Carin Bondar: Eggs and the genius of bird moms Laying eggs may seem like a simple way to reproduce compared to human birth, but biologist Carin Bondar says bird moms are the micromanagers of the animal kingdom.

Eggs and the genius of bird moms

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It's the TED Radio Hour from NPR. I'm Manoush Zomorodi. And on the show today, Birds And Bees - the buzzy, busy, amazing cycle of reproduction that keeps all life clucking along.


CARIN BONDAR: So eggs are amazing.

ZOMORODI: This is Carin Bondar.

BONDAR: We just think about scrambling them for our breakfast - and fair enough, 'cause there is a great source of protein. But the thing about eggs is they enable a couple of really spectacular things.

ZOMORODI: Carin is a biologist, science writer and very eggcited (ph) about eggs and the birds who lay them.

BONDAR: Moms make these little, beautiful packages of - you know, there's - certainly, the DNA is in there, OK. We've got the embryo. But what else we got in there? We've got food. We've got nutrition. We've got hormones. We've got immune factors. And each one of those factors can be individualized for each egg.

ZOMORODI: Tweaked, refined and personalized. Carin says that bird moms are the micromanagers of the animal kingdom, and they can tailor their eggs in ways you would never imagine.


BONDAR: For example, if a mother is experiencing a resource shortage, if times are really tough, if food is not plentiful, this will make its way into the eggs that she lays. In a lot of cases, mummy (ph) birds will lay just a few extra eggs. And those will be almost insurance eggs that will almost certainly - you know, if they develop at all, they will develop into runts of the litter that won't make it.

ZOMORODI: Oh, OK. So these mommy birds are hedging their bets. Like, she has to weigh all the factors around her as she determines where or which egg she should invest her time and energy in.

BONDAR: Yeah, that's right. It's like - and those decisions have to be made quite early. But, you know, there's other things that are to be taken into consideration, too. And a really interesting example of this comes from the Galapagos Islands...


BONDAR: ...From the blue-footed booby. Now, these ladies, or mothers, also base their egg contents on the identity of dad. In the blue-footed booby, you have these males...


BONDAR: ...Who have blue, gorgeous feet, and this is an indicator of health. And this is something that a female would use as a proxy by which to say, OK, yeah, this is a good male. I like this male. I want to reproduce with this male. Therefore, I'm going to invest a lot into these eggs. So researchers manipulated the system of the blue-footed booby. Females will lay two eggs. So they'll mate with a male, and then, about 36 hours later, they'll lay the first egg. Then, another three days later, they'll lay the second one. It almost always works out this way. And so researchers during that interval, they painted a male's feet...

ZOMORODI: (Laughter).

BONDAR: ...A dark grey - right? - as if to say, OK, hang on, we're taking his health away. We're taking away his beauty. Lo and behold, the second egg came along with fewer nutrients, fewer hormones. Mum provisioned the egg according to the fact that dad was no longer a high quality. And isn't that just an incredible demonstration of the ability that she has to allocate at such an early level of development? It blows my mind.


ZOMORODI: It is bananas. So another thing about bird eggs that you talk about in your book is size. Size matters, like, a lot.

BONDAR: The size of an egg is actually highly correlated with the success of that egg. So the ones that are somewhat larger, generally speaking, we can assume that that has, you know, more food in there to make for a successful hatching. The latter, or the later eggs, will often be smaller, which makes sense if since she's running out of energy. But sometimes what mums will do for these late-laid, smaller eggs is they'll actually load them up with some more androgens, or the hormones that help for the chicks to be a little more aggressive. In some cases, this will make the difference for them between life and death. Because once they hatch, if they are not aggressive enough - I mean, the saying is true; the loudest bird gets the worm. (Laughter) And so if the late-hatching, little guys don't fight for their share, they simply won't eat, and they will then die.

ZOMORODI: So in some ways, she's saying, OK, I get that you little guys are at a disadvantage to start with, but I'm going to give you a boost of androgen, or testosterone, so that you can fight to the death if needed.

BONDAR: If needed.



ZOMORODI: ...Fight your older sibling?

BONDAR: Yes, absolutely. I mean, siblicide is quite commonplace in a lot of these nesting birds. But from a biological point of view, that really is the most advantageous thing to have. The older chicks are going to be the ones that are most likely to survive anyway. And for her to receive or realize any biological fitness, she's got to let those smaller ones go.


ZOMORODI: So we talked about the size of the eggs, but I didn't realize that there was also a way for birds to modulate when they lay their eggs, that - like, tell us about the process of actually, how long does it take to lay an egg? How much time is there between eggs? Does it vary for each species?

BONDAR: Yeah, there - actually, there can be quite a lot of variation there. But generally speaking, for most species, it takes between 20 and 30-ish minutes. However, there's a whole other set of birds. These ones are called brood parasites. And these are mothers who swoop into someone else's nest entirely - a different species, in fact. And they lay their eggs in someone else's nest, and then, they swoop away again. So they essentially drop off their children in a permanent daycare situation...


BONDAR: ...And off they go.


BONDAR: We've got someone like the brown-headed cowbird. She can lay it in about 41 seconds...


BONDAR: ...Which is respectable. But the shiny cowbird can actually lay hers in seven seconds. And the bronzed cowbirds can lay them in five seconds.



BONDAR: Could you imagine being in labor for five seconds? (Laughter).

ZOMORODI: That, well...

BONDAR: I could handle that (laughter).

ZOMORODI: Well, yeah, I guess so. So - but she has to fly there first. She has to fly in...

BONDAR: Fly in, swoop in, lay the egg, and swoop out again. I'm - it is...

ZOMORODI: And then, get the hell out of there.

BONDAR: Right?

ZOMORODI: (Laughter).

BONDAR: It is just - it's a drive-by laying.


ZOMORODI: OK, so these brood parasites - I imagine that this whole scheme only works if they can fool the host parents. And you say in your book that cuckoos can mimic other eggs, like, lay them so that they match down to the shade of color.

BONDAR: Oh, yeah. I will often do public lectures where I throw a slide up that has eggs in a nest. And it's nearly impossible for the audience members to tell which ones are laid by the parasite and which ones are laid by the actual mum of that nest. They can copy the colors, the pigments. Sometimes, there's even little patterns on the eggs, some speckles.

ZOMORODI: But how do they know? Like, are they casing the joint...

BONDAR: (Laughter).

ZOMORODI: ...And then returning to their own, you know, nest and saying, like, right, OK, I'm going for a Tiffany blue with some brown speckles? Like...

BONDAR: Right? (Laughter).

ZOMORODI: ...How do they know?

BONDAR: That's a great question. And that is a question that we - you know, that - we don't necessarily have a full-on answer to that as far as the cognitive parts of it and the physiological parts of it. But what we do know is that, yes, they will make sort of pre-drop-off flights where they observe the positionality of the nests that they're going to parasitize. They look for nests that are in an appropriate location, that they'll be able to make a quick getaway. But then, you know, what these birds will do - the parasitized birds will then actually say, OK, well, we're going to put our nests right beside this predatory bird as a means by which to scare you guys off. So the predators hopefully won't eat us, but they will eat these cuckoos who are coming incessantly by.


ZOMORODI: It does beg the philosophical question of, how does a cuckoo bird know it's a cuckoo bird if it was raised by a magpie, right?

BONDAR: You have to think that the identity (laughter) - the personal identity of these birds has got to be - right? And they don't even get to meet their parents ever.

ZOMORODI: (Laughter).



ZOMORODI: Can we talk about dads? Is that OK?

BONDAR: (Laughter) Let's do it. Let's talk about dads...

ZOMORODI: Let's do it. Let's talk...

BONDAR: ...Because, yes, you know, the bird world is really where we see some of the best fathering, I would argue.

ZOMORODI: Really? The bird world?

BONDAR: Yeah. And that's just because of that truism that, in mammals - I, me and every other female mammal keeps that baby inside until that baby gets born, right? So as soon as that sperm has made its transfer, once that embryo has been fertilized, dad's job is done. There really isn't anything biologically relevant that dad can do. But in the bird world, that's not the case. In the bird world, once mum has laid that egg, dad can legitimately sit on that egg, and that is worth something. This is why we do see a lot of biparental care in the bird world.


MORGAN FREEMAN: As the fathers settled into their long wait at the breeding ground, the winter's second storm arrives.

ZOMORODI: I feel like the best example or the first time I kind of became aware of bird parenting was that penguin documentary...


FREEMAN: As they move about, the fathers will balance their eggs like tightrope walkers.

ZOMORODI: ...Where you saw these amazing emperor penguins passing an egg back and forth between themselves to keep it warm.

BONDAR: Yeah. So there's a need for adults and specifically the dads to sit on these eggs to keep them warm in these frigid environments. And rocks, for example, large rocks to create these nests are something that's quite a hot commodity in these environments. So a lot of sexual favors will take place by females towards males in order to grab onto some of these rocks to be able to build a nest. There's a good amount of homosexual sex that happens between males who are, you know, also dutiful dads but certainly doing their own activities on the side. So, I mean, there's a lot of interesting parental and, I guess, in a way, familial stuff that happens because of the nature of these really harsh environments where the penguins live.


ZOMORODI: My daughter's favorite bird is the peacock. And...

BONDAR: Oh, OK. Yes.

ZOMORODI: The male peacock, full feathers...

BONDAR: Oh, yes.

ZOMORODI: ...These showy birds who kind of put on a display for their - the female, does that indicate in any way whether or not they will also be caring for the eggs and the chicks? Is there...

BONDAR: It sure does.

ZOMORODI: Oh, really?

BONDAR: Yeah, that's a great question. And generally speaking, the more beautiful he is, the less he's going to help. And that's because...

ZOMORODI: Why does that not surprise me?

BONDAR: (Laughter). I know, right?


BONDAR: That's because, you know, there's only so much biological energy to go around. So we'll often see in species that are lek breeders, or species where there is a male and his job is just to give out sperm - they're going to be very pretty because they are trying to give out as much of that sperm as possible. That's their only job.

We also would see that in something like a bowerbird. So these are the birds that create these beautiful homes, almost like bachelor pads, with really beautiful feathers. Sometimes, they find pretty things that are plastic from the human world, and they decorate their nests with these things. They're doing that to entice her into coming to his, you know, bachelor pad and only to receive some of his sperm. And once she has received that sperm, that's it for him. He's looking for the next female.


ZOMORODI: Just to wrap up, you seem to - and I don't know how you'll take this, but you seem to relish the darker side of our conversation...

BONDAR: (Laughter).

ZOMORODI: ...Just as much as you relish the beautiful - the gorgeous feathers and songs and well-built nests. There's a...


ZOMORODI: There's a little bit of a psychotic side to birds, it feels like. Or am I attributing psychological things to an entire world that...

BONDAR: No (laughter).

ZOMORODI: ...Doesn't deserve it?

BONDAR: I - you know what? I love it. I delight in the macabre and the sort of darker sides because I love that nature is messy and that it isn't what we've been told is a nuclear family or a monogamous relationship or whatever kinds of things that humans have decided should be happening in whatever species or for our own species. But I really do delight in the fact that there's so much about nature that we maybe glaze over.


BONDAR: And it's not necessarily even dark. It's just all a matter of individual survival. And when you - you know, when you consider how that sort of plays out across the human world as well, for me, it just makes us all that much more a part of the animal kingdom. We are animals just like anyone else is, or any other living thing is. And I guess maybe that's where some of my delight in these ridiculous and crazy things comes from. It's like, of course they experience these things. Animals are not these perfect, little, happy family units and stuff. I mean, they just - they go through it just like we do.


ZOMORODI: That's Carin Bondar. She's a biologist and author of several books including "Wild Moms" and "The Nature Of Sex: The Ins And Outs Of Mating In The Animal Kingdom." You can see both of her talks at


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