The Science of Birds - The Pecking Order: Dominance Hierarchies in Birds

Episode Date: March 25, 2022

This episode—which is Number 48—is about the “Pecking Order” in birds. Or, to use the more technical term, dominance hierarchy. Our focus will be on dominance hierarchies that we see among birds within a single species. White-crowned Sparrows beating up on other White-crowned Sparrows, for example, as opposed to White-crowned Sparrows beating up on another species, like Lincoln’s Sparrow. The latter would be an example of interspecies dominance. But today, we’re talking about intraspecies dominance. Meaning within species.~~ Leave me a review using Podchaser ~~Links of InterestPukeko videoPukeko fighting videoLink to this episode on the Science of Birds websiteSupport the show

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, which is number 48, is about the pecking order in birds, or to use the more technical term, dominance hierarchy.

Our focus will be on dominance hierarchies that we see among birds within a single species. White-crowned sparrows beating up on other white-crown sparrows, for example, as opposed to white-crowned sparrows beating up on another species, like Lincoln's sparrow. The latter would be an example of inter-species dominant, But today, we're talking about intra-species dominance, meaning within species. This all has to do with bird behavior, some bad behavior, some good behavior, the whole range.

It's pretty interesting stuff, so how about we get into it? For most birds and birds and and other animals, living wild and free out in nature, there just isn't enough food to go around. The harsh reality is not every individual can get all the food it needs. The same is true for shelter, nest sites, and mates. Vitally important resources like these are limited. That's one reason that evolution by natural selection works, right? The fittest individuals in the population are the ones that get the most of the limited resources. And then those guys are more likely to survive and pass on their genes.

Animals and plants have all sorts of clever adaptations for being more fit. No, not fit like they've been working on their glutes at the gym, or fit meaning sexy in British slang, but fit as in having evolutionary fitness, having what it takes to survive and pass on your genes. Birds, in addition to having anatomical and physiological adaptations, have behavioral adaptations that can give them a competitive edge. One common behavior that can work well when resources are limited is territoriality. Consider a songbird like the oak titmouse of California,

biolophis in Ornatus. This is a cute little gray bird with a crest on its head. A mated pair of oak titmouses, titmice, defend a territory covering one to three hectares, which is 2.5 to 7.4 acres of oak woodland. This patch of prime real estate includes things like tasty seeds and bugs, and microhabitats for foraging, nesting, and roosting. Because these resources are limited, the titmice need to defend their territory from competitors

by using songs, displays, and physical aggression. They need to patrol the perimeter. And we're talking about California here, right? So the real estate prices are through the roof. These little titmice got to protect their investment. But being territorial has its downsides, its costs. It takes a lot of extra energy to sing, to patrol, and to chase off trespassers.

and those oak tit mice can make themselves easy targets for predators if they're not careful. Birds will evolve to have territorial behavior like this only when the benefits outweigh the costs, when it increases their fitness. For many other bird species, however, territories just don't make sense, at least not at all times of the year. For them, the costs of this behavior would outweigh the benefits. This could be for any number of reasons. For example, when food resources are too spread out across the landscape, a territory in that

case would need to be so large that any small bird would find it impossible to defend. It's sort of like that one theory for why the Roman Empire collapsed. The Empire had grown so expansive, it covered so much territory that the Roman military was spread too thin. They couldn't defend their lands against the invading barbarians, all those scruffy Goths and vandals. An alternative behavioral adaptation to the problem of limited resources is the pecking order. This is the dominance hierarchy that we find in many bird species, mostly in species that gather in groups, as in flocks. Hanging out with other birds in a flock is a smart move for several reasons.

Perhaps the biggest advantage is that there are all those extra eyes and ears to detect sneaky predators. And if a predator does attack the group, there is a decent chance that it will single out one of your buddies instead of you. Better him than you, right? Safety and numbers and all that, jazz. But then there's all those hungry mouths to feed, all those beaks. If food supplies are limited, as they so often are, how do birds in a flock decide who gets to eat? That's right, they establish a pecking order. This is a social order where some birds in the flock are dominant and others are subordinates. In other words, we say some are alphas and some are betas. Alpha birds pretty much do what they want. They don't care what other birds think. They get to eat first and eat the most. They get the best

perches, the best mates, and they can talk as loud as they want at the movie theater. No one is going to be brave enough to shush them. Betas, the lower-ranking birds, are submissive to the alphas. They have to wait their turn for food and other resources. Betas have to make do with lower-quality roosting sites and foraging areas, and they don't have as many opportunities for mating. In its simplest form, the dominance hierarchy in birds runs in a straight line. from the most dominant individual all the way down to the most submissive.

In a linear hierarchy like this, the top bird dominates everyone below it. The next bird below dominates everyone except for the top bird, and so on down the line. Most birds in the flock are dominant to lower-ranking individuals while also being submissive to higher-ranking individuals. Every bird knows its place in the linear hierarchy. they know better than to step out of line. Or at least they should. So just how do birds sort all of this out?

How do they establish and maintain these dominance hierarchies? Well, there's no one thing that decides a bird's position in the pecking order. The determining factors vary from species to species, and they can vary with circumstances too. But the general pattern is this. Larger birds dominate smaller birds, adults dominate immature birds, and males dominate females.

Not always, but in general. Large body size tends to be an advantage in a fight. Duh, right? The more fights you win, the higher your social rank. For example, there was this study of silver eyes, Zosterops Lateralis, living on a small island off the coast of Australia. Silver eyes are small, yellow and gray songbirds that have a ring of crisp white feathers around each eye. The researchers in this study measured body size for a bunch of individual silver eyes on the island.

Then they observed the birds as they fought each other over food at feeding stations. The outcome of each fight was recorded. each natural agonistic encounter, as the researchers put it. It turned out that, yes, indeed, the silver eyes with the heaviest bodies and the most robust bills were the ones that tended to win fights and end up as dominant birds. As a side note, the researchers conducted this silver eye study, published in 2003 in the Journal of Evolutionary Biology, because they were interested in the widespread phenomenon of small birds evolving into larger forms on isolated islands. Larger than their mainland counterparts,

that is. Silver eyes living on this small island on the Great Barrier Reef are much larger than silver eyes on mainland Australia. This is the phenomenon called island gigantism. The researchers speculated that, in general, competition for limited resources among the members of an island population could result in larger, dominant individuals surviving longer. They could have more opportunities to mate and pass on their genes. If so, body size in the overall population should increase from one generation to the next. And this 2003 study did find that the birds that won the most fights were not only bigger, they also tended to have better chances of survival.

So, something like this might, might, be what has happened again and again for different birds around the world, where the ones on islands evolved into monstrous giants compared to their mainland cousins. Interesting stuff. Islands are just the coolest. But I digress. Because nature is complex and beautifully messy, there are exceptions to the general patterns of dominance. Sometimes a small-bodied pugnacious bird with good fighting abilities can become dominant to larger birds. And there are also exceptions when it comes to sex differences. There are plenty of examples where females, rather than males, are the dominant sex. In some bird species, dominant status isn't something you earn. It's something you're born

with. Take, for example, the white-throated sparrow, zonotrichia albacolus. This North American bird has two color morphs that occur in roughly equal proportion. There's a white-striped morph and a tan-striped morph. Adult white-striped males are more aggressive and more promiscuous. They sing more and they tend to be the most dominant members of the flock. And little-known fact, these birds were the band name inspiration for the white stripes. Not true at all, unfortunately, but what if, right? Whether you're born as a white-striped or a tan-striped sparrow is determined by an inherited

difference at the chromosome level. In other words, it's genetic. So the social behavior and status of these birds is sort of already baked in when they hatch out of their eggs. Anyway, once the pecking order has been established in a flight, then comes the job of maintaining it. This whole dominance hierarchy thing was first described scientifically in the early 1900s by a Norwegian zoologist named Thorlife Shai-do-Thorelife-Shelder-up-ebaeba.

That's a hard one to pronounce. I'm sure I mangled it, sorry. Thorlife paid close attention to the chickens on the farm he visited on summer vacations. He observed that the birds have a consistent social order, an order maintained by pecking. So Thorloft named this behavior, hakordnung, which, I'm told, is German for pecking order. My wife and I have five pet chickens that we love dearly. These little buggers established their linear pecking order years ago, and it hasn't changed. Our white chicken, chowder, is the alpha hen.

And last place is Binks. a gentle-hearted, submissive girl that gets pecked on by everyone. Dominance hierarchies in birds are often maintained over long stretches of time. And that stability is generally a good thing for all individuals in the flock. If everyone just keeps cool and nobody challenges the status quo, there's no need for more fighting. No need for any natural agonistic encounters. Nobody gets hurt and nobody needs to get all stressed out.

out. In my backyard chicken flock, we rarely see much aggression or fighting anymore. Order has been maintained, and things are pretty peaceful. Now, here's something you may have been wondering about. If birds are going to establish and maintain a dominance hierarchy in their flock, don't they need to be able to recognize each other as individuals? A bird needs to know, specifically, who to bully and who to cower in the presence of, right? Well, the answer is, yes, birds often do use individual recognition like this. Sure, when you and I look at a flock of, I don't know, uh, here, let me just push the random bird species button.

African forest flycatchers? No. No, let's try again. Let's push the button. Snow Mountain Munya, Longura, Montana. That's better. When you and I look at a flock of Snow Mountain Munias, which live in the highlands of New Guinea, we find that these little finches all look pretty much identical. Like somebody selected one bird, hit copy on the keyboard, and then paste, paste, paste. But, most likely, they can tell each other apart. They recognize each other by subtle differences in plumage pattern and color,

by body size, voice, and maybe even smell. So birds in a flock might fool you and me into thinking they're all the same, but they can't fool each other. Fool me once. Shame on you. It fool me, we can't get fooled again. But some research shows that this whole individual recognition thing works well only for small flocks.

A small number of birds means a small number of faces to remember. But there's another way that birds size each other up and recognize their competitors. They use badges of status. For example, research on golden-crowned sparrows, zonotrichia atrocapola, suggests that these birds recognize each of their flockmates as individuals. These birds forage in small, stable groups in winter. But when they encounter golden-crowned sparrows from another unfamiliar flock, uh-oh, stranger danger!

In this case, the birds switch to using badges of status. For golden-crowned sparrows, the badges are patches of gold and black feathers on the head. The larger the badge, the more dominant the bird. So even if you don't recognize that new, strange bird, as an individual, you can at least see that it has a big, flashy patch of gold on its head. That means you're looking at a dominant alpha sort of character. Best keep clear of that guy if you don't want to throw down and have a bloody battle. Badges like this, along with other displays and bluffing behaviors, help to minimize violence

between individuals in many species. This way, birds can sort out the pecking order without too much actual pecking. It's a more peaceful, more passive approach. Another bird I'd like to highlight is the Australasian swamp hen, Porphyrio Melanodos. The name for this species in New Zealand is Pukiko. I like the sound of that better, so that's what I'll use, Pukiko. This bird is in the rail family, Raleigh. It lives across Australia, New Zealand, and a bunch of other islands in the Western Pacific. Pukiko have black, purple, and blue plumage, and they have bright red bills. The bill extends upward into a fleshy red shield, which spreads across the forehead and goes up to

the crown. The shield is shaped kind of like a guitar pick. Pukiko are strongly social in many of their populations. They often breed in communal groups, where multiple males and multiple females control. tribute to the pile of eggs in a single nest. They also defend their territory fiercely against other groups of Pukiko. But within a group, there's a stable dominance hierarchy. This pecking order stays mostly the same from one year to the next. It's reinforced regularly in squabbles over things like food and incubating duties. When two birds fight, they peck at each other and they use

their long legs and claws to attack. It's pretty wild and pretty dramatic. I'll put a link in the show notes to a video of Pukiko fighting. The most violent encounters occur between members of two different flocks or breeding groups. But within the group, violence is mostly avoided because each bird doesn't step out of line very often. They don't try to go against the social order. When there is a confrontation, there are several postures that the birds might assume. Usually a dominant pukko holds its head high in an aggressive, upright posture. A submissive bird will bow and keep its head low. It will often show the fluffy white feathers under its tail, its undertale coverts.

So this is sort of like a white flag of surrender, a signal that no blood needs to be shed with beak end claw. The subordinate bird is like, simmer down there, tough guy. I'm not questioning your authority. You're the boss and everything's cool. Here, here, look at my fluffy white butt. It's very fluffy, see? See, does that make you feel better? Sure, it does. Sure it does. Are we cool? Are we cool, man? That fleshy frontal shield is an important feature for the social lives of Pukiko. The larger an individual's shield, the more dominant the bird is. Researchers have found that shield size and color are strong signals of dominance, and these traits are also correlated with the size of the testes in male Pukaco. Larger testes usually means more

testosterone is flowing in the bloodstream. So males with the largest shields are also hopped up on testosterone. They're extra aggressive. And here's what I think is the really interesting thing. Shield size can change significantly in as little as a week. If Apuco loses its dominant status after getting beaten up by a rival say, the loser's shield might shrink. And that shrinkage is likely to reflect decreased levels of testosterone in the bird's system. So, unlike Apuco's plumage, its feathers, which can change only once or twice a year, the size and color of the fleshy shield can change in a matter of days. So it might be the best, most meaningful badge of social status in Pukiko.

There are benefits and costs to all of these behaviors, risks and rewards. You might think it's always good to be the king. That the most dominant birds have it the easiest. And yes, whether you're looking at silver eyes, sparrows, or swamp hens, the alpha birds generally get the benefits of more food, better food, less exposure to predators, better mating opportunities, and so on. But scientists have found plenty of evidence that being dominant also comes with downsides, some costs. One potential cost for dominant birds is just the risk of injury, you know, from all the fighting. Another cost might be high levels of stress. These guys have to assert their dominance regularly,

and they sometimes have to deal with insubordination by violent means. Alpha birds have to look over their shoulder constantly, like they're in Game of Thrones or something. Who knows who's going to stab you in the back next time? Better to trust no one, as my grandma always used to tell me. Continual stress is hard on the body and might decrease. the health of a dominant bird, or shorten its lifespan. So it seems high levels of stress could reduce the overall fitness of an alpha bird.

Researchers have measured stress levels in many social bird species. From what I can gather, however, there doesn't seem to be a universal pattern where dominant birds are always more stressed out. It varies from species to species, from place to place, and from season to season. Similarly, subordinate birds don't seem to be more stressed out than their dominant flockmates, at least not in all cases. Life as a subordinate isn't always a total bummer. Sure, these birds have to wait their turn for food and they don't get to mate as often.

But they also don't get as many cuts and bruises. If they keep their heads down and don't rock the boat, they avoid fights and get to stay in the flock. They get the benefits that that entails. Safety from predators, most importantly, and at least the chance of hooking up with a mate. Overall, the costs and benefits of different social ranks in birds seem to vary a lot. There have been mixed results in the many studies on this subject. Sounds like we need to do some more research. Let's get some grad students on that right away.

But like I said, regarding territorial behavior, dominance higher hierarchy behaviors should exist only because, in the evolutionary sense, the benefits outweigh the costs. If these behaviors affect the gene pool of a population by increasing fitness, well, that's all that really matters. So, just like door buster deals at your local Walmart on Black Friday, resources are limited in the natural world. Birds have come up with the behavioral strategies of territoriality and dominance hierarchies to make the best of it. These behaviors are definitely not mutually exclusive, since many species use both strategies at one time or another. Often, birds will be territorial during the breeding season and then

gather in hierarchical flocks in the non-breeding season, or sometimes vice versa. There's so much more we could talk about on this topic, but let's call it good for now, shall But here's some homework for you. The next time you're watching a group of birds, see if you can notice some of the social dynamics. Spend a little time watching a flock of dark-eyed junkos if you're in North America, or blue tits in Europe, or silver eyes in Australia. You might see the little bluffs, battles, and passive displays of dominance and submission. See if you can sort out the pecking order. The birds may be adorable, even comical. from our perspective,

but from their perspective, this is some serious drama with very high stakes. As always, I hope you enjoyed the episode. And perhaps more importantly, I hope you learned a little something. I really appreciate that you care about birds

and that you spent some of your valuable time with me today. My supporters on Patreon are really helping to make the science of birds possible. So thank you to all of my wonderful patrons. And my newest patrons get a shout out today. So welcome Scott Mon, Lynn Stern, Drew Brunner, Asimonas, Aurora Murphy, Jean Rosenmeier, Jose Pablo Castillo, Samrat Vupalanchi, Dave Rodriguez, Dan and Alex and James Garrett. Thank you all so much. If you're listening at this very moment, moment and you are curious about supporting this podcast, you can check out my Patreon page at patreon.com forward slash science of birds. You can also shoot me an email if you have something

you'd like to share with me, a comment about me, about the show, or maybe you want to tell me about one of your many agonistic encounters. How'd that turn out for you? Whatever the case, my email address is Ivan at scienceofbirds.com. You can check out the show notes for this episode, which is number 48, on the Science of Birds website, scienceofbirds.com. I'm Ivan Philipson. I hope life is great for you, and I'll catch you next time. Cheers.