Blog #470 Engineering a Nest
An Osprey’s nest is a wonder of nature’s engineering, a seemingly disordered bundle of sticks woven into a cohesive structure. Ever wonder how strong and resilient an Osprey nest is? My interest was piqued during trips to see the Caribbean Osprey (Pandion haliaetus ridgwayi) in the Turks & Caicos islands located a hundred miles north of Hispaniola. This subspecies is much paler than our northern Ospreys and has a weakly defined to no eye stripe across the eye.
In 2014, my husband and I surveyed every Osprey nest we could find in five days on Providenciales, the largest of 75 small islands and isolated cays, and North and Middle Caicos. We documented details of materials, general design, substrate, habitat, and activity. Transportation to the other islands and cays, where Osprey were known to nest, was not possible during our limited stay.
Akin to their northern brethren, the Caribbean Osprey nested on a variety of substrates including two on nest platforms in a park and a harbor designed especially for them. Others were on mangrove cays, utility poles, the roof of an abandoned beach house, light poles, and even on the nautical beacon signaling the most northern point of Providenciales. The most scenic nest was located atop a sea mount near Pirate’s Cove barely containing two hearty youngsters ready to fly. Their ever-attentive mother permitted us to climb remarkably close without being perturbed.
All birds build with materials determined by habitat and availability. The Caribbean Ospreys use available woody materials, palm fronds, beach grass, and other available vegetation along with a vast amount of anthropogenic garbage such as food wrappers and plastic shopping bags. One nest on a dedicated Osprey platform contained a small laundry basket, several tire inner tubes, plastic shopping bags, and a rotary telephone with its coiled cord dangling. (Ospreys world-wide are incredible pack rats.) On dry tropical islands, debris dominates when natural nesting material is unavailable. Materials such as fishing line, twine, and plastic string may strengthen nest structures but are dangerous entanglement traps for the birds.
We surveyed the nests again in 2017 after Irma, a Category 5 hurricane with winds of
156 mph, ripped off roofs, twisted metal, and destroyed the islands’ communications infrastructure (photos below). Amazingly, the totally exposed Osprey nests appeared intact with only minor damage. How could that be? Were they not just piles of plant materials and garbage? These questions nagged at me.
Bearing in mind the extreme weather patterns that an Osprey nest is exposed to, what are the structural properties that give their nests such resilience? The structural engineering and design factors present in bird nests are the same one’s humans use to build their dwellings, buildings, and aircraft. Basically, a nest must be engineered to be structurally sound to support the weight of an incubating bird and its entire brood of nestlings, which quickly become adult sized. The larger the bird species, the heavier its reliance on woody materials for structural integrity.
Corey O’Hern, of the Yale University notes, “Birds are able to build nest structures with remarkable strength and durability out of readily available and not particularly strong materials.” O’Hern and his colleague Richard Prum use computational and theoretical techniques to understand the internal structural and mechanical properties of bird nests. “Though it may surprise most people, we do not yet have a fundamental understanding of such interwoven and entangled composite structures, and in that regard, we can still learn a lot from birds.”
O’Hern studies the yield stress (the resilience or sturdiness) of materials by testing the vertical force needed to crush them. Using abandoned birds’ nests from a variety of species as subjects, O’Hern employs x-rays and CT scans to analyze the nest components before measuring the applied vertical force necessary to crush the different nests.
Ceramics and metals are known to have a high yield stress. What O’Hern found when measuring the yield stress of nests was remarkable! The yield stress necessary to break a Robin’s nest cup made of twigs and mud can be as much force needed to break ceramics. Moreover, the density of the nest was at least 10 times less than that of ceramics. Nests without mud also have high yield stress. Don’t underestimate nests; they may appear fragile and carelessly constructed, but they are exceptionally sturdy.
O’Hern and his colleagues meticulously took the nests under study apart, measuring each piece and examining its engineering properties. Computer models replicating these nests were developed and continually refined to reflect the resilience to pressure exhibited by each nest. Included in the models are factors such as the aspect ratio of the length of a twig to its width are key to having a solid, cohesive structure, as well as the ability to withstand vibrations. Advance studies of the nest components and the construction processes used by birds may spur the development of strong lightweight materials for human use. Unfortunately, getting an osprey nest into O’Hern’s lab for analysis presented too many challenges.
From exams of a wide variety of stick nests, O’Hern found that the more sticks and twigs in the bird nests, the higher the number of entanglements and crossings. All the seemingly random ways the sticks cross back and forth and lock together give nests their impressive structural integrity. Osprey nests can contain hundreds of sticks—the more sticks, the more stable they become.
A rare Osprey nest in a larch tree, courtesy of Getty Images.
“When nest building, a bird won’t go for any old twig. Somehow, birds pick and choose material that will create a cozy, sturdy nest. That’s just totally mystifying to me,” says physicist Hunter King of the University of Akron in Ohio. “Birds seem to have a sense for how the properties of an individual stick will translate to the characteristics of the nest.”
King measures the structural stress and force needed to distort nest materials, one of many engineering properties ascribed to bird nests, by compacting them with a piston. A nest is
essentially a pile of sticks with many points of contact as the sticks lay on top of each other. King’s experiments show that when a pile of sticks is compressed the number of contact points along each stick increases, restricting its bending and making it stiffer with increased compression. The sticks inside the nest retain static friction, which helps prevent them from sliding. The friction between strands and the distribution of the strands’ points of contact play a decisive role in determining a nest’s properties.
What makes nests self-supporting? Weaving, of sorts, is the secret. Tidy nest-building birds are known to bend and tuck twigs that poke out of nests under construction. This form of “weaving” twigs continues as the bird moves about the outside of the nest in a circular fashion, pushing twigs in to pack the nest tighter. Osprey nests may be sloppier on the outside than other stick-nests, but female Ospreys keep order inside the nest by continually fitting twigs, larger sticks, plastic bags, and plant material into the inner nest walls by pushing, pulling, repositioning, and bending it if possible. Each twig is flimsy but together they are strong much like a woven garment. Reinforcing nest walls could give rise to building self-supporting structure. Smaller platform nesters such as robins add strength to their nest walls by packing them with mud.
King and others suggest that birds select materials during the various stages of nest-building based on their differing properties. The overall design of most bird nests reveals that the placement of materials is not random. The base of the outer nest is composed of significantly thicker, stronger, longer, and more rigid materials compared to the side walls. The side walls in turn are significantly thicker, stronger and more rigid than materials used in the cup. The rims must be especially strong to support the adults perching on it.
Ospreys construct nests using weaker and more flexible branches for the lining and more rigid, thicker ones for the main structure. Ospreys line the nest and egg cup with softer materials such as algae, leaves, grasses, and moss. This reinforces the idea that Ospreys can distinguish between materials based physical properties and are selective in their use during nest construction.
How does a naïve Osprey become proficient in the act of nest building and choosing appropriate nest locations and materials? Is it instinctive or learned behavior or a combination of both? The basis of birdsong, for instance, is an innate ability, but the songs are modified over time. By contrast, little is known about the nest building process and what young birds innately know and how much must be learned. One important factor in nest building, the dexterity essential for manipulating materials with the beak or “mandibulation,” is learned and improved with experience. Older Osprey nestlings learn and practice mandibulation by helping their parents arrange sticks and other materials brought to the nest. Once fledglings, they continue manipulating building materials by practicing how-to pick-up sticks and other materials off the ground and flying over the water to grab floating algae as part of their play.
Juvenile Osprey dragging its feet in the water and trying to catch a stick, courtesy of Cindy Sedlacek.
Eyes to the sky!
Candace
Candace E. Cornell
Salt Point Natural Area
Cayuga Lake Osprey Network
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TO HELP PROTECT OSPREYS:
• Stay 100-300 feet away from Osprey nests during the breeding season.
If the Osprey vocalizes or flies off the nest you are too close!
BACK OFF IMMEDIATELY.
• Carry binoculars to view wildlife from afar.
• Dispose of used fishing lines, twine, nets, and plastics which
can kill Ospreys and other animals of the lakeshore.
• Become a nest monitor for the Cayuga Osprey Network: cec222@gmail.com.
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