The Nesting Instinct

Photo by Georgia Lill.

With over 130,000 specimens, Yale’s Peabody Museum of Natural History is home to one of world’s largest collections of birds, eggs and nests. The collection resides deep in the heart of a brick Gothic building. The corridors leading to it are lined with fossils, a stuffed llama and dozens of photos of birds. Ornithology professor Richard Prum leads me there. He briskly leads me out of his office, down the hall, through a bird taxidermy operating room and into a cavernous warehouse. The room hums as the ventilation system pumps in chilled, low-humidity air. Row after row of glossy white metal cabinets stretch from the floor to the 15-foot-high ceiling. The cabinet doors, with their continuous white façade stretching into the distance, keep their contents hidden.

It is here that the nest collection is stored, filled with sticks, structures and secrets. Despite their incredible variety, ingenuity and resilience, for some reason nests have been treated as novelties. Scientists have not studied them as rigorously as birds themselves; the nests of 20 percent of all bird species remain undescribed. Scientists know very little about how most nests are constructed, and even less about the physical principles responsible for their strength. “Biologists always point at these things and say, ‘Wow, look at how beautifully designed and structurally sound they are!’” Prum observes with a grin. “Yet those notions have never really been put to any scientific test. There has been no quantification, no anything.” Now Prum and a team of ornithologists, mathematicians, and physicists are looking past the beauty of the birds’ nests and starting to explore how such tiny creatures became some of nature’s greatest engineers.

When we enter the collection, Prum darts straight for the nearest cabinet. He slides open a drawer, revealing row after row of brilliant blue birds. Then he slides open another drawer. And another. Each tray is lined with a grid of seemingly identical specimens. Prum carefully picks up a little blue bird. Its feathers glimmer purple in the light. Tied around the bird’s leg is a tiny slip of aging paper. Prum peers through his thick circular glass and reads the beautiful cursive handwriting aloud: “Cotinga maynana. Alta Mira, Peru. 1962.” He puts the bird back and smiles. “I just can’t function unless I have 100,000 dead birds across the hallway,” he jokes as he closes the drawers and begins to open another cabinet.

The door swings open, revealing a forest of nests, stored in clear cases. There were nests built with hair, and nests built with thorns. There were nests shaped like bowls, and nests shaped like tunnels. The nests were sorted by species just like the birds themselves, yet no two nests in the collection — not even those constructed by the same species — looked the same.

It’s this variety that gets Prum going. Instead of simply adapting by changing their bodies, birds have also adapted by constructing ever improving homes. By studying the architectural history of nests, Prum believes we can gain insight into more than just the clever ways birds hid their eggs from predators: we can see the way that birds’ minds have evolved over time.

These remarkable structures are the work of craftsmen who spent their days carrying out the vision in their minds. Once they are kicked out of the nest, birds are forced to make one of their own without any help from their folks. “They have no opportunity to learn. It has to be innate,” argues Prum. Birds seem to have built-in blueprints. Instead of mapping out every detail like an instruction manual, these plans seem to contain a few general engineering principles that help birds decide which materials to use and where to put them. Even though the exact layouts are never the same, ornithologists can still identify the species that built a nest just like humans can use architectural features to distinguish between Roman and Gothic buildings.

The history of the ornithology collection showcases how science itself has evolved. Most of the nests in the collection date from the Victorian era, at the close of the 19th century. In those days, “egging” was all the rage. Everyone from schoolboys to scientists scavenged the countryside, searching high and low for nests. But nests were but containers for the real targets: eggs. The humans would snatch the eggs, drill a tiny hole in the bottom, drain their contents and place the shells in their collections. Egging was regarded as a respectable scientific pursuit. Legions of curious citizens catalogued the world around them and reported their discoveries to the academy. They then showed off their collections of eggs to their friends. Nests, on the other hand, rarely appeared in Victorian collections. They simply weren’t as glamorous. Egging’s similarity to predation ultimately spelled its downfall, as more and more governments criminalized the disturbance of nests. As citizens lost interest in egging, natural history museums like the Peabody began acquiring their personal collections. The nature of science was changing, too. The gulf between amateur and professional scientists grew as biologists turned their attention away from the treetops and onto the hidden worlds of cells and genetics. Nest collections in museums worldwide gathered dust.

Prum is the latest in a long line of eccentric individuals to build and incubate the nest collection. Much of it was assembled by Henry O. Havemeyer. Havemeyer was the heir to his family’s sugar business, which controlled 80 percent of the U.S. market. He dedicated his spare time to stocking a prolific bird nest museum in the comfort of his New Jersey mansion. Havemeyer’s collection was acquired for the Peabody by another character with private extracurricular activities named Sidney Dillon Ripley. Before he became the Peabody’s director for two decades, Ripley was sent to India to gather intelligence for the Office of Strategic Services (the predecessor to the CIA). Normally, spies posed as ornithologists to distract attention from the time they spent in the bushes with binoculars. Armed with a shotgun and taxidermy kit mailed to him by the Smithsonian Institute, Ripley used his sneaky skills to collect birds behind enemy lines. Now the collection has fallen into the hands of Prum. Prum got his start studying feather evolution, and won a MacArthur genius grant in part due to his discovery that dinosaurs had feathers. His interest in how feathers get their color brought him into close contact with engineers, who have inspired him to think of nests differently.

To begin my tour of the collection, Prum walked me through the architectural lineage of the weaver family. The birds start by finding a fork in a tree. Like a human with a needle and thread, the birds use their curved beaks to weave sinuous fibers in and out, forming a dense fabric. Once they bridge the gap between the branches, they build a pouch to hold their eggs. In moderate climates like those in Connecticut, these pouches are a few inches deep. Yet as the weather becomes tropical, the pouches stretch down as if melting from the heat. The result is a work of striking architectural beauty: a 3-foot-long pendulum.

The collection is also home to mysterious nests. One monolithic nest dominates an entire cabinet: the towering home of the firewood-gatherer. The South American bird gets its name from the size of the branches it can be seen dragging along the forest floor. The nest’s architect stitched together a formidable tower. The keep’s circular outer wall is barbed with thorns. The only opening is a small entrance at the very top. No one knows what is inside. Most scientists who have attempted to cut apart firewood-gatherer nests have failed to dissect the constructions before they fall apart, taking their secrets with them. Yet the few who have succeeded report finding a spiral passageway that winds back and forth en route to the inner sanctum. “Perhaps someday we will get a robot to go inside of it,” muses ornithologist Kristof Zyskowski, who captured the nest during a 2002 expedition to Uruguay.

Not all birds build fortresses to protect their eggs. Some, like the rose-breasted grosbeak, try to make their nests disappear. The bird uses tiny twigs that are thinner than spaghetti noodles to form a tangled web, and then lays its eggs atop it. The result: eggs that appear to float in the air.

Although ornithologists were originally interested in the most intricately crafted nests, Prum’s attention is shifting to ones that seem to defy a conventional idea of structure. The next nest Prum grabs is similar to the grosbeak’s: it looks more or less like a handful of twigs dumped into a box. It was built by a brown bird with a short curved beak called a crissal thrasher.

At first glance these nests lack clear signs of craftsmanship. Grosbeaks don’t stitch the twigs together into a fabric like weavers, nor do thrashers build castles like fire-gatherers. Instead, the twigs in these nests look like a cross between a wood pile and a tangled mess. Yet this apparent structural anarchy is what fascinates Prum: how did birds learn to make such strong homes so effortlessly? Or as an engineer would say, how did they become so efficient?

The answer may be that there is something special about piles of sticks themselves. In 2010 a team of physicists led by Scott Franklin at the Rochester Institute of Technology started to explore why we can easily run fingers through a bucket of sand, but not through a bucket filled with nails. They put a bunch of chopstick-like glass rods into a bucket and shook it. When they peered inside they saw something magical: all of the rods were stuck together in a spontaneous rigid structure that could be removed in one piece by a single string tied around a single rod. Prum describes it as a “chopstick hairball.”

To uncover what mysterious principle held the hairballs together, Franklin tried the same experiment using sticks of different sizes. They found that structures made with shorter sticks fell apart, while those composed of longer sticks were strong. They soon realized that what mattered most was the ratio between the diameter and the length of the rod, called the aspect ratio. The resulting paper caught the eye of Yale engineering professor Corey O’Hern.

Last year Prum invited O’Hern to visit the nest collection. The engineer was struck by the resemblance between chopstick hairballs and bird nests, and started to wonder: had birds discovered the wonders of the chopstick hairball millions of years before humans did?

If birds have indeed evolved to understand aspect ratio’s engineering significance, Prum predicts that birds will instinctively use sticks with higher aspect ratios to build their nests. In order to explore this hypothesis, engineers would have to first measure the shape of every twig and then map all of the places where branches intersect. Here a problem arises. To measure the twigs, they would have to deconstruct the nest, making it impossible to then measure the interconnections.

In order to get around this sticky problem, Prum and his team put the nests through a CT scanner.  They carried a selection of structurally remarkable nests to the Yale-New Haven Hospital for testing. The lab technicians were thankful for the change of pace. “You would not believe how enthusiastic they were!” said Zyskowski. He tried to use the scanner to peer inside of the firewood-gatherer’s citadel, but it was too big to fit in the scanner. Next up was the crissal thrasher’s nest. After adjusting the machine to scan wood instead of human tissue, the result was what the bird lovers had been hoping for: the world’s first precise 3-D image of a bird nest.

Now the hunt for the secrets of bird nest blueprints is on. Mathematicians like Georgia Lill ’13 are working to transform that set of images into a virtual model. Prum hopes that soon we will be able to start quantifying the thrasher’s engineering knowledge in terms of the aspect ratios it prefers.

Yet aspect ratios will never be able to explain the amazing variety of nests. Prum finds this exciting. He believes that biologists’ attempt to reduce life to a series of numbers and base pairs has been a distraction. “Without the phenotype, the genotype is just library science,” he argues. He believes that genetics is just a fad. “Soon it will all be done. Literally. We’ll have the genome of all the vertebrates. That’s going to happen in 10 years. And then what are all these people going to do for a job?” he asks. “In fact, they’re going to be studying the phenotype.” Prum sees nests as evidence that the world is still filled with mystery and complexity. “Science has reached the limits of its ability to reduce us … and I think this is an incredibly liberating message for the planet.”