Birds' nests vary a lot.

From the massive holes that  mallefowls dig in the ground, to ovenbirds, who are literally named  for their pizza-oven-style nests. And most of the time, it’s pretty straightforward to see why a nest is built a certain way.

Say you’re a Crested Auklet living on the old lava flows on the Aleutian Islands of Alaska. You’re probably not going to find  many leaves to help build your nest. Or maybe you’re a Toucan with a beak  up to four times the size of your head.

That makes it pretty difficult to maneuver  tiny sticks into an intricate nest. Scientists have tried, as scientists do, to develop a general explanation  for bird nest variety. But they keep running into trouble.

Because there’s just no perfect way  to explain all those unique styles. [♪ INTRO] All birds lay their eggs in a nest of some kind. Even if it is super basic. And many species of chicks tend to hang out in that nest for a  while after they’ve hatched.

Despite the many forms nests might take, we can generally think of them  as a cozy little container that protects future chickies from  the rough, wild world out there. That means survival, in part,  depends directly on the nest. So nests are a prime target for our  friend evolution to modify their shape, based on environmental pressures like  climate, foliage type, or predators.

Like we saw with the crested auklet  and toucan, birds are constrained by the available construction  materials and by their anatomy. And some species, like Siberian Jays,  will build their nest in a covered area if they think there’s a high chance of  predators swooping in to snatch an egg. So far, so good.

These examples make sense! But of course we know that  nature isn’t as simple as ‘oooh, sticks! I’m going to  make a nest out of sticks’.

So once scientists start searching  for common factors between lots of different nest styles and across many  species and environments… it gets trickier. What we would love to understand is  how all those environmental factors— climate, predators, material availability— influence a nest’s location, shape, size, and how it’s attached to external structures. But even trying to explain different nest styles within similar environments can be tough.

For example, Northern Gannets and Black Guillemots both nest on similar rocky  cliffs on the British Isles. The Northern Gannet builds huge nests up to a meter tall to support their  eggs on the precarious cliffside. On the other hand, Black Guillemots just  try to wedge their eggs in a crevice, or they plop them in an abandoned puffin burrow.

While the location, building  materials, and predators are mostly the same for both species, they’ve  ended up with vastly different nests… It might seem that researchers  have set themselves too big a task by trying to understand all bird nest methods. It seems too varied a problem to  explain with just one hypothesis. But it’s pretty common in evolutionary  biology to search for these kinds of patterns using a statistical method called  multivariate phylogenetic analyses.

So that’s exactly what one  study from 2025 tried to do. But before I can tell you  about what they found out, we’ve gotta keep our nest  funded, so here’s a quick ad. You watch SciShow.

You’re curious about the world  and the universe you live in. You think it’s good to learn new  stuff, whether you’re in school or not. And you’ll do it all online if you can.

If you’re the average SciShow viewer, you’re  also the average Brilliant subscriber. Brilliant is an online learning platform  made for everyone from age 10 to 110. They offer courses like the  Digital Circuits course, which includes step-by-step  interactive lessons and personalized practice opportunities built in.

So you can learn by doing. To learn for free on Brilliant for a full  30 days, go to brilliant.org/scishow, scan the QR code onscreen, or click  on the link in the description. Brilliant’s also given our viewers 20%  off an annual Premium subscription, which gives you unlimited daily  access to everything on Brilliant.

Researchers collected data about  nests from as many species of birds around the world as they could,  as well as information about the environment for each one,  and created a huge dataset. Then they used statistical analyses  to make a model that could explain bird nest variations based on  factors that could be driving the evolution of those traits. Things like climate, potential predators, or whether or not the birds lived on an island.

Remember, surprising evolutionary  stuff happens on islands! But their model could only find  evolutionary explanations for about 4% of the variation in birds' nests. Which is, you know, not a lot!

Climate could explain around 2.5% of  nest variation, according to this model. This mostly came down to whether the environment was dry and windy or cold, wet and still. In the dry and windy case, the  harsher environment tends to drive birds to build their nests in  cavities, like the Mountain chickadee, or an underground burrow  like Leach’s Storm-petrel.

Researchers could explain another  1.6% chunk of nest variations based on the combination of  kinds of vegetation nearby, predator presence, and whether  the birds are on an island. We are really splitting hairs here, folks. That “universal bird nest model” clearly  did not find many overarching themes!

You might be thinking that there  must be some unifying factor missing from the model, since it’s  not finding many useful connections. Perhaps birds that are more  closely related have similar nests, or maybe their position on the  evolutionary tree comes into play! But it turns out relatedness also wasn’t a great predictor of what a nest might look like.

Take the group Furnariidae, those cute  little fellas with the pizza oven nests! Even though species within this  group are closely related — being in the same genus and all — these birds can be found in all  sorts of different environments. They live anywhere from rocky shorelines, to deserts, to alpine areas and deciduous forests.

Their nest diversity reflects  the range of their environments: they make nests of mud and plants, or  cactuses, or spiderwebs and grasses. Sometimes inside a woodpecker hole  or a burrow deep in a cliff face. So a lot of different real  estate options for ovenbirds!

Since ovenbirds kind of throw  out the relatedness hypothesis, researchers pivoted to thinking perhaps nests are an “all roads lead to Rome” situation. Like, if conditions are broadly similar,  then maybe it doesn’t matter too much exactly how birds protect their  eggs, as long as they survive. Remember those Northern Gannets and  Black Guillemots we mentioned earlier?

It probably helps that their eggs aren't  totally helpless victims to their surroundings. The embryos inside the eggs can cope with some of the fluctuations going  on outside their cozy confines. These eggs can withstand temperature swings of up to 20 degrees celsius in a single day.

So it’s not as important for the nests  their parents built to be perfect. And there are a couple other factors that could be making it so hard to explain nest variety. For one, in most studies, researchers  talk about the environment as, well, the fairly big space that the nest sits in.

But the scientists in the 2025 study say  that maybe the area just around the nest, or its microenvironment, is just as  important as the broader environment. It’s not too hard to imagine that when  it comes to hiding from predators, how tightly packed the tree  branches are around a nest might matter more than how many trees  there are in the forest overall. We can also look at the Tibetan  ground tit as an example.

They nest in pre-made Pika burrows, in crevices, or in burrows they build themselves. At least one study found  that, within the same habitat, Tibetan ground tits change the shape  and entrance size of their burrows depending on how windy and cold  their particular patch of tundra is. If their burrows faced the strong afternoon winds, they’d have a longer entrance  tunnel to keep warm for longer, whereas if they were facing the sun, they’d build shorter tunnels to pick up  as much heat from the sun as they could.

But unfortunately, microenvironment  information isn’t captured in big databases like the ones  researchers used for the big nest survey. Other scientists have suggested  that nest building could be a form of culture for birds, and that  there’s an element of creativity to building. At least one study found  that individual zebra finches build nests with their own sense of style.

These birds choose different  materials based on preference, say for a particular color, or due to  constraints of their microenvironment, like selecting shorter fibres to  fit through a narrow nest box hole. But maybe the biggest thing  that the scientists learned from this story was that we haven’t  actually been very good at describing nests! There’s tons of glorious nest nuance out there, but it just hasn’t been captured by the broad, standardized descriptors that  scientists have been using for years.

So, for now at least, scientists don’t  have a unified theory of bird nests. But with a little more searching  and some better vocabulary, we may one day weave together an explanation. [♪ OUTRO]