than an expression of intelligence. I don’t recall where I heard that; it was part of the ambient conventional wisdom, which held that birds enter the world with the knowledge somehow encoded in their brains, then construct their nests in a rote enactment of those innate instructions. Over the past decade or so, however, a new wisdom has challenged the conventional.
Instinct does play a role in nest-building, but so much more seems to be involved. Entwined with instinct are reason, learning, experience, insight, and perhaps even a sense of aesthetics. And when next you encounter a nest, you might see it not only as beautiful, but also as a physical embodiment of a rich and still-mysterious intelligence.
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One spring morning I visited the Tilden Regional Parks Botanic Garden, a 10-acre native plant paradise perched in the East Bay Hills and a fine place to contemplate the subject of nests. Inside the visitor center—below the awning of which a flycatcher has sculpted a nest from daubed mud and moss—is a display case containing nests from three species: American robins, Anna’s hummingbirds, and bushtits.
Nests take forms as diverse as birds themselves: the shallow ground scrapes of arctic terns and pendulous globes of hooded orioles, the saliva cups of edible-nest swiftlets and the sometimes refrigerator-size communal apartments of monk parakeets. Robin’s nests are archetypal, though, the sort of nests that come to mind when I think of the word. A closer look at one reveals an intricate bowl about eight inches across, its outermost layer composed of coarse stems interwoven with thicker twigs and reinforced by mud; inside that is a layer of finer stems, and in the center is a cup made from the finest stems of all. At the bottom rests a smattering of baby feathers from the hatchlings for whom the nest was so painstakingly built.
If I saw such an object without knowing anything of birds or of nests, I would assume that someone had crafted it with conscious skill. That such a complex object could result from a thoughtless behavior seems counterintuitive. The great naturalist Alfred Russel Wallace, who was Charles Darwin’s peer in conceiving of evolution through natural selection, thought much the same.
“Birds, we are told, build their nests by instinct, while man constructs his dwelling by the exercise of reason,” wrote Wallace in 1867. “Yet I have come to the conclusion that not only is it very doubtful, but absolutely erroneous; that it not only deviates widely from the truth, but is in almost every particular exactly opposed to it.” Nearly 40 years later, ornithologist Charles Dixon called bird nests “the most palpable example of those reasoning, thinking qualities with which these creatures are unquestionably very highly endowed.”
As the 20th century progressed, the view espoused by Wallace and Dixon largely faded from science and from popular culture. Why this happened isn’t entirely clear; actual scientific evidence for the primacy of instinct is as sparse as a pigeon’s nest. (If you’ve never seen one, do an internet search for “pigeons are bad at building nests.”) Susan Healy, a biologist at the University of St Andrews in Scotland, points to poorly interpreted experiments from the mid-20th century on canaries who, without any prior experience with nests, still tried to make them.
Healy also suspects that research on seemingly mechanistic nest-building in invertebrates led to the assumption that similar processes occurred in birds. Whatever their provenance, instinct-centric explanations prevailed. In Sharon Beals’s beautifully photographed book Nests: Fifty Nests and the Birds That Built Them, for example, nest-building is described as “purely instinctive.” And when researchers recently discovered family-specific building traditions in southern African white-browed sparrow weavers, the New York Times said the finding “upends longstanding assumptions that nest building is an innate behavior.”
“We know song is learned, and when a boy bird sings the same song as his dad, we know he learned it from his dad. Whereas if he was to build the same nest as his dad, people would say, ‘He didn’t learn it from his dad. He had genes for it. He just, you know, produced it,’” says Healy. “That’s kind of where most people still think nest- building is at.”
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Healy, who specializes in cognitive evolution and oversaw the research on sparrow weaver culture, has spent much of the last two decades studying nest-building. When we talk by Zoom, she shows me several nests made by zebra finches—a species native to Australia but commonly used as so-called model organisms for studying bird behavior—in her lab.
The birds were given pieces of colored yarn with which to build. Thanks to the colors, patterns are easily visible: One finch’s nest looks like something imagined by Jackson Pollock, with a riot of evenly scattered hues. Another nest is neatly organized: a green section, an orange section, a white section. One finch decided to use only blue yarn, which he teased apart before arranging in a poofy mass that resembles cotton candy. Were nest-building entirely instinctive, the finches’ nests should be uniform. Instead the individuality of the birds can be seen.
That doesn’t mean instinct is unimportant: Pick up a field guide to nests, says Healy, and the species-typical nests depicted can be reliably used to identify others, suggesting commonalities that exist within species. Given the profound importance of nests to reproduction, the crucible of natural selection was bound to bake some aspects of their construction into avian heredity. But instinct does not act in isolation.
“Do they have some mental template that they’re trying to match? If so, does that mean it’s innate—or does that mean it’s learned?” says Lauren Guillette, who studies animal cognition and nest-building at the University of Alberta. “The mental template can be innate, but assessing what they’ve done, and how that relates to where they want to go—that still involves cognition.”
Studies point to the importance of learning. In one experiment at Healy’s lab, zebra finches were provided with both flexible and stiff string; after working with each, birds preferred the stiff, ostensibly because fewer pieces and less effort were needed to complete a nest. This may sound simple, but it’s subtly significant: The finches acquired knowledge of structural properties and made choices based on experience. A UCLA study described how nests fashioned by young village weavers tend to be crude and loosely knit, while older birds make compact, tightly woven structures. Their craft is acquired by practice.
The role of experience is also seen in an experiment by Guillette’s team, which found that when babies hatched successfully, zebra finch parents built their next nest with a similar selection of materials—but when eggs were removed by researchers, simulating a nest failure, the birds took a different approach with subsequent nests.
That was a lab-based finding, and results from studying zebra finches should be understood not as universal insights but as suggesting what may be possible in other species; still, Healy’s team observed the same dynamic in wild blue tits—a Eurasian relative of chickadees and titmice—who responded to nest failures by changing the amount of insulation used. Northern flickers and collared flycatchers have also been seen changing nest locations after failure.
Learning in this way might again seem simple, but it involves considerable mental sophistication. “A bird has to remember what they built with. They have to be able to assess the outcome. And then, when it comes time to build a new nest, they have to remember those things and have those memories be reflected in the choices they make,” Guillette says. “That’s pretty wild.” For Guillette, the evidence suggests that birds who learn from past nests are also planning for the future—a sophisticated, scientifically contested capacity that many scientists were reluctant to attribute even to primates whose high-level cognition is easily recognized.
All of this doesn’t point toward one absolute rule: that, say, nest-building is one-thirds instinct and two-thirds reason, reflection, and learning, or vice versa, nor that the ratio holds true across the more than 9,000 bird species who build nests. Some species may be more cognitive and others less. “I’d predict variation,” says Healy, who thinks that longer-lived species are more likely predisposed to learn from experience.
Iliana Medina Guzmán, a behavioral ecologist who studies nest evolution at the University of Melbourne in Australia, likewise thinks that “it really depends on the type of bird.” Some of her own work involves species who build enclosed nests, like the hanging orbs of the bushtits in the visitor center display case. The structural complexity of these nests allows for more individual variation, Medina-Guzmán says, but it’s unclear whether those species are in fact more cognitively developed than birds who build open nests. What does seem clear, though, is that instinct alone is not a universal explanation for nests.
At the entrance to the botanic garden was a shrub, and when I told the visitor center aides about my article, they suggested I look in its still-bare branches for an Anna’s hummingbird nest. One of their job’s small satisfactions, they said, is watching hummingbirds gather spiderwebs from office windows to use in their nests.
Even knowing the nest was there, and with the shrub’s leaves not yet unfurled, it took a while to find: a perfect dollhouse teacup of web-bound moss, covered with flakes of lichen that made it blend into the dappled branches.
Did the hummingbirds know that the lichens provided camouflage from prying eyes? Or were they compelled by some rote impulse to fashion their nests that way, with no deeper awareness of the behavior’s function? Healy’s lab investigated this question with the zebra finches, whom they provided with strips of paper that either matched or contrasted with the color of their enclosure’s walls. Most of the birds chose to match, which Healy and her colleagues interpreted as evidence of intentional camouflaging.
Nest Materials
These questions touch on matters of aesthetics. It’s also possible that birds take visual pleasure in a well-camouflaged nest; the sensibility could motivate the behavior. It’s an unstudied idea—even Charles Dixon, so open-minded to active intelligence in nest-building, denied that birds could find nests beautiful. Scientific attention to aesthetic tastes in birds revolves around mate choice, as with peacock plumage or the sculpture gardens of bowerbirds. That a hummingbird could delight in her nest’s appearance is rarely considered.
Even so, some scientists who study aesthetic cognition think a sense of beauty is simply an evolutionary adaptation that, by attaching positive feelings to useful perceptual stimuli, nudges animals toward behaviors that help them survive and reproduce. The notion is intuitive to Healy, who says that it would be advantageous for natural selection to make certain objects more attention-grabbing than others by linking them to neurological reward circuits.
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Healy references experiments in color preferences with her zebra finches. “It turns out that male zebra finches have a view about pink and orange. They basically love the pink or they love the orange,” she says. “They have these color preferences. I don’t know where from, but they do start and they are strong.”
Might visual preferences include not only color, but geometric features too? If, say, the symmetry and orderliness of a nest is linked to its function, could natural selection have encouraged a bird to enjoy and pursue those qualities in shaping a nest? “That’s a very good question,” says Medina-Guzmán. “We don’t really know.”
It was at least something to think on as I strolled the botanic garden’s paths. At one point a red-tailed hawk flew overhead—the same hawk, perhaps, as mentioned in a birder email alert about a pair seen carrying material to their nest on nearby Vollmer Peak. They called to mind a study from Spain of another raptor, black kites, who adorn their nests with strips of white plastic that are thought to signal physical prowess, discouraging would-be competitors.
By a pond I saw a mallard couple feeding. Mallard mothers line their nests with down plucked from their own breasts—as moving a testament to maternal love as could be. A rabbit hopped by; many birds use fur as insulation in their nests, and I imagined the industry that must go into finding that precious material.
Near a manzanita I saw a finch gathering what looked like a sprig of rosemary. The botanic garden is devoted to native California plants, so it almost certainly wasn’t rosemary, but the encounter gave me a moment’s pause: the birds knew the garden’s plants better than I did. This also brought to mind research on birds lining nests with herbs—and, in one famous study of house sparrows in Mexico City, cigarette butts—that discourage pathogens. As with the hummingbirds’ lichens, the use of medicinal plants raises the question of what birds know about them.
Perhaps, having been raised in nests containing those plants, they seek them as adults. Guillette thinks birds may simply like how the plants smell or look—or, if they happen to use a certain plant and their babies thrive, they’re able to make an association. “Isn’t it astonishing what associative learning can get us?” she says. “I think so much seemingly complex behavior can be explained by a simple process that humans engage in. That every animal engages in.”
Among the questions Guillette is beginning to study is whether various nest materials affect birds’ moods. Healy is especially interested in mistake-making: When a bird messes up while building a nest, do they have insight into what went wrong?
Healy’s research on cultural traditions in sparrow weaver nests also continues, and much remains to be learned about the role of culture—information passed between individuals, accumulating by learning rather than heredity—in nests. Few researchers have studied this, perhaps because nest-building was so long considered instinctive. Social learning is widespread among birds, though, and so too might be as-yet-undiscovered nesting traditions.
The use of human-made materials may offer an accessible glimpse into nest cultures. In one little-known experiment conducted a century ago, the naturalist Henry Smith Williams hung strands of red, white, blue, purple, orange, and yellow yarn for the orioles in his Connecticut backyard. At first the birds had varied color preferences, but within a decade the entire colony used only white yarn in their nests. Patterns subsequently emerged among other neighborhood birds, evidently in imitation.
More recently, strips of blue plastic have been seen in the nests of many species. Who were the first to incorporate that strange—but apparently desirable—material? Did others learn from them? “It could be a case of culture,” says Guillette, and one that most anyone can see while walking in a park or past some strip-mall shrubbery. For that is another joy of nests: miraculous as they are, they’re also common.
No matter where you live or work, nests abound. Each is a work of art. They speak to how instinct and intelligence combine in lived experience, and of how a behavior can be shaped by millions of years of evolutionary history and also by an individual’s fancy. We are surrounded by other minds, marvelous and mysterious. If you ever need a reminder, just look for a nest.
