Why don’t birds’ nests blow out of trees?
Avian architects at work
Winter has arrived here in North America, with a bitter arctic cold arriving this week on the crest of strong winds whipping out of Canada. Even the slightest breeze is frigid when the temps are in the 20s, but when the wind is over 30 mph, it’s truly brutal. Branches litter our lawn and dead trees take a sudden rest across our local trails. Any leaves that had been clinging to their branches have given up the fight and blown to the forest floor (except for beech trees and oaks).
These hefty winter winds have me wondering – why don’t we find more bird nests on the ground after windy days like this? I envision a beautifully constructed nest perched at the fork of two branches – that’s all fine when the weather is warm and parent birds are feeding their babies. But what about when storms arrive? How does the nest stay put? Some tropical birds weave nests, which seems like a sturdy solution – do our native birds create some sort of woven foundation that connects the nest to its tree?
Why don’t birds’ nests blow out of trees?
Bird nests are built out of an amazing array of materials. Some, which I learned are called “adherent nests,” stick to their foundations because they’re built out of mud and organic materials. Think of swallows and phoebes, who leave a messy splatter of mud on our eaves each spring as they fly back and forth to find the best source of dirt. But what keeps the mud from crumbling as it dries?
Bird spit.
The birds use the mucus within their saliva to bind together the mud particles in their V-shaped nests, a design which structurally allows for the best success in maintaining these birds’ weight.
But only lightweight birds can construct these nests of mud and spit – no storks or predator birds live in mud nests. Instead, larger birds live in platform nests, oftentimes at the very tops of trees where they’re definitely exposed to harsh winds and weather.
How do these nests stay in place? Generally, by virtue of their sheer size. Eagles make the biggest nests, reusing them year after year and simply adding to the material each time. Last week, this video of an eagle weathering a storm in Louisiana demonstrates how stable the nest is even in tornado-watch-strength winds.
Storks also build large platform nests reaching 9 feet across in many cases. Their nests are so large that other birds sometimes build their nests in the crevices within them, like an apartment building with a huge waterbird in the penthouse above.
Weaverbirds are one of the more creative nest builders, weaving their nests out of 1,000 strips of long grass. Walter Tschinkel has written a wonderful issue of his newsletter detailing the process these birds use to create their homes. You can also watch the 5-minute video below, narrated by David Attenborough, which shows the nest building from start to finish, beginning with a clever half-hitch knot that anchors the entire nest. The process seems instinctual and natural until the camera begins to follow a juvenile bird evidently attempting a build for the first time.
The male weaverbird is in a race against time to build these nests as they try to attract a mate. The female must choose their nest while the grass it’s made of is still green. If it turns brown, it’s too late and the male has to begin all over again.
Then there’s the Tailorbird of Asia, that doesn’t weave but rather sews its nest. Literally. It punctures holes in the leaves, weaves grass through, and then builds a nest in the resulting cavity (click to watch).
All of this is incredibly impressive, but what about birds typical of North America that build “cup nests” such as thrushes and finches and jays? Why don’t more of these nests blow out of trees in a heavy wind? Researchers have studied the nests by reverse engineering them (meaning, taking them apart, twig by twig), but they’ve also scanned them with CT scanners and electron microscopes in an effort to look for details about how they’re built.
One reason these nests are strong? Birds tend to build them in the crooks of branches, often next to the trunk where the wind has less sway. Nests of many species are also built using a “cup within a cup” formation, meaning the outer nest uses heavier, interlocking twigs while the inner nest is made of soft grasses and moss for comfort.
The nests are bottom-heavy and interlock carefully with a high number of contact points among the materials (as you might imagine, someone has written a research paper after calculating these contact points and the nests’ ability to resist the pressure of a piston. On second thought, you might not have imagined that).
The strength of bird nests has inspired human architecture as well. There is, of course, the Beijing National Stadium as well as a hanging house with retractable stairs that looks to be inspired by weaverbirds. Meeting rooms have been built to resemble nests, as have dining areas for restaurants. There’s also this horse arena at Stork’s Nest Farm in the Czech Republic:
Overall, birds are amazing architects and can be incredibly creative with their ability to turn natural materials into a comfy home for raising their young.
And then there’s Honolulu’s manu-o-ku.
These birds just lay their egg directly on a branch, incubating it for about 35 days and hoping it doesn’t blow off or fall. If the baby hatches, it learns at a very young age to hold onto branches tightly, even through strong storms.
If you can’t be creative, be tough.
I work at a wildlife sanctuary in Virginia. In early spring hummingbirds appear around our building to collect spider webs from under the eaves for their nests. The blue-gray gnatcatchers build works of art, also using spider silk, lichen and other materials. There is a pair that nests in the same tree, same branch every year. Some years the nest has looked like a porcelain bowl up in the tree. One year we had heavy spring rains. The nest was so thoroughly, thickly woven that the water did not drain easily from the nest and the nestlings drowned. It is hard when you become vested in the lives of individual animals in the park, and then tragedy occurs.
Who doesn't like to collect bird nests, eh? Many are such compelling objects even if they don't blow out of their trees. I liked the mud nests with the image showing the saliva glue. In southeast Asian caves (Borneo, Thailand, Phillipines) swiftlets build cup nests made entirely of saliva. These are prized for bird nest soup in Chinese cuisine, and have been for centuries (https://en.wikipedia.org/wiki/Edible_bird's_nest) (https://en.wikipedia.org/wiki/Swiftlet). Collecting such nests in several of Borneo's huge caves used to be a dangerous way to make a living, but with bird nests selling for over $4000 a pound, people now build concrete structures to induce the swiftlets to nest in them, in effect farming bird nests. I have seen swiftlet nests in a cave in Borneo that also contained a flowing creek. Very neat!