In this episode, you’ll hear about:

• The difference between social and genetic monogamy in birds

• Why “monogamous” birds engage in extra-pair copulations (a.k.a. extra-curricular behavior)

• How males try to avoid cuckoldry with mate-guarding and other tactics

• Whether birds “cheat” in secret or right out in the open

• How researchers use DNA and multiple-paternity tests to see who really fathered which chicks

• 00:33 - Growing Up by the Lake

• 01:28 - Exploring Bird Monogamy: Science vs. Fairy Tales

• 09:43 - The Cognitive Abilities of Chickadees

• 16:26 - The Complexities of Bird Monogamy

• 18:57 - Exploring Bird Monogamy and Mating Systems
  
  Timestamp Disclosure
  These timestamps were generated using AI and may contain errors or omissions. They are provided for accessibility and reference purposes only and may not perfectly reflect the original audio.

Dr. Carrie Branch (Excerpt)
You know, you might see a male that has multiple females, you might see a female who has multiple males, or you might see multiple pairs actually mating with each other. And like everything in between.

Dr. Scott Taylor
I was a really lucky kid.

I grew up on the shoreline of Lake Huron in Ontario, Canada, where the lake was my backyard and the cedar forest was my playground. And I spent a lot of my time watching birds. Every year, my family put up a nest box for purple martins that was inevitably occupied by tree swallows.

But waiting for the first swallows of spring and listening to their bubbly songs was a highlight of my childhood. I'd stare up at that box and wonder which cavity they'd pick and whether the house friends would show up and ruin everything.

The first time I actually watched a pair of birds raise a family, though. It wasn't swallows at all, but a.

Song sparrow nest tucked under dead grass near our front door. I watched the female incubate the eggs, the male sneak in to give her food.

Both parents shuttling meals to chicks that went from tiny, pink, helpless blobs to loud, demanding fledglings. And I got invested. I romanticized the pair, and in my head, they were a married couple who'd chosen each other for life.

Years later, science complicated that story. Which brings us to today's question. Okay, but is bird monogamy just PR?

We'll ask Dr. Cary Branch, an ornithologist and assistant professor at Western University who studies bird behavior and extra pair mating. We've all heard the line, they mate for life. It shows up in wedding toasts, on fridge magnets and a suspicious number of greeting cards.

Swan necks forming a heart, penguins in formal wear, lovebirds doing PR for the idea of soulmates. Many birds do form pair bonds. They share a territory, build a nest, raise chicks together.

But across the avian world, extra pair copulation, the scientific term for stepping out on your mate, is very common. In some species, a third or more of the chicks in a nest may have a different father than the one tirelessly bringing in food.

It's less storybook romance, more Maury Povich. You are not the father with feathers. So why bother with a pair bond at all if exclusivity isn't guaranteed?

Because what looks like cheating through a human lens often looks like strategy through an evolutionary one. Birds are responding to conditions. Food, neighbors, predators, timing, even genetic compatibility. They're not acting out of morality play.

Even the word monogamous hides nuance. There's social monogamy, the pair you can see, and genetic monogamy, the DNA version. Those two don't always line up.

And that gap is where the drama and the science really live. In this episode, we're going to pull on that thread not to kill the romance, but to move it closer to reality.

How common is this monogamish middle ground? When does a pair bond hold tight and when does it flex? And what are the birds actually optimizing when two individuals decide to build one nest?

If you're worried this is a takedown, you can breathe. Birds don't need our fairy tales to be impressive. Sometimes pairs really do come back season after season. Same partner, same choreography.

After the break, Dr. Carrie Branch joins us to separate the love story from the branding and to show us what they mate for life actually means when you bring in data, DNA and a little less pr. Stay tuned.

Well, welcome back, everyone. I'm really excited today to be joined by Dr. Carrie Branch, who's with us from Western University in Southern Ontario, Canada. To dive more into this question about really what is bird monogamy?

Obviously, it's worked as really good PR for a long time, but there's more to it and Carrie studies it. So we're excited to have you here today.

Dr. Carrie Branch
Awesome. Excited to be here.

Dr. Scott Taylor
Yeah, great, thanks.

It would probably make sense to start out with just really talking about, like, what is when we use the term monogamy in birds and studying birds, what do we mean?

Dr. Carrie Branch
Yeah, for sure. So monogamy can mean a couple of things, right? So to us, it means one partner forever.

And sometimes that's true in birds, but oftentimes we just, we differentiate between what we call like genetic monogamy versus social monogamy. Right. So a lot of birds or most bird species, as you know, are socially monogamous. So that means that they pair up with another individual to breed.

And that might be, you know, for a few months. It might breed for a full year and they'll breed together.

But they're not necessarily genetically monogamous, which means that the offspring they produce, so the eggs they produce aren't necessarily from that male or from that same partner that they're raising. Right. So they'll seek what we call extra pair copulations outside of their social pair.

Dr. Scott Taylor
Right. So they form these social pair bonds that last different lengths of time. Right. So there are some that form a social pair bond just for one season.

There are others that can form multi year social pair bonds. But then in all of those cases, yeah, that genetic piece is non monogamous.

Or females are seeking out these extra pair copulations from other males, maybe males that are nearby. What's the benefit to that? Why form a strong social pair and then still step out, as it were?

I mean, that's anthropomorphizing, which we try not to do. But stepping outside of this social pair bond, what have we figured out about why that's so common?

Dr. Carrie Branch
Yeah, so there's the biggest hypothesis really, or the prevailing hypothesis is that females are seeking extra pair males to enhance genetic compatibility or genetic quality. Right. So maybe their social pair male is a really good parent, he's a really good father, but maybe he's not like the sexiest male on the scene.

And so then she might want to seek extra pair copulations with a higher quality male or a male that's sexier, so to speak. I put that in air quotes for those that aren't seeing the video.

In order to like sort of boost the fitness or the, the likelihood of survival of their, of her offspring and. Or like the sexiness of her offspring. Right.

Like if she mates with a sexy male, then that means more likely her sons will be sexy and then they'll get matings from other females.

Dr. Scott Taylor
And it goes on the sexy sons hypothesis.

Dr. Carrie Branch
Yeah, exactly.

Dr. Scott Taylor
So, but what is. I guess that adds another question. Like what is sexy to a bird? Yeah, like what is it that they're seeking out? Is it brighter colors?

Is it something more relevant to survival?

Dr. Carrie Branch
Or I think it can be all of the above. Right. I think it depends on the species. Just like so much of what we do is dependent on the ecology and biology of the particular species.

But yeah, oftentimes that's, you know, something like bright coloration, a really well performed song, which, you know, has something to do with how they developed in the nest right. When they were young.

So if they had really good access to nutrition, then maybe they're able to have really big brains, healthy brains, and then produce these brilliant songs they're able to imitate from their fathers. Right. For us, I mean, I could talk about chickadees, the chickadee of it all.

Dr. Scott Taylor
Definitely talk about the chickadee of it all.

Dr. Carrie Branch
Yeah. Because that's what's really neat, I think for us.

Because oftentimes, you know, we talk about high quality or low quality quality males or variation in male quality or condition. And it seems to be like this catch all term, right? Like females prefer high quality males.

But then we get in this feedback loop, right, where it's like they prefer high quality males, the males are high quality because females prefer them and they are preferred because they're high quality, you know, so we get into this like feedback loop where it's like, what does that even mean in terms of like health? Right.

It might be that they're larger, maybe they're more dominant, they're able to get better access to resources, whether that's, you know, territories or mates or what, whatever it might be.

But for us, a higher quality male or a high quality chickadee is actually one that is quite clever, so to speak, that can perform really well recovering their food stores. Right. So these are little tiny birds, about 12 grams, and they live in these harsh winter environments all year round. Right.

They don't, they don't bail, they don't leave and migrate like a lot of other species do. They hang around all year. And so, yeah, they experience some pretty harsh winter conditions and they store food when it's abundant.

So like in the fall or summer, fall, and then they recover all those food items throughout the winter and that's how they survive. But they put them in all these different locations.

So unlike a lot of squirrel species, right, where we think of them like putting them all in one location and then they can come back and find all their food in their, in their little home chickadees put them all throughout their territory. And so they really rely on specialized spatial memory to actually recover those food stores.

And so what we've shown recently is that females are picking up on this.

Like they can assess and discriminate among males that perform better or worse on a cognitive task, which is our measure for their spatial cognitive abilities. And that's linked to survival as well. Right.

They're more likely to survive, live longer, and the females more recently thinking about extra pair and like social monogamy because they're socially monogamous, these chickadees, just like some 90% of birds are socially monogamous, right?

Dr. Scott Taylor
Yeah, yeah, I think most birds are socially monogamous. And there's a couple other really interesting mating strategies.

Dr. Carrie Branch
Yes, yeah, I know. There's a whole host. Right. It's so much more complex than I think we ever thought it was before genetics came on the scene. Right.

But yeah, so we've shown that, you know, comparing their extra pair male, so the one they stepped out with versus their social male, that the extra pair male actually performs better on these cognitive tasks than their social male.

So it seems like for a chickadee seeking a higher quality male or the benefit they're trying to get for their offspring is this, is this better spatial cognitive ability.

Dr. Scott Taylor
So that's really, really cool. I think, you know, we often think of Coloration is this very obvious male character that's indicative of quality in birds.

But the idea that female chickadees could.

Evaluate the spatial memory of a male that's not even their social partner is really wild to me. I mean, I don't even have a good sense of my own partner's spatial memory.

And we live together, let alone like watching the neighbor and figuring out like, oh, you actually have better spatial memory than my social mate and I should seek you out for extra pair copulations. How are they doing that? What are they observing? Like, yeah. What do you think is going on there?

Dr. Carrie Branch
This is a great question and one that we still have. We have some evidence that it may be something about what we call. So it's. They're called secondary sexual traits. Right.

So it's not the primary sex trait like the sex organ itself. Right. But the secondary trait, which might be something like song production for songbirds or plumage variation.

So like how, you know, we're talking about chickadees here. So they're achromatic.

They actually don't have colorful plumage, but they have very contrasting plumage between these, like, white and black patches on their heads. And so that's where the secondary sexual trait could come into play and might be associated with these cognitive abilities.

So something that females can actually overtly assess and like sort of compare among individuals that then is for whatever reason linked to better cognitive ability. So that's one option. We have some evidence that that's true that, you know, smarter males or males that perform better on this facial task.

I'm going to say smarter just for, you know, ease of chatting about it. But these smarter males sing more throughout the day. So we do know that.

We also have some evidence that there's like more contrast in their head regions, which is pretty cool. So females could be using that to ID the males. But the reality is they spend all winter with these other birds. They end up, like you mentioned. Yeah.

So they, you mentioned they often seek like nearby males for their extra pair copulations because, you know, the female's having to leave her territory and find these other individuals and she doesn't have all the time in the world because.

Dr. Scott Taylor
Right.

Dr. Carrie Branch
The social media things to do gets suspicious. Right. Like there's evidence that they will change their feeding behavior and parental behavior if the female's gone for too long. Right.

That's another species, not in chickadees, but so they're aware, right. They're like, where's my female? She's been gone for a long time. These may not be my chicks. So I'm not going to feed them as much. But anywho. But yeah.

So she seeks these opportunities with these neighboring males. Right. And they are ones that she most likely has spent all winter with because they winter in these flocks.

They're already paired up and they're little social pairs in those flocks. And so the likelihood is that she's pretty aware of how good they've been able to recover their food stores.

So, you know, part of me wonders if it's just direct observation. She's like, look, I know the reality is Jerry has got the best cognition. I wasn't able to mate with him.

He was already paired up for the, for the winter or for the summer. Right. But I could have extra pair mates with him and get his jeans. So.

Dr. Scott Taylor
Yeah, it's a good point. With chickadees, like, they form those, those winter flocks that are quite stable and have these dominance hierarchies.

And those dominance hierarchies have been shown to be super important in terms of who gets to mate with who. They do spend a lot of time. Together over the winter and they do, I guess. Yeah, that direct observation piece is a really cool idea.

Dr. Carrie Branch
Yeah, that's. Yeah, that's kind of what we've thought about because it's interesting.

Like, you know, when we say choice with female choice, it's like they don't really have the choice of any male in the forest. Right. They're limited and constrained. Who's not mated yet. Right. So, yeah, absolutely.

Dr. Scott Taylor
Yeah. One of my other questions which you kind of started touching on was like, how do males respond?

I mean, we know that extra pair copulations or extra pair paternity is variably common across the bird tree of life. But in some cases, you know, you can have 30, 50% of the chicks in a given nest are not chicks of that social, that social father.

There's some somebody else's chicks that he's taking care of.

So you alluded to this a little bit, but can you give other examples of how males respond in these situations where it may be that they're not feeding their own offspring?

Dr. Carrie Branch
Yeah.

So, I mean, I think luckily for the female, the evolutionary timescale or, you know, selection hasn't quite caught up with them yet because oftentimes males are not able to like, recognize their own chicks. Most female females can't even recognize their own chicks. Right, yeah, that's a good point. Some can.

You know, that gets into the whole brood parasitism of it all. But even among Females and other species. Think of American coots, right?

And so the females will prioritize certain chicks over others and they can tell the difference in their other females that will lay eggs in their nests. Right. And they'll actually kill those extra chicks that aren't theirs. Right. So they can recognize them.

But males, as far as I know, I don't think that they can. So they don't. They wouldn't be able to discriminate within the nest, like who's theirs and who isn't.

But as I mentioned, there have been these removal studies of other species where, yeah, you take the female away for a certain period of time and the longer she's gone, you see the reduction in his parental provisioning or how much food he's bringing, how many visits he's making to the nest.

Dr. Scott Taylor
Yeah, definitely. I've.

Yeah, there's joint singing behaviors that seem to reinforce pair bonds and reduce the frequency of extra pair copulation. There's definitely mate guarding during the fertile period that comes.

Dr. Carrie Branch
Absolutely.

Dr. Scott Taylor
I guess males have their ways of trying to make sure that it's their chicks, they're taken care of, but they can't watch the female all the time.

Dr. Carrie Branch
Oh, yeah, there's always. Yeah, there's absolutely mate guarding. And yes, you're absolutely right.

They do, like, keep an eye on her and make sure that she's not off somewhere and that's where that comes in. When she's gone for too long, it's like, what's going on, the jig.

Dr. Scott Taylor
Yeah, yeah. The suspicion.

Dr. Carrie Branch
Yeah, yeah, yeah.

Dr. Scott Taylor
Okay.

So we've been talking about monogamy in birds and the fact that social monogamy is quite common in birds, but genetic monogamy is rare. But in order to know that, we needed new tools.

And so could you walk us through, like, how we actually evaluate whether a brood contains chicks from another male.

Dr. Carrie Branch
So, you know what's interesting and I guess, but like, each egg that a female lays is fertilized, like one at a time. Right. So the day before, you know, however many hours before, that's when it's actually fertilized. So they're not all fertilized at once.

So that opens that opportunity up for, you know, multiple paternity in there.

And so for birds, we can just take blood from them and it makes it pretty easy and like, pretty reliable in terms of getting really high quality samples from individuals. And.

Yeah, so then we just, you know, we have to sort of target certain areas of the genome again because it's a reduced representation is what we call it's called DD rad. So it's like a double digest, reduced representation analysis. Not that that matters. Write that down. We'll do a quiz later.

Dr. Scott Taylor
I mean, basically they digest the DNA with these enzymes. Right. That cut certain parts of the genome. And then we sequence out from those cuts sites and compare the genetic variation from those cut sites.

Dr. Carrie Branch
Exactly right. And so like, how many, you know, of these thousand SNPs, for example, you know, how many differ between like a purported male and the. The.

And the chick. Yeah.

So then we use these different softwares that actually can tell us, you know, combined the, the mother, the chick and the father, who we think it is, like sort of. What's their score? Score of relatedness. And if it's too low, then we're like, oh, it's probably not the correct father.

And then you can put in other options to these programs. Once you have all of the sequences of the different individuals, then you can kind of assess that that way.

Dr. Scott Taylor
Again, thinking about the genetics of it all, our ability to ask these questions has really changed even since you and I were graduate students, which feels very recent.

Dr. Carrie Branch
But it was so recent, Scott. It was so recent. I. Early career.

Dr. Scott Taylor
Yeah. Well, I'm mid career, I guess. You're mid.

So mid. But it is interesting that now we can sequence whole genomes relatively in a cost effective way and ask these questions about who's related to whom.

And it's shedding some really interesting light on the decisions birds make and the evolutionary pressures that shape them. We've been talking about this idea of social monogamy in birds.

The fact that many birds are socially monogamous, so they have one social mate, but they step out of that pair bond. And different species have different rates of extra pair copulation. You know, house wrens have really high rates of extra pair copulation.

Most seabirds, it's pretty low. But why might it vary? What evidence is there that these behaviors vary across environments, across individuals or across populations?

Dr. Carrie Branch
So it's a really interesting species, the Dunnock, which is a European songbird, which, like, looks quite drab when you.

Dr. Scott Taylor
Super drab. Yeah. When I show them to my ornithology students, they're always surprised. It's just this pretty drab little brown bird.

Dr. Carrie Branch
Yeah. But they're up to all of it, man. They, like, have all of the. They. So there's different mating systems, if you want to think of it that way. Right.

So there's like, you know, social monogamy, genetic monogamy, males that mate with multiple females, females that mate with multiple males, et cetera, et cetera. And dunnocks seem to express all of these different behaviors.

So they will show all these different mating system types, like, depending on resource availability. So, like, how much food is available, how good the territory they were able to get that year is.

So they might, you know, you might see a male that has multiple females, you might see a female who has multiple males, or you might see multiple pairs actually mating with each other and like everything in between.

Dr. Scott Taylor
And that's called polygonandry, right?

Dr. Carrie Branch
When everything. Polygonandry, yes.

Dr. Scott Taylor
Everyone's doing everything all at once.

Dr. Carrie Branch
Kind of like, like polygynous, polyamorous, like, you know, polyandrous. So polygonandrous. I know that's always like a mouthful for my students too.

Dr. Scott Taylor
It's like, what I mean, it's amazing that within that one species you see all that variation, because you'd never see that kind of variation in chickadees.

You might see, like from season to season, different numbers of extra pair young in a nest, but certainly not an entirely different, like, from social monogamy to polyandry. Polygamy and polygonandry.

Dr. Carrie Branch
I think it's.

Dr. Scott Taylor
The history of the story of the Dunnock is funny too. Cause I think a lot of the initial insights into Dunnic breeding behavior came from, I don't know if he was a priest or he was a man of faith.

Anyways, he really thought the Dunnock was this very humble creature, and he thought that that was a great trait to encourage in his parishioners. But if he knowed a little bit. If he knowed a little bit more.

About the Dunnock's behavior, depending on resource availability, I don't think he would have been encouraging parishioners to. To engage in polykinandry or these other things.

Dr. Carrie Branch
Yeah, What a square. Yeah.

Dr. Scott Taylor
All right, so we're at that part in the podcast that we like to call that's BS or that's bird stuff. And this is an opportunity for our guests to tell us something about the topic that.

A myth they want to bust or something that ruffles their feathers in this case, about this idea of bird monogamy.

Dr. Carrie Branch
I think that our temperate bias shows a lot. And I speak, I'm speaking to myself included, because related to sort of mate choice, we think of males as being the ones that primarily sing. Right.

But we know that the evolutionary history of song, actually females also sang originally the original songbird that all the songbirds radiated from saying as well. And actually female song is actually quite common below the northern hemisphere and even in the northern hemisphere in a lot of instances.

So that's something I like to bring up to remind folks that our categorizations are almost always wrong. Every time we try to make a category, it's like, ugh, nope, here's 17 exceptions.

Dr. Scott Taylor
Exactly. Biology is the exception to the rule. I totally agree with you.

I think pointing out that, you know, who has studied bird biology really has dictated how we've thought about it. And this idea that female birds don't sing probably comes from this history of really mostly men studying birds.

But female bird song is common and probably very well. It is very important as people are finding. So I think that's an excellent myth to bust in the context of mate choice.

Females sing too, and they make choices. It's not all passive, as we have been led to think for many years.

Dr. Carrie Branch
Oh, yeah, absolutely.

Dr. Scott Taylor
Well, thank you so much for joining us, Carrie. It's been really a pleasure to chat about bird monogamy and the lack of monogamy and all the cool tools we're using to figure it out.

So thanks for joining.

Dr. Carrie Branch
Awesome. Thanks so much for having me.

Dr. Scott Taylor
Okay, so birds are dinosaurs, and around here we like our snacks. So we end every episode of the podcast with what we call the Dinosaur nugget.

Today's dinosaur nugget is that while 90% of birds are socially monogamous, which means that a pair together raises a nest, very few of those socially monogamous birds are genetically monogamous. So in every one of those nests, you're likely to find individual chicks with a different father. So that's a wrap on.

Okay, but is bird monogamy just pr? Thanks for listening or watching. Please, like, subscribe, leave a comment, and be sure to tune in next week for another episode.

Okay, But… Birds is hosted by Scott Taylor, with production and creative by Zach Karl.

Transcript Disclosure
This transcript was generated using AI and may contain errors or omissions. It is provided for accessibility and reference purposes only and may not perfectly reflect the original audio.