The Science of Birds - Mate Choice and Sexual Selection in Birds

Episode Date: April 27, 2021

This episode is all about how birds choose their mates and the evolutionary outcomes of those choices.A lot of what we’re talking about today has to do with sexual selection, which is a special form of natural selection. In sexual selection, individuals compete for mates. This is when females and males choose their partners based on specific traits. Traits like plumage color or song, behavior, etc.This choosiness has resulted in some spectacular features in birds. Many of the things we love most about birds are consequences of sexual selection.~~ Leave me a review using Podchaser ~~Links of InterestVideo of Long-tailed Manakin DisplayLink to this episode on the Science of Birds websiteSupport the show

It's early morning in a Peruvian cloud forest. Here in a damp ravine, a special event is about to occur. Hopping around in the deep green foliage of the trees, there's a gathering of about 12 bright orange birds. They are males of the species Rupa Cola Peruvianus, the Andean cock of the rock. In the right lighting, these birds seem to glow from within, with a red-orange radiance.

Each male sports a weird, disc-shaped crest of feathers on his head, extending all the way to the tip of the bill. It looks to me kind of like an orange slice glued to the bird's head. In any case, this species is unmistakable and amazing. Male Andean cocks of the rock gather in the early mornings and late afternoons in special places like this forested ravine for one purpose only, to convince females to mate with them. This gathering site is what we call a leck, L-E-K.

Males in the leck perch on branches five to 15 meters off the ground. They pair up, one male with another, and then act out ritualized displays for each other. They jump, bow, flap their wings, snap their bills, and make a general racket. All of this activity ramps up and reaches a fever pitch when a female enters the leck area. The female has a shape similar to the male, and she even has her own disc-shaped crest. But her plumage is much less showy. She's more of a brownish or deep red color.

Females visit the leck to select their mates. Each female assesses the appearance. occurrences and performances of the males, and eventually chooses one. After mating, she will head off to build a nest out of mud at the entrance to a cave. She raises her chicks all by herself. The male just gets back to preening his orange-tastic feathers and gets ready to put on another frenzied performance. Bring on the next female!

These birds have a polygynous breeding system, where at least some males mate with more than one female, but each female mates with only one male. The male offers no help in raising his offspring. In the human world, we generally frown upon this sort of behavior, to put it mildly. What does a female Andean cock of the rock look for in a mate? And how does her choice influence the evolution of her species? Why does she put up with a mate who won't help out around the house or even help build the house in the first place? I mean, this deadbeat doesn't even lift a finger, or lift a feather to raise his own kids. How did this seemingly unfair, unbalanced arrangement come to be?

There are so many questions here. Such is the wonderful mystery and complexity of mate choice and sexual selection in birds. Female and male Andean cocks of the rock have their own agendas in the game of reproduction, and we can say the same for every other bird species on the planet. Hello and welcome. This is the Science of Birds. I am your host, Ivan Philipson. The Science of Birds podcast is a lighthearted, guided exploration of bird biology for lifelong learners.

This episode is all about how birds choose their mates and the evolutionary outcomes of those choices, generation after generation. Most of what we're talking about today has to do with sexual selection. You probably have a good understanding of how natural selection works, survival of the fittest and all that jazz. Well, sexual selection is just a special case or a special form of natural selection. In sexual selection, individuals compete for mates. This is when females and males choose their partners, not at random, all willy-nil. but based on specific traits, traits like plumage color or song or behavior or something like that. This choosiness, this non-random selection of mates, has resulted in some pretty

spectacular features in birds. Many of the things we love most about birds are consequences of sexual selection. So, my dear student of ornithology, if we really want to understand birds, it behooves us to learn something about sexual selection and mate choice. Let's see if we can make some sense of it all. Birds, like all animals, have only so much time and energy to spend. Maybe they can afford to goof around now and then, to have some fun, but mostly birds are consumed by their need to survive and their drive to make babies. The problem is, it's rarely easy for a bird. to strike the perfect balance when allocating time and energy to survival, growth, and reproduction.

If you spend most of your energy trying to reproduce, for example, you're going to lower your chances of survival and growth. Alternatively, if a bird spends all day, every day, stuffing its face with food and hiding from predators, well, sure, it might survive to grow old and fat, but it's also going to die childless and alone. Who's going to cry at its funeral? No one. That's who. So there's a trade-off between producing offspring on one hand and surviving on the other. Reproduction is an energetically expensive affair. A bird has to go through the trouble of finding and choosing a mate, building a nest, feeding and protecting chicks, buying them boxes of Legos for Christmas, paying for piano lessons, and eventually bailing them out of jail after they start hanging.

hanging out with the wrong crowd. The point is that babies are an enormous investment for a bird. But the level of investment is often very different for females compared to males. Most fundamentally, females have to invest a lot more energy because they produce eggs. There's no way around that. And the female is nearly always involved in building the nest, incubating the eggs, and caring for her little ones until they're independent. For the males part, he might, at best, share in all the duties from nest-building onward. At best. Even in that case, his energy investment may fall short of what the female puts in because he doesn't make eggs. And then there are the males of many species that spend all their energy on the front end.

They do everything they can to score a mating, and then leave their mates to do all the rest of the work. So these guys make basically no investment in any particular brood, other than to, bestow their genes upon them. The Andean cock of the rock is one example. Male hummingbirds, too, mate with multiple females, if they can, and are conspicuously absent when it comes to nesting and raising chicks. Female hummingbirds must do everything on their own. Take the female Anna's hummingbird, a familiar species in western North America. She builds a nest of soft plant material, spider webs, and lichen. Each of her eggs weighs 12.5% of what her tiny body weighs. If a 160-pound human produced an egg of this relative size, it would weigh

20 pounds, and our Anna's hummingbird lays not one but two eggs within about 48 hours. She incubates her eggs for 16 days or so. She cares for her two chicks for almost a month before they leave the nest, and maybe another two weeks before they're independent. So, yeah, female birds in situations like this make huge investments of time and energy to reproduction. Across the avian tree of life, that is, among bird species, males are more variable than females in their level of parental investment. For example, at the other end of the spectrum from polygynous males are those males who incubate the eggs on their own and raised the chicks as single dads. But such male-only parental care is found in only

about 1% of bird species. This degree of male investment is the exception rather than the rule. The rule, for about 80% of all bird species, is bi-parental care. Most birds pair up to share the duties of child care, often for only one season, but sometimes for life. Looking at these monogamous pairs, we might think they've worked out a lovely, efficient little system of cooperation. From our biased human perspective, monogamy and birds can seem heartwarming. I have to admit that it makes me feel all fuzzy inside. I'm like, aw, they love each other. If this arrangement of monogamy with biparental care is so widespread among birds, it must work pretty well, right? Apparently, yes, in the evolutionary sense.

But behind the facade of domestic bliss, there might be more going on between a mated pair of birds than you think, because, as usual, nature is complex. You see, there's often a conflict between the interests of a female and her male partner. Each individual bird does what's in its own best interest, from a genetic fitness perspective. Each bird selfishly wants to pass on as many of its own genes as possible to the next generation. The other parent, be damned. Biologists call this sexual conflict. This occurs when the two sexes have different optimal reproductive strategies,

and these strategies aren't entirely compatible. The male would be better off if the female did all the work, and vice versa. By hanging around to care for his babies from one female, a male is missing out on opportunities to mate with other females. So maybe he's feeling a little. little phomo, a little fear of missing out. The female faces a similar trade-off. But because she has to lay eggs, she's more limited in how many offspring she can realistically crank out in a given

season. So what's good for the goose is definitely not good for the gander. I mean, who comes up with these sayings anyway? Whoever it was, they obviously weren't thinking about sexual conflict and optimal fitness strategies in an evolutionary context. Geez. So, to review. 1. Reproduction in birds is a major investment. 2. The level of parental investment often differs a lot between males and females. 3. Monogamy with biparental care is by far the most common breeding system in birds.

And 4. Sexual conflict is a thing. Even in seemingly harmonious monogamous pairs. Since reproduction is such a big deal, and because it's so costly, it's no surprise that birds can be picky about who they shack up with. Female birds especially are a choosy lot when it comes to mating. Because of the whole egg-laying thing, females have more to lose if they make a bad choice. But biologists have long puzzled over why females choose certain mates. This is a complex issue. And I should point out that choosiness itself can be costly. In her quest to find the ideal mate, a female might expose herself to predators or to the elements.

And if she's too picky, she might not find a mate at all. No bueno. The benefits of being choosy, however, must generally outweigh any such costs. Otherwise, natural selection would weed out choosy females left and right until that behavior would be eliminated from the gene pool. What benefit is a female hoping to gain by choosing the quote-unquote best male? Is it a short-term benefit, long-term? Let's say you're a female long-tailed duck, clangula hyamalus. You're paddling around in a coastal lagoon in far eastern Russia. It's winter. Before you is an assortment of male. You're long-tailed ducks to choose from. They're mostly white with black breasts and wings. They have big black neck patches and gray cheeks. And sure enough, they have long, elegant tail feathers

that curve gently upward. Handsome fellows, for sure. Here's one now, singing a song at you. You decide that that guy, the one who's saying so beautifully, is the lucky winner. Maybe that long-tailed duck has some long-term genetic benefits to offer you. Biologists have three major hypotheses to explain the instinctual motivation behind female choice. The first is the good genes hypothesis. The idea here is that a female bird chooses a male based on traits that. that accurately signal his genetic fitness. Traits like songs, plumage, fancy dance moves,

these can be indicators of health and survival skill. We'll talk more about those traits in a few moments. Males with good genes are those that produce healthy babies with high rates of survival and reproductive success. So by choosing the most attractive male, a female gets the indirect benefit of getting more of her own genes in circulation in the next generation and beyond. The second hypothesis has to do with direct benefits.

This one is a little more straightforward. The direct benefits hypothesis suggests that a male uses his flashy traits like badges to advertise his ability to provide a female with child care, protection, food, or other material resources. Genes aside, the female in this scenario chooses a male who will improve her own chances of survival and or increase the number of babies she can raise successfully.

This is sort of like the bride price that men pay to women in some human cultures, both historical and contemporary. A bride price is a signal of material wealth and commitment. And I suppose a fancy diamond engagement ring is basically a bride price, isn't it? It's an indicator that the fiancé can provide resources, for his mate. Direct benefits. The third hypothesis is the arbitrary choice hypothesis. This one is

pretty wild. It works like this. For no meaningful reason, females begin to find a particular male trait especially attractive. Might be a long tail, a shiny patch of feathers, an oversized belt buckle, a gold tooth, something like that. But this trait doesn't actually signal anything beneficial to the female. It has nothing to do with good genes or direct material benefits. Maybe these females are hardwired with a pre-existing sensory bias, like an attraction to the color red because they like to eat red berries. A male with a patch of red feathers or red legs might look sexy for this very arbitrary, irrational reason. So the females start choosing males based on an arbitrary trait.

Their babies inherit both the trait and the preference for the trait. Generation after generation, the male trait gets amplified and more extreme. And the female aesthetic preference for the trait gets more and more ingrained in the population. Every generation, the most fashionable superstar males get to mate more often and pass on more of their genes.

This hypothesis is invoked to explain explain how some male traits become exaggerated over many generations, because the process I just described is actually a positive feedback loop. Arbitrary mate choice can lead to what biologists call runaway selection. This is where male ornaments, like long tail feathers, can evolve to become comically, ridiculously out of proportion. It's basically a fashion craze, like bellbottom jeans or shoulder pads. Except this trend plays out over millions of years. In some cases, the male trait may have actually started out as a genuine signal of good genes or a direct benefit. But once the feedback loop gets spinning, look out,

everybody, selection is off and running away, leading to some bizarre male features that may no longer provide the female with any benefit. Except that they do, right, in a way? Unless I'm missing something, these traits do provide a long-term genetic benefit, because in this runaway selection scenario, if a female chooses the most fashionable, desirable male, her male sons will also tend to be sexy dudes that the ladies swoon over. So the female's genes get spread around in the next generation, because she chose the most aesthetically appealing mate she could find. The phenomenon I just described has a name. It's the sexy son hypothesis. A sexy son may or may not have genes that directly reflect his quality or health. The sexy son hypothesis includes some elements of both the

good genes hypothesis and the arbitrary choice hypothesis. There's still a lot of debate among ornithologists and other biologists about these hypotheses and about sexual selection in general. None of the hypotheses we just discussed applies to every bird in every situation. Again, nature is complex and there are many unsolved mysteries here. In any case, female birds are generally choosy and they base their choices on something. Once a female decides who she wants to mate with, she will often make a copulation solicitation display. This is a sort of come-hither posture where the female raises her tail and exposes her cloaca. This signals her readiness to mate. She has chosen.

And the male is like... You choo-choo-chose me? Mate choice isn't always about females choosing males. There are cases where males are just as discriminating. Consider the Rock Sparrow, Petronia Petronia. This streaky brown bird is in the family Paseridi. You'll find it in open habitats from Spain to Turkey to Mongolia. Researchers in Italy studied a population of rock sparrows to see if males choose more attractive females. Attractiveness was measured by the size of a female's yellow breast patch. The highest quality females are known to have the largest breast patch ornaments. The researchers tested the nest defense behavior of males in 28 pairs.

What they found was that, quote, male rock sparrows defend their brood from predators with an intensity directly proportional to the size of the ornament of their mate, end quote. The implication here is that if a male chooses a high-quality female, and he knows she's high quality, he is going to fiercely defend their mutual offspring, because those youngans got some good genes in them. But how do the researchers actually test the male's nest defense against predators in the field? They used a dead stuffed weasel and placed it right next to the nest. I can just imagine a poorly taxidermied weasel with a toothy grin, ratty fur, and

bulging plastic eyes looking in different directions. It was probably scarier looking than a real live weasel. So the male rock sparrow is coming home from a long day of hunting caterpillars for his chicks and, hold up, what's that sitting next to his nest? Look out, it's a zombie weasel. Save the children. Ornithologists overlooked male choice and competition among female birds for a long time. But more and more, they're discovering cases where females compete for males, or more often for resources such as nest sites. And now think about this. If mate choice and sexual selection

cause birds to have colorful outlandish plumage and all that, what's the deal with species where both sexes have dramatic ornaments or complex songs or elaborate displays? Well, these things can help in species recognition. You want to be able to quickly identify

members of your own species for breeding, if nothing else. But sexual selection is another possibility, another explanation. It can be mutual between sexes. It doesn't have to be so one-sided all the time. Take, for example, the crested ocklet, Ethia Christatella. This chunky little seabird is in the same family as puffins. Crested ocklets breed on remote islands in the frigid North Pacific and Bering Sea. In their breeding plumage, both male and female ocklets have a tough of black feathers that forms a crest at the base of the upper bill.

Males and females in this species both compete for mates and for nest sites. They form monogamous pairs and perform elaborate mating displays for each other. Research on these ocklets in the wild found that both sexes prefer a mate with a large crest. So this is a good example of mutual sexual selection. As I said earlier, many of the features we love most about birds are the ones that have to do with sexual selection. If sexual selection didn't exist, maybe birds would be just a sad bunch of no-frills beasts with dull brown feathers. Maybe they wouldn't be colorful or have any ornamental plumes. They wouldn't sing and they wouldn't perform any dances that amuse and delight us.

Boring! There's another Simpsons reference for you. So, hooray for sexual selection, right? Female choice isn't the only game in town, but it's the most common mechanism of sexual selection. Just remember that males too can be choosy and that mutual sexual selection is a thing. So up to this point,

I haven't really elaborated on the myriad ways that males each try to be the chosen one, how they try to win, with females. This is the fun stuff, in my opinion, and now I want to reward you for paying attention this far. You were paying attention, right? Good. It's fun to look at the weird and wonderful traits males have for impressing females. There's a bewildering assortment of these traits found among the world's approximately 11,000 bird species. We can group most of these traits into several categories.

Acoustic signals, visual signals, and olfactory signals. Males of a given species may use one or more of these signal types. Courtship displays, for example, typically involve sounds in addition to visual elements. And look out, ladies, a triple threat male can do it all. He sings, he dances, he smells like a fresh-baked apple pie right out of the oven. Or, since we're talking about birds, he probably smells like a greasy sardine or roadkill, or really whatever floats the female's boat. Now, I'm not saying there are any birds that actually smell like that.

I'm just kidding around, but, well, we'll get to the olfactory stuff in just a moment. Let's actually start with acoustic signals. The best-known acoustic signals used by males are obviously their songs. Males sing songs to impress females, and they also sing to mark the boundaries of their territories. Females assess male songs in different ways, depending on the species. The most important criterion might be the complexity of a male's song, or its volume, its frequency, its duration, or any combination of these. For example, in the cute little zebra finch of Australia, females prefer males that sing faster, and more consistently.

In general, female birds prefer males who can nail the most difficult vocal performances. These are the males that sing harder, better, faster, stronger. That's right, just like daft punk, the French electronic music duo. Besides vocal sound, better, faster, stronger. Besides vocal sound,

like songs, males of some species use non-vocal acoustic signals to attract females. Ruffed grouse, for example, make deep low-frequency drumming sounds by flapping their wings, essentially creating little sonic booms. And some hummingbirds make loud sounds with their tail feathers during high-speed dive displays in front of female spectators. Here's a male purple-collared wood star in a dive display. This is a type of hummingbird. Listen for the little toy trumpet sound he makes. Those are some examples of acoustic signals. Now let's have a look at the visual signals that males use.

These signals commonly involve showy plumage and or ornaments like crests, inflatable pouches, fleshy waddles, patches of colorful skin, and so on. The way a male moves his body also qualifies as a visual signal. A female might choose a male whose display is the fastest, most acrobatic, or most vigorous. Now, I'm pretty sure it's a federal crime to bring up the topic of sexual selection in birds without mentioning the peacock. I can't go to prison, man. I'm too delicate for prison. So here we go.

The peacock. We all say peacock. right? But hey, fun fact, the actual name of the species is Indian pea fowl. Its scientific name is Pavo Christatus. I'm pretty sure it's pronounced Pavo, pavo, tomato, tomato. In any case, the male is the peacock, and the female is the pea hen. This bird is such a familiar denizen of parks and gardens around the world that it's easy to take it for granted, to overlook how insanely, impossibly gorgeous it is. The display of the peacock, when he fans and waves his enormous bronze green train with all those eye spots, that is a true marvel of nature. It's amazing. Here's another fun fact. That big

fan of feathers on the peacock, the ones that form the NBC logo, those are not tail feathers. They're actually modified upper tail coverts. On birds, the upper tail coverts are between the tail feathers and the rump. A peacock has decent-sized tail feathers, but they're literally overshadowed by that long train of upper tail coverts. We often attribute the evolution of these spectacular feathers on peacocks to the effects of runaway selection. The Indian pea fowl is a leck breeding species. the peahens visiting a lech prefer males with the most elaborate trains. Elaborate ornaments like this aren't just for polygynous leck-breeding species. For instance, consider the resplendent Ketzal of Central America.

The male's appearance is stunning. He's mostly a luminous green color with a bright red belly. Trailing out behind him is a pair of super long green feathers. These two are upper tail cover. Like all other members of the Trogon family, the resplendent Ketsal is a monogamous breeder. The male and female both share in building the nest, incubating the eggs, and bringing food to the chicks. During courtship, the male makes a dramatic flight display. He flaps up above the forest canopy, circles around while calling,

then floats back into the trees. This display shows off his long feathers and bright colors. So even in a monogamous species like the resplendent Ketzal, female choice seems to be driving the evolution of male ornaments. How about a couple more examples of visual signals? In forests of the eastern Himalayas, there's a species of pheasant called Teminks-Tragapan, which has the scientific name Tragapan Taminkia.

These birds are generally monogamous, but dad doesn't stick around to raise the chicks. Females of this species are cryptically colored. They're well camouflaged to blend in with their surroundings. They're brown with little spots and streaks. The male tragopans plumage, however, is a rich reddish orange with large white spots. He has a mostly black head with bare electric blue facial skin. That's pretty flashy already. But wait, there's more!

The male Temmink's Tragopan does something extraordinary during his courtship display. He inflates a flap of that blue facial skin so that it unfurls into a huge appendage that looks like a beard or a bib. It's bright blue with symmetrical scarlet patches on the sides. This structure is technically called a Lapit, L-A-P-E-T. This word Lappit comes from the word for a decorative flap on a headdress or other item of clothing. But wait, it doesn't stop there. The male also inflates a pair of fleshy horns that stick up over his eyes.

They look like antennas or like the eye stalks of a snail or something. The overall effect, to me, looks like an alien creature you'd see sitting around a canteena in Star Wars. It's really the other way around, isn't it? Star Wars creatures take their inspiration from real-world animals. I think the animals deserve more credit. It's like when people say I look like a more handsome version of Brad Pitt. What they should really say is that Brad Pitt looks like a less attractive version of me. And for another species in which the male wears an ornament that serves as a visual signal,

I give you the standard-winged night jar, capromulgus longipenus. This is a medium-sized nocturnal bird that lives in the savannas of the African tropics. Males and females are both speckled brown and gray, but in his breeding plumage, the male has something special. One of the primary feathers on each wing is wildly elongated. The bird is about nine inches or 22 centimeters long, but each of these ornamental feathers is about 20 inches or 50 centimeters long. The feathers have a naked shaft except for the tip, which expands into a sort of paddle shape, so it looks like each wing has a little flag trailing out behind it. The common name of standard winged nightjar doesn't imply that the

wings of this bird are just, you know, pretty standard, nothing special. Standard in this case refers to the word that we use for a ceremonial or military flag or banner. This species is polygynous and breeds in lecks. Males fly around in circles at the leck, showing off their impressive wing feathers to each other and to the watching females. Those elaborate feathers aren't all that colorful, which, if you think about it, makes sense. These are nocturnal birds, and they display in the low light around dusk. colors wouldn't really show up in these conditions.

So some male birds, like these night jars, have ornamental feathers on their wings, rather than on the tail or on the head. The last thing I want to mention, specifically about visual signals, is that many females also appear to prefer males that show strong symmetry. By that I mean that the two halves of the male's body or of a specific ornament are very similar in shape and size, symmetrical. Think again about our standard-winged night jars. Females at the Leck are sizing up the displaying, swooping males.

If the elongated wing feathers of a particular male are lopsided, asymmetrical between the left wing and right wing, maybe he's likely to get passed over. He's going to be stuck in the friend zone. biologists think that symmetry is attractive for many animals because it's a visual signal of good health and perhaps good genes. That goes for us too. Attractiveness among humans is strongly influenced by facial symmetry. At long last, we come to the third category, olfactory signals. The sense of smell in birds has often been underrated or overlooked until fairly recently. We now know that at least

some species have a superb sense of smell. But regarding mate choice and breeding, one of the coolest examples is a bird we've already talked about today, the crested ocklet. These little buggers produce a special odor in the breeding season. Does it smell like fish or roadkill? Nope. It smells like tangerines. How delightful. During their courtship displays, crested ocklets of both sexes sniff the feathers around their mate's neck. The citrusy smell comes from a pheromone that the birds secrete. Ocklets are highly attracted to this smell. It's like a natural perfume. Ornithologists aren't sure how many other species use olfactory signals like this, but studies have shown that some species like the spotless starling and the dark-eyed junco

can distinguish the sex of other members of their own species using odors emitted by the euripigial gland. That's the prine gland. Have a listen to this. Those are the vocal sounds of two male long-tailed mannequins in Costa Rica. They're performing an elaborate, synchronized display for a female spectator. Each male mannequin is about 4.5 inches or 11.5 centimeters long. He has two tail feathers that are almost as long as his entire body.

His plumage is mostly black, but he has a bright blue mantle on his back and a red crown. The long-tailed mannequin, cheroxifia linearis, is a species that demonstrates some of the extremes of evolution by sexual selection, especially when it comes to display behavior. We could just as easily have picked birds of paradise, widow birds, busters, lyre birds, bower birds, wild turkeys, or any number of other birds. But I'm going with this little mannequin. This species has a lecking mating system. Pairs of males perform in a location called a court. One male is the alpha, the other is the beta.

There are also some subordinate birds lurking around on the edges of the court. There are between 3 and 15 of these gamma males at any given court. The alpha and beta males share a strong social bond. Not only do they perform their mating rituals together, they spend most of the year together, and they stay bonded for multiple years. They need to work together for a long time like this so they can perfect their dance moves.

When breeding season rolls around, the alpha and beta males head to their court. Gamma males usually float between several courts. The alpha-beta pair at a court call out to attract a female. When she arrives, the alpha and beta take their positions on a horizontal branch or Leanna, one to five meters off the ground.

Then the show begins. The dynamic duo has two main dances. The first is the popcorn dance, where the alpha and beta alternately jump up and down. They flutter and hover for a moment at the top. When he jumps, each male makes a buzzy little call. The second dance move is called the cartwheel variant. One male jumps backward on the branch,

and the other male leaps forward to take his place. Jazz hands and repeat. Here again is how this performance sounds. Eventually, the beta male exits the stage. If the alpha male is lucky, he then gets to mate with the female. Beta males almost never get chosen by a female. They don't get to mate. And the gamma males? Forget about it, no chance. Females prefer pairs of males whose display is tightly synchronized. A female only visits a few courts over the season, and she's often loyal to one particular court from year to year. Very few

males have the opportunity to mate in a given year. For every 50 or so male mannequins, in the population, only one is the alpha. So you have to wonder, why the heck the beta and gamma males participate in this whole thing if they never get to mate? What's in it for them? Well, never say never, as they say, the alpha male has to die at some point, right? If he dies or leaves for any reason, the beta is in a position to take the alpha's place. So the beta male helps his alpha a buddy, and they perfect their routine over several years, sometimes for over 10 years. But the beta is just biting his time. He's hoping his day will come. The same is true for those gamma males hanging out on the periphery. Each is hoping to eventually become a beta, and then,

with any luck, an alpha. This is an example of what biologists call reciprocity. By cooperating with an alpha male today, the beta male may eventually be rewarded when he inherits the alpha's court and social status. So the beta isn't being altruistic or anything. He's still got his own best interest in mind from a genetic perspective. I'll put a link to a video of the long-tailed mannequin's incredible display in the show notes for this episode. I want to take just a moment to thank the very first patrons of the Science of Birds podcast. I recently started a Patreon page and this gives my listeners a way to support the show by giving a small donation every month.

This is incredibly helpful and with enough support from patrons, I'll be able to have the independence to keep creating this content for you in a sustainable way. Patrick, Lena, Katie, Teresa, Stephanie, Shinrui, you guys were the first of my listeners to become patrons. Thank you so much. It's so cool that you jumped right in to support the show. You guys rock. If you are interested in helping out,

you'll find a link to my Patreon page in the show notes, or you can also just go to patreon.com forward slash science of birds. As I've been saying, female birds are a bunch of choosy-susies, and we talked about the possible reasons for that. In making their choices, females evaluate males based on one trait or another or on a suite of traits. Ideally, these traits are signals of a male's ability to provide a female with benefits, such as food, nesting territory, protection, or genetically superior babies.

But how can a female be sure of this? Does that flapping blue lap it on a male temnix tragopan really say something about the question? quality of his genes? How about the speed of a male Zebra Finch's song, or the upper-tail coverts of the resplendent Ketzal? It's not like a female can just click over to Amazon.com to peruse the reviews for the males she's considering. Huh, this guy only has 3.8 stars. One reviewer wrote, This is the worst male I ever made it with. Very disappointed. Genetically inferior product and the customer service was terrible. One star. This brings us to the concept of honest traits or honest signals of fitness.

The idea is that all these flashy ornaments and exaggerated courtship displays actually have costs to the males that possess them. They're liabilities that make it hard for males to survive day to day. Bright colors and oversized feathers not only take a lot of energy to produce, they also make males more conspicuous to predators. and they can make it harder to escape predators. Imagine how the massive train of a peacock must hinder his speed and maneuverability, both on the ground and in flight.

Logically, then, natural selection should remove males who have sexual ornaments but who don't have the strength, stamina, speed, or intelligence to survive long enough to mate. On the other hand, if a male can survive with the handicap of having all that blame, and drapery hanging off him, well, that dude must have good genes. Countless generations of evolutionary tug-of-war between good-old survival of the fittest and sexual selection

should ensure that whatever sexy traits we see in a male today are honest signals of his genetic fitness. In Peacocks, for example, one study found that males with higher numbers of those eye spots on their train feathers have stronger immune systems. And it turns out that females prefer males with the most eye spots.

Bam! Honest signal! Female house finches prefer males with the most red color in their plumage. That color comes from carotenoid pigments in the bird's diet, since birds can't make their own carotenoids. So better diet, more red color. And research has found that the redder you are, the healthier you are. By choosing the reddest males, female house finches are selecting the males who are healthy and well-fed. Bam!

Honest signal! There are many other examples, but I think you get the idea. But perhaps you're wondering, are all songs, ornaments, displays, and other signals in the honest category? Are there dishonest signals? I couldn't find much in the way of examples, but there's a sense. a study that was just published that points to a possible example of dishonest signaling. The study was conducted by a team of researchers mostly based at Harvard University. It was published in the journal Scientific Reports.

The researchers studied the coloration of eight tanager species and one subspecies in the genus Ramphocelis of the family Thraupidae. These guys are found in Central and South America. Males of these species are some combination of red and black. Females are much less colorful. In this study, the carotenoid pigments in both males and females were measured in the lab. Interestingly, within a species, there isn't much difference in the amount of carotenoid pigment between the sexes. The cool and unexpected thing that the researchers discovered is that,

at a microscopic level, the feathers of males have very different structure. compared to those of females. The females' feathers have a typical microstructure, but male tanager feathers have unique microstructures that interact with light to increase their color saturation. The resulting colors are called super black and velvet red. Now we're getting to the lying, cheating, dishonest signaling part. Because normally, in other birds like those house finches,

we associate brighter reds with more carotenoid pigments and therefore higher fitness or quality in the bird. But with these ramphocelus tanagers, the brighter color doesn't come from having more pigment. It's just a trick of the light reflecting off those special feathers. As far as we know, this modified feather microstructure has nothing to do with the male's fitness or his quality. Seems like a dishonest signal to me. Sexual selection in birds has been going on for millions and millions of years. The diversity we see among birds today is just a snapshot in a long, dynamic, complex interplay between natural selection, mate choice, and sexual conflict.

The most common compromise birds make. to balance all these factors seems to be social monogamy. This is the breeding system of the vast majority of birds, about 81% of all species. I've heard social monogamy described as a stalemate in the sexual conflict between male and female birds. The qualification of social is important here. Socially monogamous birds form pairs that stick together for the most part, even if just for a season. They often share parental duties and so on. But most socially monogamous species are not actually sexually monogamous. With songbirds especially, we now know that they're frequently out there mating with other birds on the sly. This is called extra pair copulation,

or if babies result, extra pair fertilization. All's fair in love and sexual conflict, I guess. Ornithologists didn't appreciate just how rampant this extra pair copulation behavior was until modern genetic techniques became available. Genetic data and analyses can tell us exactly who's the daddy and who's the baby mama. There are so many other subtopics I could drill down into here. Some of them deserve their own podcast episodes. But this episode is already turning out to be a long one. There's nothing wrong with a nice long episode, but man, I got to get outside for at least a little while. I don't want to just write about birds and talk about birds. I want to see and hear those beautiful little beasts. So how about we go out into nature soon with all of this

knowledge about sexual selection percolating in our brains? Hopefully, you'll go out there with a greater appreciation for what mate choice and sexual selection have done to increase the diversity of birds. Their shapes, songs, colors, and dance routines. Thank you so much for listening and for learning about birds with me today. If you haven't subscribed to the Science of Birds podcast, you might want to find that subscribe or follow button and give it a nice little tap. That'll help you keep up with the episodes as they come out. If you have something you'd like to share with me, perhaps your thoughts about the podcast, or an amusing anecdote about a zombie weasel, just shoot me an email.

The address is Ivan at Scienceofbirds.com. As always, you can check out the show notes for this episode, which is number 27, on the Science of Birds website, Scienceofbirds.com. This is Ivan Philipson. Thanks again for listening, and I'll catch you next time. Peace. Thank you.