E34: Okay, but what's the deal with nests?

Scott Taylor: I'd never really thought about mammal nests before, but it turns out most of them build one. They're pretty different from what a majority of birds make, though. They're usually for sleeping and only sometimes for raising young. You've probably seen a few a squirrel dray up in a tree, or a mouse nest in your woodpile. You even have a nest, technically, and you crawl into it every night. Mammal nests can take a lot of forms, but bird nests can take many, many more. When I say bird nest, I'm guessing most of you picture our robins, mud and grass, an open cup, bright blue eggs. Not all birds build nests, but plenty do, and I think when most of us picture a bird, a nest isn't far behind. What we don't always picture is how wildly they vary. Some birds weave them, some birds build ones you could park a car under, some nest on the ground, some in cavities, some right out in the open. And whatever the shape for the birds that build them, a nest is where the eggs go and where the chicks grow up. So your nest matters. And it turns out it has a lot of work to do. Why is there so much variation? Does a single bird build the same kind of nest year after year? Which brings us to the topic of today's episode. OK, but what's the deal with nests? To dig into this strange and very cool world of bird nests, we'll talk with a friend of mine, Dr. Vanya Rohwer. Vanya is the curator of Birds and mammals at the Cornell University Museum of Vertebrates, housed at the Cornell Lab of Ornithology. He and I first met at a Halloween party in Kingston, Ontario back in our grad school days. After the break, we'll get into all things nests and we'll show you a genuinely wild variety of them. Video on if you can stay with us. Well, welcome back to the podcast, everyone.

Scott Taylor: I'm really excited to have my friend Vanya here today to talk all about nests. Thanks for joining us, Vanya.

Vanya Rohwer: Well, thank you for having me. It's a real treat.

Scott Taylor: Of course, I thought we could start with just like, why build a nest? So birds aren't the only organisms right that build nests, but they often have what I would consider the most complicated ones. So what's the deal with birds in building nests in the first place?

Vanya Rohwer: We think of birds as endotherms, but at the at the egg stage and at the early nestling stage they can't make any of their own heat and they are totally reliant on a parent to to stay warm and for development.

Scott Taylor: Yeah.

Vanya Rohwer: So in that case, nest construction is the thing that could keep them warm, especially if the parent is going to take care of itself. Like, parents still need to eat, and sometimes that means leaving the nest to find food and then the babies are going to cool down, potentially so. A nest also has to survive it. It can't be eaten by a predator because of that. That means that there's, there's like a lot, there's a lot of innovation behind what type of nest you build and where you place your nest. So you can have birds that will place nest at the very tips of tree branches just cuz it's really difficult to access. You can have birds that will bury eggs under underground even or, or that nest in holes. And so there's all these different strategies of where to put your nest or how you build it that will give it better odds of, of not being found by a, by a nest predator. Nests are really attractive to a lot of a lot of bugs that like to suck the blood of of baby birds and adult birds.

Scott Taylor: Yeah.

Vanya Rohwer: That's another challenge for birds to overcome during breeding. And so they can get ahead of those challenges by choosing some materials over others. You know, they can use like green leaves, they can use really aromatic plants. These edible-nest swiftlets will build their entire nest out of saliva. Just their own spit.

Scott Taylor: Just their own spit. That's very cool.

Vanya Rohwer: You've got a lot of swallow species that will grab mouthfuls of mud and, and stick that mud to, to rock faces or bridges and, and build an entire nest, at least the exterior of the nest out of, out of mouthfuls of mud. Other nests that I think of as almost like, like a third grade kind of science fair where it's like, you know, as as a third grader, you're given a whole pack of straws and you're like, OK, build a bridge or build a tower. And, and for, for species like chimney swifts, they, they build their nests out of these little sticks of wood, maybe anywhere from 2cm long to three, maybe 4 centimeters long. And, and that's it. It's wood and saliva. And it's like, how do you, how do you build a nest out of those materials? Like what a challenge.

Scott Taylor: Yeah, that does seem like something a third grader could attempt though. Like is saliva the most surprising material to you? Or are there other surprising materials you find in nests?

Vanya Rohwer: I remember reading this description of a, of a hammerkop's nest. They are a bizarre species. They're throughout kind of southern central Africa and they build these massive nests. It is like a, a wood fire pizza oven that is placed in a tree and it has a little opening their entrance hole and, and, and they're huge and they are made of surprising materials. OK, you ready?

Scott Taylor: Yes.

Vanya Rohwer: So this this list is from a hammerkop's nest that blew out of a tree, and it includes the following A pan brush, a broken cassette tape, a glove, a plastic dish, a plastic cup. 2 peacock feathers, chicken feathers, 2 socks, rabbit fur, 45 rags, 4 corn cobs, One piece of glass, 4 bits of wire, a plastic comb, one pair of underpants, nail, a typewriter ribbon, a piece of leather belt, 4 bits of stocking, 2 bits of tin, 2 bits of foam rubber, 7 bits of hose pipe, 9 bits of plastic pipe, electrical, 6 bits of asbestos roofing, 11 miscellaneous bones, T-bone, etcetera. 12 pieces of sandpaper, 4 bits of insulation tape, 10 plastic bags, 9 pieces of paper, 56 scraps of tinfoils, 6 bicycle tires, 6 lengths of insulating wire, and about 100 kilograms of twigs, sticks and grass.

Scott Taylor: Wow, what a MacGyver of a bird. It's like I'm going to use twigs, sticks and grass mostly, but then I'm also going to use men's underpants, etcetera.

Vanya Rohwer: Yeah, yeah, I love the detail in that description.

Scott Taylor: Yeah, what's the deal? Why are there so many things in their nests? Are they just weird little like collectors?

Vanya Rohwer: Like I think so. I think because their nests are so big and and both males and females will build the nest.

Scott Taylor: OK. That's kind of unique.

Vanya Rohwer: Yeah, and and they continuously add to nests and are just continuously building nests throughout the year.

Scott Taylor: Oh, throughout the year. Is it because hammerkops are, I assume they're non migratory.

Vanya Rohwer: They're just in the same territory year round so they can keep adding to this nest as they experience the year. That's really kind of unique.

Scott Taylor: We've gotten into this a little bit, but when we say bird nests, most people probably picture something like a robin's nest. So some mud, some pieces of sticks. Not they were not going to picture a hammerkop nest kind of spans all the way from I'll have one feather on a cliff with my egg to something like the hammerkop. But can you tell us a little bit about what's in between there?

Vanya Rohwer: Yeah, yeah, OK. There is a lot in between. At one extreme you've got you've got no nest, you've got like a whip-poor-will that will just lay two eggs right on the ground. At the other extreme you've got you've got species like hammerkops, you've got species like Cape penduline tits. And these birds will build these like just fantastic nests. They look kind of like a little bit bigger than a water balloon and the material is totally felted. It's like plant material. And then they build these nests with false entrances. If you look at the nest, you're like, oh, that's where the bird goes in and out. But it is just this, this little blind entrance that that stops about maybe an inch and a half down. Whereas the real entrance is this, this thin little flap that the birds can open and close. And she's like, what a what? An ingenious little nest.

Scott Taylor: And the false entrance? What's the purpose of the false entrance? For folks who don't know what happens to most baby birds, yes.

Vanya Rohwer: Yes, the false entrance ideally is to, you know, fool a nest predator. Could be a snake, could be a small mammal.

Scott Taylor: Yeah.

Vanya Rohwer: Keep the nestling safe and the real nest entrance can can be closed by the bird when it's inside the nest.

Scott Taylor: Oh really?

Vanya Rohwer: It's really tough to to figure out how to get into this thing.

Scott Taylor: That's so cool.

Vanya Rohwer: Yeah, you have spectacular hanging basket nests like oropendolas, caciques. These, these are nests that can be 4 feet long and they're just, they're super obvious, but they're placed at that very, very tips of tree branches and that kind of placement makes them really difficult to access. If a, if a monkey's climbing out the tree or thinking it's going to get an easy meal, you've got, you've got nests that float on water. You think of, there's a lot of grebes that would use bulrushes and, and, and vegetation that floats and they'll just create these big floating mats. And, and those are, those are really cool because you, you've got this floating nest. It's out in the middle of a lake and it's, it can be kind of obvious. And so when a, when a, when a parent will leave the nest, they'll cover up their eggs with some of the nesting material and, and conceal the eggs.

Scott Taylor: Yeah, which is super neat.

Vanya Rohwer: You've got South American ovenbirds that build these like really, really unique kind of just like, like this mudden. It it, it kind of swirls around with the nest entrance, almost like a almost like a snail shell.

Scott Taylor: Yeah, and they'll like just put them on the tops of fence posts and stuff. They look kind of like a pizza oven, even made of this mud in a really interesting way. And I don't know, it's fascinating. Like I think when most people picture birds nest, they picture this simple nest which works, you know, like a simple cup nest works well for a robin. They're generally like kind of hidden, but don't have these anti predator defenses. And the reality is like nest predation. So the chicks getting eaten by some kind of predator is very common, like extremely common across the globe, but even more common in these tropical areas in all of this, like some of these are super complex. The hammerkop is kind of working on its nest all year, whereas other birds arrive on the breeding grounds and they've got to get their nest ready in order to lay their eggs so that their chicks hatch and are the right age when there's insects, etcetera, etcetera. So has anyone kind of quantified the cost of nest building?

Vanya Rohwer: There have been some studies that have looked at this. It's a, it's a really tricky thing to look at, you know, because birds will travel really variable distances for certain materials. There's some more direct ways of looking at it, and that is like counting the number of trips that a bird will make to build the nest. And I remember doing this with yellow warblers that were breeding between like a colder subarctic site and then A and then a much warmer, more temperate site. And, and yellow warblers build different nest morphologies between these two sites. Your colder site they they built just a a larger, better insulated nest.

Scott Taylor: That makes sense.

Vanya Rohwer: Yeah, yeah. And when you look at the number of trips it took a female to build the nest, they were incredibly different. So those those larger nests, they are about 1200 trips to build wow, whereas whereas those smaller nests are about between 3 and 400 trips to build.

Scott Taylor: That's a huge amount of differential energy investment in building that nest.

Vanya Rohwer: Yeah, it's a huge difference. And so there's similar studies for cliff swallows. You know, it's estimated to be about 1400 trips to build that nest.

Scott Taylor: Wow.

Vanya Rohwer: It's a tricky thing to evaluate. We did something similar with the yellow warblers. We We did these nest transplant experiments where we.

Scott Taylor: Remember, yeah, this was your graduate work, right when we were in Queen's.

Vanya Rohwer: Yeah, that's right. And, and and this is just related because it involves like swapping nests. And so we, we took nests from from each location and we transported them to the opposite location. And then we would, we would give them the old, like, switcheroo. And what was so interesting is how these birds would respond when they returned to the nest.

Scott Taylor: Yeah.

Vanya Rohwer: Like some birds would. It was like nothing ever happened. And they would just sit right back down and continue incubating.

Scott Taylor: Yeah.

Vanya Rohwer: Whereas other birds would be like something different. You know, they'd be pretty cautious about approaching the nest. And, and you know, they would, they would approach it, they would sit on it, they would get off and they would look at the nest again and, and, and come back, try sitting on it again. But so they clearly noticed something was different. But they eventually continued incubating.

Scott Taylor: What you were trying to get at is like how important is local nest morphology for fitness, right? Does it matter that you're building this bigger nest in Manitoba versus Ontario and what did you find there? I mean it makes it's a very cool but very difficult to ask question with this nest swapping thing, but what was the major outcome?

Vanya Rohwer: You know, going into the project, the the first thing you think about is, well, there's differences in climate between these two sites and that those large nests are, are probably driven by a temperature just breeding in a colder environment requiring birds to build a better insulated nest. So we we found support for that idea when we measured nest temperature inside the nest and then also when we measured nestling growth rates when it was cold, those better insulated nests stayed warmer and then nestlings grew at a faster rate during cold temperatures only.

Scott Taylor: OK, yeah.

Vanya Rohwer: But then.

Scott Taylor: Common in Manitoba than Ontario during the breeding season, right?

Vanya Rohwer: Yeah, that's right. But then the the really surprising finding was in Manitoba, your, your local nest, your larger nest, your better insulated nest suffered higher rates of nest predation, which which, you know, I certainly didn't expect that going into the into the study, But but it was on the order of like your, your experimental nests, were fledging about one chick more on average compared to your, your, your bigger, better insulated nests.

Scott Taylor: Wow.

Vanya Rohwer: Yeah.

Scott Taylor: That's really interesting. I guess like, I've seen your photos of those bigger, better insulated nests and if I was a visual predator I would find them easier. I mean, when you were nest searching, where was it easier to find their nests in Manitoba or Ontario?

Vanya Rohwer: Oh yeah. Hands down Manitoba, right?

Scott Taylor: So as like a pseudo nest predator, I think you kind of proved that outcome yourself.

Vanya Rohwer: Definitely in terms. Of being able to find the nest, but it's amazing how important like even what we might consider to be minor differences in nests are for nestling survival.

Scott Taylor: Like as you alluded to, these for altricial chicks anyways, which is most birds that build like complicated nests have these altricial chicks or chicks that are very helpless when they're born. But in these altricial situations when the chicks are basically ectotherms, like you said, it's so important that they keep warm enough that they keep hidden and predators are out there looking for these helpless little blobs. We call the chickadees the day they hatch the gummy bear stage because they're just like a little pink gummy bear with no eyes, no, no feathers. And then they've got to go from that to like a flying bird in, you know, three weeks, which is faster than some birds. It's kind of insane.

Vanya Rohwer: Yeah, sociable weavers are tiny birds. You know, they're like smaller. They're, they're about the size of a house sparrow, maybe a little bit bigger, but they build these nests that can weigh like a metric ton or you know, two and three metric tons. They, they nest in huge colonies and they build such a big nest in, in an area that is super hot. We're, we're talking like southern Africa. And so these nests offer this incredible habitat to a bunch of different species of birds. It's like African pygmy falcon is one of the like most symbiotic with these, with these sociable weavers. They nest inside the chambers, but then you have such a big nest that's casting a large shadow on the ground and then you have incredible amounts of just guano because these nests are used for 50 years or something. They're, they're massive colonies. You'll get several hundred birds breeding there. And so underneath these nests are like these little, these, these little oases of, of nutrient rich soil of shade of cooler temperatures. And so you get a lot of different, different, different plants and different organisms that are that are attracted to these areas. You can, you can almost think of them as like the ecosystem engineer.

Scott Taylor: Yeah, almost.

Vanya Rohwer: Almost like a beaver of North America, except in a in a bird way.

Scott Taylor: In a bird way in an African tree. That's the I really love that. That's cool. I knew social weaver nests were big. I didn't know they were metric tons big. But I have seen pictures where the trees they're in have just fallen over because they're so heavy. So.

Vanya Rohwer: That's totally cool.

Scott Taylor: Break the trunk big.

Vanya Rohwer: Yeah.

Scott Taylor: And in terms of weird materials, we haven't talked about this yet, but some of your work has looked at not just what the birds use to build the actual nest, but then other things that they use. I would. I don't know if decoration is the right word, but what's the deal with birds decorating their nests with snakeskin?

Vanya Rohwer: Yeah. Yeah, snakeskin is such a unique material.

Scott Taylor: Yeah.

Vanya Rohwer: You know, I think about like walking around outside how frequently I find shed snakeskin and it's not that often.

Scott Taylor: No, it's every once in a while if you live in a place with like a pretty big snake population, right, Yeah.

Vanya Rohwer: Right, right. And yet there there are certain species that you know about 80% of their nests will have snakeskin in it. The the the classic example is great crested flycatcher.

Scott Taylor: Great crested flycatcher, Yeah, I love great flycatcher and great name and very cool behavior, yeah.

Vanya Rohwer: So we we ended up amassing about 3 independent data sets and they were all providing the same picture, which is that like species that nest in cavities are more likely to use shed snakeskin in nest construction.

Scott Taylor: And did you figure out good evidence for why it's so common?

Vanya Rohwer: Our best idea right now is that that snakeskin in the nest will reduce nest predation from some nest predators, but not all of them. And, and the nest predators that snakeskin seems the most effective at like scaring away are small mammal nest predators. And, you know, ecologically it, it made sense to us in the sense of like, well, snakes eat a lot of small mammals. And that history of interaction probably makes the decision of whether or not you're going to enter this dark cavity as like a as a mouse or a chipmunk. Like, is it worth the risk if I like, if I think there is a snake in there?

Scott Taylor: Yeah.

Vanya Rohwer: And and so that's a little.

Scott Taylor: Calculus the chipmunk is making looks like I really want to eat those eggs and chicks, but am I myself going to get murdered by a snake as soon as I put my face in there?

Vanya Rohwer: OK, yeah, but so, so snakeskin is a is a pretty unique material. Yeah, there's a lot of really bizarre materials where it just seems like birds are going out and they are like curating their nests, you know, I think of hummingbirds and gnatcatcher nests. So beautiful. Exterior of those nests are stunning. These these birds are finding little flakes of lichen and and fastening fastening those lichen bits to the outside using spider silk.

Scott Taylor: I know it's the most delicate ever. I saw like a hummingbird harvesting lichen once at work. There's this rock wall with lots of lichen. And I noticed a broad-tailed hummingbird just flying along and was like gently taking a tiny little thing and then flying away and coming back. And she came back and forth like 30 or 40 times while I was sitting there. Just, you know, I could imagine her going back and sticking it all around her nest. And yeah, it's pretty amazing. The chickadees we work on now, we're quantifying like how often they're using owl pellets, which again, like snakeskin, How often do you just randomly stumble across an owl pellet? Not very frequently, but if you need fur for your nest, it turns out they're probably a pretty good source of fur. And you know, many more than 60% of our local mountain chickadees put owl pellets in their nests, which is just such like, it's interesting to think about a chickadee flying around the woods being like, where are the owl pellets? Like having a search image. And then it gets even cooler because mountain chickadees do that, but black-capped chickadees don't. So very similar species living in the same forests with really different search images for what they want for their nests.

Vanya Rohwer: OK, there's another really like fantastic example of a of a unique material. And this is a bird that is going out and finding stuff like mongoose and jackal poop and bringing that to their nest. It's like, why in the world are birds doing that? And and and they're, they're, they're probably using it to reduce nest predation, But that's a. That's a really fun example.

Scott Taylor: White-breasted nuthatches use scat in their nests too. They're pretty gross. When you first open the nest box, you get this, like, nose full of usually fox scat, I think. Or they'll use weasel as well.

Vanya Rohwer: It's interesting to think about disguising your nest as the latrine of a predator so that nobody will nobody will eat your babies.

Scott Taylor: Literally. To make it smell like everyone leaves you alone.

Vanya Rohwer: Yeah.

Scott Taylor: What's your approach? Festival Portable.

Vanya Rohwer: Yeah, exactly.

Scott Taylor: Oh my gosh.

Vanya Rohwer: OK, there's, there's a couple like I think really neat examples where nesting materials are not only unique, but also helpful for conservation. And, and I'm thinking about materials that are, that are supplied to birds by, by conservation biologists or by scientists, by humans. And, and there's a really neat example of Galapagos finches. They've taken cotton and like soaked it in permethrin and then just offered to the birds and the birds like happily accept it and use that material and and gain this benefit of having almost no kind of fly ectoparasites in their nests.

Scott Taylor: Right yeah and there's this invasive fly in the Galapagos called Philornis downsi, that the maggots of the fly eat like they burrow into the chicks faces basically and eat their flesh and it's really gnarly and it ends in like most chicks die when it happens. But yeah, it's there is a native Galapagos cotton, so it makes sense that the Darwin's finches would just use offered cotton because there's a lot of that already in their environment. And then the permethrin prevents the flies from killing the chicks. And it's been quite successful. My understanding is.

Vanya Rohwer: Yeah, definitely. Yeah, yeah. So it's cool, Yeah. Even in the context of like conservation, if you think carefully about what birds might need for their nests, you can actually affect some help, which is pretty amazing.

Scott Taylor: Yeah. Right. So, I mean, you're on this podcast with me because you've handled nests for years. And like I said, when I see bird nests and if there's something weird about them, I often think, Vanya would think this was cool, So you've spent a lot of time looking at nests, but what about nest building still surprises you? Or do you think would change how someone else sees bird nests?

Vanya Rohwer: I think finding a bird nest is like always surprises me because it's, it's like you've found something that you shouldn't have found. You, you think of the, the efforts that birds go to keep their nest from being found. And so when you find them, it's like a little, a little gem tucked away and, and it's like such a carefully crafted little gem. There's more and more plastics that you're finding and, and you know, like local bird nests around where you live. At first I just thought this was so depressing. I just thought like, man, what a what a terrible reflection of how we humans are interacting with our environment. We're just, you know, we're just casual and just leaving garbage. And, and yet when you, when you find a nest with trash in it, it's, it's still a really beautifully constructed nest, like even if there's trash in it. And I, I think that that part is what is what was a little bit surprising. And so I think of that as like, man birds are just so resilient. They're using materials that they've, you know, only recently encountered and and making the most of them and.

Scott Taylor: Yeah, I've wondered about that. And I know in some cases they have quantified some negative side effects. But we have some chickadees that if they can get into the inside of someone's wall, they will remove the insulation and use that in place of in place of fur or moss. And I can imagine the the the chicks are toasty. But we've reached the part of the show we call that's BS or that's bird stuff where we give our guests an opportunity to debunk a myth that ruffles their feathers. So Vanya, what do you want to call BS on?

Vanya Rohwer: OK, I want to call BS and it, and it's maybe just a partial BS, but this notion that like a bird's nest is its, is its home, it's where it lives. And, and I, I think for many, many species of birds, we have to think of those nests as like this is a temporary structure. This is where you lay your eggs. This is where your nestlings grow. And as soon as they are old enough to leave that nest, that nest is no longer needed. I don't know. I feel like people are often like, oh, they, they live in their nest. They sleep in their nest. And yeah, you know, and it's like, well, they do it during the breeding season, but that's it.

Scott Taylor: Right, Exactly, exactly. If you think about your bed as your nest, which technically it is, as a human, you definitely spend a lot more time in your nest than a bird ever spends in there.

Vanya Rohwer: And there's, I mean, maybe unless you're a hammerkop and you're just a little klepto wandering around and building your nest.

Scott Taylor: Well, thanks so much for joining us on the podcast, Vanya. It's always really fun to talk about everything but nests in particular, so I appreciate you taking the time.

Vanya Rohwer: Oh man, well, thanks so much for having me, Scott.

Scott Taylor: Of course, birds are dinosaurs, and around here we like our snacks, so we end each episode with the dinosaur nugget. Today's nugget is a nest isn't a bird's home. It's typically just a place to raise young. For most birds, it's temporary with one job. Get the eggs and chicks to the point where they can leave. Then it's done. But into that one job, birds pack an astonishing range of solutions, and they'll build with whatever works, right down to the things we leave behind. So next time you spot one, don't think of it as a house, but a temporary and very special masterpiece. That's a wrap on this week's episode. OK, but what's the deal with nests? Quick ask before you go. This show is brought to you by two guys, and in many cases, our personal friends help us out by leaving a rating of five stars. Duh. Or a follow. It's got to matter if we ask every episode, right? We'll catch you next time. Byeee.

This transcript was generated using AI-assisted transcription and may contain errors or omissions. Please refer to the audio or video episode for the most accurate representation.