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The reason why male birds display to court females — even if their displays hurt their ability to survive — has long been a scientific mystery. The answer may turn out to be simple.

In mating season, male birds compete in contests. The contest can be about songs or colors. They can be about who has the longest tail, or whose tail fans out the widest, or who has the best dance moves, or who can do the best aerial acrobatics.

The judges at these contests are the female birds.

But why do things like this matter to the female bird?

According to the theory of natural selection, a female animal wants a mate who has the best genes to pass on to her children. The “best genes” means the genes that can best help her children to survive and thrive. If she mates with the strongest male, her children will inherit his genes to be strong. If she mates with the fastest male, her children will inherit the genes to be fast. They will be better able to escape danger, or to catch prey, depending on what kind of animal she is.

When the young grow up, the females of that generation will again choose the males who can give their children the most advantageous genes, and so on. Some male animals have contests of strength. For example, male bighorn sheep crash their heads together until one of them wins, proving he is the strongest. The females mate with the winner, and every generation is a little stronger.

In each generation, the children with the best genes for survival live longer and produce more children themselves, and so each generation will be stronger, or faster, or able to jump higher, or they may have a little better camouflage, or better hearing ability, or better ability to live on the kind of food available in an area — some characteristic that helps them survive better. This is called natural selection; creatures with the best survival characteristics reproduce the most, and, as a result, over the generations, the species develops those characteristics more and more.

Male birds have contests to impress the females, but, unlike the bighorn sheep, the birds don’t compete with strength. Instead, male birds compete with songs, or plumage, or both. Their contests are about beauty.

But that doesn’t fit the theory of natural selection. Even if the children inherit genes to be beautiful, how could beauty help them to survive?

In fact, fancy plumage can actually hurt a bird’s ability to survive. The more colorful a bird is, the harder it is for him to hide from predators. And an extra long tail or other fancy ornaments can make it harder for a bird to hide from predators.

Take the peacock and his big tail. The tail makes it harder for him to move around, especially in the dense foliage of the jungle. The tail makes it easier for a tiger to see him, and if a tiger is chasing him, that tail can slow his getaway and offer the tiger something to grab.

The dancing and displays of the male Sage Grouse helped inspire powwow fancy dancing. Besides fanning out his tail and making great dance moves, he can make loud booming sounds by inflating the air sacs he carries in his chest. He’s quite a showoff. But if a coyote shows up, the Sage Grouse loaded down with the fanciest accoutrements will become coyote lunch first.

So logically, the female bird shouldn’t want to pass on to her kids ornaments like that. Plainer birds would survive better, and females should choose the best survivors to mate with, so her babies would survive better too. Logically, the birds should have become plainer over generations.

In some cases, the handicap is inherited by the females too. The Club-winged Manakin, a songbird of Ecuador, uses a sound made with his wings to court females. Over generations, this has caused the Club-winged Manakins to develop deformed wings that interfere with their ability to fly. Not only males, but females inherit this handicap. So why would a female Club-winged Manakin want a male with this characteristic, if her criterion for choosing a mate is all what will give her children the best chance at survival?

The fact that many female birds prefer characteristics in males that will disadvantage the survival of their children has been a puzzle to biologists ever since they first came up with the theory of natural selection.

Biologists have come up with several hypotheses to explain birds like the peacock.

The “health hypothesis” says that the female bird is looking for the healthiest male, and that fancy feathers are a sign of overall health.

But plumage is not an indication a bird has especially healthy genes. Feather condition is a sign of overall health, because a healthy bird preens more and takes better care of his feathers, and bad feather condition can be a sign of too many parasites. But plumage — the color and length and fanciness of the feathers — says nothing about health of a bird’s body. The more “eyes” in a peacock’s tail, the more attractive he is to females, but the number of “eyes” says nothing about his general health, or whether he has parasites. Plumage is inherited, and inherited separately from other characteristics. A bird with a longer tail could have a weaker liver or lungs than a bird with a shorter tail, and a bird with duller colors could have stronger bones or heart than a bird with brighter colors.

The “handicap hypothesis” proposes that a male who is handicapped by a display like a large tail is showing a female how strong he must be, because he has managed to survive in spite of that handicap.

Proponents of the handicap hypothesis cite examples like the springbok antelope of Africa, who, rather than running off when he sees a lion, leaps straight up into the air, over and over. The springbok is saying to the lion: don’t even think about going after me, I am in such good shape I can waste energy jumping up and down and still have plenty of reserves to outrun you. A female springbok is presumably impressed because she will want her children to have that pro-survival ability. That may make some sense in the case of the springbok, but the handicap hypothesis doesn’t explain why a bird should want her children to inherit an anti-survival characteristic that would reduce their chances of survival.

And the fancy male might not in fact survive long. That male peacock might be eaten by a tiger an hour after mating, and he might be dying at a young age.

And the handicap hypothesis doesn’t account for the iridescent colorfulness of the peacock’s tail. As a handicap, a gray tail would have worked just as well. And the handicap hypothesis doesn’t account for birds, like most songbirds, whose beautiful characteristics don’t handicap them.

The “sexy son” hypothesis says that the female chooses the fanciest male so that her sons will be fancy too, which will get them more mates. But that is circular reasoning. It doesn’t explain why fancy, impractical characteristics should be sexy or desirable to females in the first place. It doesn’t explain why females would want their sons to inherit “desirable” characteristics that will be bad for their survival, or why the sons’ future mates would be attracted to such anti-survival characteristics. And it doesn’t explain beauty in birds who pair up one to one, who are 80% of bird species.

The “beauty happens” hypothesis says that birds simply love beauty and that is all there is to it. And, as I elaborate in my book How To Make Friends With Wild Birds, I believe that birds do love beauty. Love of beauty is something we humans share with birds, and it is part of the common language through which we can communicate with birds — much as loyalty to family and pack is part of the common language through which we communicate with dogs.

But even if birds love beauty, why do females of a particular species find a particular characteristic beautiful? And why do females of different species find different characteristics beautiful? Male cardinals and male bluebirds are both beautiful. Why is a female cardinal attracted to a bright-red male and a female bluebird attracted to a bright-blue male?

The “arbitrary choice” hypothesis says that there is no particular reason for females to prefer any particular characteristic, but once the females of a particular species started choosing something, the males of each generation inherited more of that characteristic, and the females of each generation inherited a stronger preference for that characteristic. So once the ball started rolling, over generations the males of the species developed that characteristic more and more.

But that doesn’t really explain anything. If displays are not about survival or reproductive advantage, why bother with costly displays at all? And why should beauty be connected with mating choice? If birds admire one another’s beauty, why not simply look at each other? We appreciate the beauty of birds without considering taking a bird as a mate. And why don’t female birds all go for the showiest males possible? If a female House Finch prefers the male with the reddest tint to his head, why doesn’t she go for a male who is even brighter red, like a Northern Cardinal or Scarlet Tanager?

Here is my hypothesis.

Let’s take the hummingbirds, maybe the most beautiful birds on Earth. The Golden-crowned Emerald and the Blue-throated Sapphire. The Violet Sabrewing and the Sparkling-throated Woodstar. The Amethyst-throated Mountain Gem and the Sapphire-vented Puffleg. The Rufous-crested Coquette and the Green-crowned Woodnymph. Male hummingbirds are extravagantly colorful, with iridescent plumage that sparkles like jewels. Some have extra ornaments, like crests, or feather tufts, or ribbon tails that wave in the breeze.

There are over 300 species of hummingbird, most of which live in Central and South America and the Caribbean. The South American country of Ecuador alone, smaller than Oregon, has about 160 hummingbird species. Humans travel from all over the world to see them. They are thrilled by every gorgeous hummingbird they see.

But if you were a female hummingbird in Ecuador, you had better not be thrilled by every gorgeous male hummingbird you see. With a hundred or more different kinds of hummingbirds living in your neighborhood, if you swooned at every colorful, ornamented male, odds are high that you’d hook up with the wrong one.

But a female hummingbird doesn’t make that mistake. Just as a female bluebird doesn’t give a second glance to a male cardinal, the female Violet-tailed Sylph doesn’t give a second glance to the male Blue-throated Woodstar, no matter how beautiful he is.

Beauty isn’t enough to attract a female bird. Beauty isn’t even the main thing she wants. Nor are signs of health the most important thing to her, either. Nor are “good genes.”

First and foremost, a female bird is looking for something far more important in her mate. Something that is absolutely essential.

She has to make sure that he is from the right species.

We don’t think about the problem of making sure our mates are members of our species, That’s because for us it’s not a problem. There are no other creatures around who resemble humans enough to cause confusion for us. So we can take it for granted that any potential mate we are considering is a fellow human.

But many birds are nearly the exact same shape as their close relatives. Warblers, for example, could easily be mistaken for each other if their colors didn’t differentiate them in spectacular ways. So could many kinds of butterflies and lizards and tropical frogs.

Our fellow mammals don’t have this problem. Even mammals with similar shapes — say, different species of squirrel — can identify each other by smell.

But birds don’t use smell to identify each other. And it can be tricky to tell closely related birds apart — what kind of flycatcher, what kind of duck, what kind of warbler.

So, to ID a bird, birders look for “field marks” – some stripe, streak, spot, patch, eyebrow or eye ring that is unique to a species of bird. The birds’ names – Black-Throated Green Warbler, Golden-Cheeked Warbler, Yellow-Throated Warbler, Yellow-Rumped Warbler – can contain clues about the field marks to look for.

Field marks are even more important for the birds themselves. A female bird needs a positive ID on her mate. She is the one who will pay the price for the wrong choice. (A “species” by definition is a group of organisms who can produce fertile offspring if they mate. Sometimes members of closely related species can produce offspring together, but the offspring will be sterile. Those offspring are called “hybrids.”) If she chooses a mate of the wrong species, her eggs won’t hatch (or, at best, her children will be sterile and she’ll get no grandchildren) and all her work of nesting and incubating would go to waste.

So health is not the most important thing a female bird is looking for in a mate. This is clearly proven by the fact that a female bird will certainly choose a less healthy male of her own species over a more healthy male of a different species. For a female bird, the species ID takes priority over anything else.

Female birds don’t have to look very distinctive, because the male birds don’t have to ID the females. Female Black-headed Grosbeaks and female Rose-breasted Grosbeaks look exactly alike, but the adult males of those species wear the different plumages that give them their names, because the female grosbeaks have to ID the males, not the other way around.

If colors aren’t enough as an ID, birds can come up with others. Songs — of course. The Willow Flycatcher and Alder Flycatcher look almost exactly alike, but their songs tell them apart. We can identify songbirds without seeing them by their unique songs. The male Allen’s Hummingbird and Rufous Hummingbird, with their coppery color and red throats, look different enough from other hummingbirds who live around them, but not so different from each other. So they use sound as an ID too. Not songs, though. Both Rufous and Allen’s Hummingbirds, when performing their acrobatic display dives for the females, make sounds with their tail feathers, but each one makes a distinctively different sound. Male woodpeckers drum on hollow logs – or metal chimneys or satellite dishes – to attract mates, the loudness of the drumming showing off their ability to dig nest holes in hollow trees and hunt insects to feed their future children, but the different rhythms of the Downy Woodpecker, Red-naped Sapsucker, and Northern Flicker are recognizable even to me, a human.

Some courting birds use dances, or other ritual movements that show off their unique characteristics. Among some birds, such as grebes, the male and female dance together when courting. Every bird’s courtship dance is unique to his species and advertises what species he belongs to.

Each bird species comes up with specific signals for the males to give. That prearranged signal is what will catch the female’s eye – or her ear.

But if all the males of the species are giving the same signal, how can one male bird stand out from the crowd?

By emphasizing that signal as much as he can. If the signal is a long tail, then grow the longest tail. If the signal is a head wattle, then grow the biggest head wattle. If the signal is a complicated song, then sing the most elaborate song. If the signal is a dance, then do the best dance moves. If the signal is the number of “eyes” on your tail, then grow the most “eyes.” Peacocks are the only birds with those “eyes” on their tails, and, sure enough, research shows that the peacocks with more “eyes” are more attractive to females.

Advertising is not a human invention.

But a big tail, long wattles, or other heavy ornaments can impede a male bird’s movements and doom him to a shorter life. Isn’t that a problem for the species? If the males aren’t so good at surviving, doesn’t that cause the population to go down?

Turns out that is not a problem.

Chickens can give us a clue. Other than Birds-of-Paradise, most of the birds with the fanciest trappings — peacocks, pheasants, wild turkeys, grouse, etc. – are from the Galliforme or “chicken-shaped” family (forma de gallina). And chickens give us a chance to observe the way of life of the fancy displayers.

Chickens can barely fly. They live on the ground. They are not territorial, but (in the wild) they keep on the move, looking for food. They belong to flocks, staying together and watching out for each other.

Unlike songbird babies, baby chickens can run around and follow their mother within hours after hatching. Mom leads them around, showing them what food is, but she doesn’t put the food in their mouths. They pick it up and swallow it themselves.

She doesn’t need help from the rooster to raise the chicks. So he has nothing to do with the kids. His job is to watch out for danger, and guard the whole flock, not just his own babies.

So chickens – and other birds of the chicken family, like turkeys, quail, pheasants – have single parent families, with mom raising the kids herself. They don’t pair up, and one rooster can mate with any number of hens. In fact, one rooster could father all the babies in the flock. So the flock can continue even if it loses most of the roosters.

The fanciest male may not live so long before he dies in the jaws of a fox, but in his short life he may mate with a lot more females and pass on a lot more genes than a plainer male who lives longer.

So, for those species, it doesn’t matter if a lot of males end up getting eaten.

A songbird species, on the other hand, can’t afford to lose its males like that.

Songbird babies hatch helpless and blind and naked. They need food put directly into in their mouths. A lot of food. And one of the parents has to stay and brood them (keep them warm) till they grow feathers, which means the other parent has to search for food. A baby songbird needs two parents to survive.

There are a few exceptions, where a female songbird doesn’t need help from her mate to feed her kids. Red-winged Blackbirds nest in marshes where insects are so plentiful and so easy to hunt around the nest that the female can feed the babies by herself. And in the rainforest, songbirds have such an abundance of insects that it is easy for most mother songbirds to find food for her babies alone. (The same is true of hummingbirds, who are so fast they can catch insects easily.) So these birds don’t pair up.

But 80% of the bird species on Earth, including most songbirds, pair one on one. To do that, a species needs to have about equal numbers of males and females. So, unlike a species in which one male can mate with many females, a species that pairs up one to one can’t afford to risk the lives of its males. Each male songbird who perishes means a female who doesn’t get a mate, a nest that doesn’t get built, eggs that don’t get laid, babies that don’t get born.

So the IDs of most songbirds don’t weigh them down or create air resistance or otherwise endanger their survival. The male Ruby-crowned Kinglet displays a red crown and the Golden-crowned Kinglet raises a tiny flame of yellow and orange, but even these small fluffs are smoothed down and hidden away when the bird is not showing off to mates. Some birds, like the American Goldfinch, wear their brightest colors only during breeding season. Color as an ID badge weighs almost nothing. And song weighs even less.

So my hypothesis is that fancy displays started out as ways that males could advertise their species identity to females, and since these advertisements are what get the females’ attention and make the males desirable, males with more prominent advertisements pass on their genes more than males with less prominent advertisements. And in some species the advertisements have developed, over the generations, till they are over the top. I call this hypothesis the “ID hypothesis.”

But the IDs are not merely practical. Most species IDs are beautiful as well. Birds didn’t have to use beautiful IDs. Some sort of avian bar code would have worked as well. And some birds do opt for something like that; for example, the male House Sparrow is distinguished from similar birds by a distinctive black spot on his throat and chest. It’s not especially beautiful, yet a male with the largest and darkest black spot is the most desirable to females. All the House Sparrows can get a mate in the end, because House Sparrows pair up one to one, but the male wearing the best ID spot gets first choice of mates, and not only that, he gets first choice of nesting spots and gets to boss the other male House Sparrows around.

But in most bird species, the females prefer something — songs or colors or something else — that is beautiful in some way. And the amazing thing is, what is beautiful to the birds is also beautiful to us. We, like the birds, are blessed with a sense of beauty, one that is close enough to the birds’ sense of beauty that we are able to recognize the birds’ beauty. Birds communicate to each other with beauty, and so do we, and through the common language of beauty, we and the birds can communicate with each other. As a Haida grandmother said to me, the reason we were placed on Earth by the Creator is to continually add to its beauty — just as the birds do.

Gayle Highpine is the author of the forthcoming book How To Make Friends With Wild Birds.

Photo credits: Peacock – Kathypdx, Wikimedia Commons; Greater Sage Grouse – USFWS; Club-winged Manakin – ryanacandee, Flickr; Wilson’s Bird of Paradise – Serhonokskay, Wikimedia Commons; Golden-tailed Sapphire Hummingbird – Joseph C. Boone, Wikimedia Commons; Golden-cheeked Warbler – Steve Maslowski/USFWS; Black-headed Grosbeak – author; Rose-breasted Grosbeak – John Harrison, Wikimedia Commons; Lady Amherst Pheasant – Henry Koh, Wikimedia Commons; Ribbon-Tailed Astrapia – Gail Hampshire, Wikimedia Commons; Rooster – Žiga, Wikimedia Commons; Northern Cardinals – nosha, Wikimedia Commons; House Sparrow – Jacob Spinks, Wikimedia Commons