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Bird Biodiversity

A look at the prehistoric proliferation of birds and their modern-day species diversity. How is this diversity distributed around the world and why is it important?

DATE:
September 19, 2020
Author
Ivan Phillipsen

Birds Illustrate Nature's Inventiveness

I am continually amazed by the diversity of birds. As a birdwatching tour leader, it’s my job to travel the world, to find as many bird species as possible. Even so, I’ve seen less than half of them. Thousands more are out there, waiting to be admired.

The variety of shapes, colors, behaviors, and ecological roles of birds is mind-boggling. Just when I think I’ve seen the most outlandish or beautiful species, either in the flesh or in the pages of a guidebook, I come across another one that’s even more impressive. They just. keep. coming.

Even the names of birds can light up the imagination and hint at their extravagant diversity: 

Horned Screamer, Marbled Frogmouth, Shining Sunbeam, Pearly-eyed Thrasher, Yellow-eared Spiderhunter, Tibetan Snowfinch, Rusty Laughingthrush, Rock-loving Cisticola, Blue Jewel-babbler… 

Wow.

This article is an overview of bird biodiversity. We'll start by looking at when birds first diversified on the planet. Then we’ll consider their present-day diversity. We’ll follow that with a discussion of what biodiversity is. Then we’ll consider how birds are distributed around the world. And lastly, we’ll touch on why all this diversity is important.

Diversity Before and After the Cretaceous-Paleogene Extinction

The first birds evolved between 150 and 200 million years ago, when dinosaurs were running amok across the planet. Birds are, of course, dinosaurs themselves.

Birds began their long history back in the Mesozoic Era and have, over many millions of years, multiplied into a dizzying array of species. 

66 million years ago an asteroid smashed into Earth and the resulting destruction killed off most of the dinosaurs. That epic catastrophe is technically called the Cretaceous-Paleogene Extinction. I’m just going to call it ‘The Big Extinction,’ for simplicity. 

Life on Earth before and after that event was wildly different. The Big Extinction makes a stark dividing line in the fossil record for birds, and pretty much all other forms of life.

Before the extinction, there were many primitive bird species. The earliest gliding and flying birds were expanding, diversifying into different forms alongside their cousins: the standard-issue, non-flying dinosaurs. Things were pretty good, and the future seemed bright. 

But then the asteroid hit and the party came to an abrupt end. From the fossil record, it seems clear that many bird lineages were extinguished in one fell swoop by the Big Extinction 66 million years ago. Avian diversity crashed.

Amazingly though, several bird lineages survived.

One of those was the ratite lineage, a group of birds that is still alive and kicking today. These are the ostriches, kiwis, rheas, and a few others, most of which are flightless. When the Big Extinction happened, the ratites had already been around a while as a distinct group. 

An Emu in Queensland, Australia.
The Emu of Australia, a member of the ratite lineage of birds. Photo by Ivan Phillipsen.

Two other lineages that survived the firestorm are quite familiar to us: ducks and chickens. Their ancestors anyway. The two present-day taxonomic orders Anseriformes and Galliformes existed before the extinction. Anseriformes includes ducks and geese and the like, while Galliformes includes chickens, pheasants, quail, etc. 

So Chicken Little could have actually been around 66 million years ago and he would have been 100% correct when he cried, “The sky is falling!”

The only other lineage of birds that we know survived the Big Extinction was the one that ultimately gave rise to the rest of the living birds. If you exclude the ratites, and birds in the Anseriformes and Galliformes orders, you are left with a big group known as Neoaves. 95% of the world’s living birds are in this group. The ancestor of today’s Neoavians was a survivor of the extinction.

There has been a debate among paleontologists and other scientists about the timing of when the earliest birds began branching off, first with the ratites, then the Galliformes and Anseriformes. I won’t get into it, but I can say that the evidence we have, from both fossils and genetic data, shows that the bird lineages I just mentioned were each distinct from each other before the Big Extinction.

Diversity in birds was at a low point 66 million years ago. But when conditions around the planet slowly settled into a new normal after the asteroid impact, life recovered, and birds hit the ground running. Or, well, they hit the air flying, I guess.

An explosion of diversification in birds began. Countless ecological niches occupied by other dinosaurs and the flying pterosaurs were suddenly vacant, free for the taking. As you know, nature abhors a vacuum. Birds began to fill the vacancies, adapting to take advantage of untapped resources. They evolved into new species and then into whole new, dramatically different lineages. 

We can trace most of the modern bird groups back to those ‘wild west’ days just after the Big Extinction. 

This relatively rapid diversification from a few ancestors is known as an adaptive radiation. A similar thing happened to mammals as they diversified after the extinction. 

Taxonomic Diversity of Birds today

Now, let’s jump in our time machine and go back to the future, by which I mean today.

Let’s consider the diversity of birds as it is now. 

There are about 10,000 bird species. 

10,000 is a nice round number. It’s easy to remember. If someone pulls you aside at a fancy cocktail party and asks you fervently, “How many bird species are there in the world?” You’ll be able to escape any crippling embarrassment because you can give them the answer with confidence, “10,000.”

However, between you and me, that isn’t the exact number of bird species. It’s actually a bit more than that. Also, the several authoritative organizations who keep track of such things report slightly different numbers.

Arguably, the most accurate and up-to-date resource is the Clements Checklist of Birds of the World, maintained by the Cornell Lab of Ornithology. That’s the gold standard. The Clements Checklist is used by the American Birding Association and is used as the basis for eBird, which is far and away the best app for people like you and me to track our bird observations. 

Today, the Clements Checklist has 10,561 living bird species listed. But that could change a bit later this year, since the checklist is updated annually.

Why would the number of species change? 

Rarely these days, a completely new bird is discovered in some remote corner of the world. That’s always a cause for celebration.

And of course, the number of species can go down due to extinction. Boo!

A more common situation is when scientists—using genetic or other data such as plumage or bird song—discover that what we thought was one species is in fact two. 

This happened in 2010 when, based on genetic data, the Winter Wren in North America was split into two species: the Pacific Wren and the Winter Wren. Or the opposite thing can happen: what we thought were two species can get lumped together as one, if careful scientific analysis suggests that they should be. 

I don’t blame you if you’d rather just remember that the number of bird species is “about 10,000”, instead of exactly 10,561. The latter number will change next year anyway… probably.

For comparison, there are approximately the same number of living reptile species: about 10,000. I guess I should say “non-avian” reptiles, because birds are, in the evolutionary sense, reptiles, even though we classify them separately. Anyway, there are approximately 8,000 amphibian species and 6,400 mammal species.

Now remember that when we classify organisms scientifically, we have that good ol’ system of taxonomic ranks: domain, kingdom, phylum, class, order, family, genus, and species. Birds fall into the class Aves.

Those 10,561 bird species are currently divided into about 2,500 genera (genera is the plural of genus), 248 families, and 41 orders.

The taxonomic rank of family is, to me, a helpful unit of organization. Birds that all belong to a particular family share many recognizable features. 

For example, the family Accipitridae includes all the world’s hawks, eagles, and kites. These birds have strong hooked bills, grasping feet with long talons, broad wings, etc. If I see a bird with those features, even if I don’t know the species, I can probably place it correctly in the Accipitridae family. That way, I can narrow down my search for its identity. I’ll flip to the appropriate section of my field guide or bird app and go from there.

As I said, there are about 250 bird families and that is a manageable number of groups to have rolling around in my head. Not that I have them all memorized, mind you. But maybe someday.

Some families contain lots of species. They’re very diverse. For example, there are several mega-diverse families centered in the tropical parts of South America. There are 349 hummingbird species in the family Trochilidae. The Tyrant Flycatchers of the family Tyrannidae, however, have the hummers beat with a whopping 422 species. Tyrannidae is actually the most speciose—in other words, species rich—bird family in the world.

A Scissor-tailed Flycatcher
The Scissor-tailed Flycatcher is in the most species-rich bird family, Tyrannidae. Photo by Ivan Phillipsen.

At the other end of the spectrum, there are depauperate families with only a few species—or even just one lonely species. When a family or genus includes only a single species, it means that species has no close relatives. That family or genus in this case is said to be monotypic, having only one type.

A monotypic bird family that comes to mind is that of the Sunbittern, which lives along forested streams in Central and South America. The Sunbittern has no close relatives and is the only member of the family Eurypygidae.

A Sunbittern bird in Brazil.
The family Eurypygidae is monotypic, having only one species: The Sunbittern.

Species diversity and family diversity—these wax and wane over the millennia and over millions of years. 

If we look at the diversity of life on Earth for the last 500 million years, there is an overall upward trend—an increase in diversity. This can be seen with birds too, over the last 50 million years.

But mass extinctions and the less dramatic but continual losses of individual species have pumped the brakes, so to speak, slowing the increase in diversity.

Taken as one big group, birds seem to be more diverse today than they have ever been, more or less. But that’s not necessarily true for each order or family of birds, when considered individually.

Families that are species poor today, like the Sunbittern family, might have been much more diverse in the past. Some of them had a glorious golden age that is long behind them.

Biodiversity

We’ve been talking about diversity in birds, mostly in terms of species richness.

But the biodiversity of birds is a bit more inclusive word. 

One fairly succinct, recent definition of biodiversity is “variation of life at all levels of biological organization.”

If we go down to the smallest, microscopic level, the ultimate source of all biological variation is mutation. Mutations are rare changes in DNA sequences that happen when there are goof-ups in DNA replication. 

Natural selection then acts upon individual variation, among individual birds say, which expresses underlying genetic differences, including those mutations. This leads to diversity among individuals, populations, species, and all the way up to ecosystems.

To keep things simple, let’s continue thinking of avian biodiversity at the species level.

Now, imagine a distinct region, perhaps a large island. It’s a nice place, with lots of palm trees and thankfully no mosquitoes or obnoxious squirrels. 

Let’s say we do a big survey and find all the bird species on the island, which means we get a great measure of their biodiversity there.

If there are 300 bird species on the island, why is it that number and not 1000? Or why not 25?

The answer is complicated. Many factors can influence the biodiversity of a given place.

The overall pattern is that species increase in a region through either evolution or immigration, and they decrease by extinction. Biodiversity is the balance of these. 

And these processes in turn depend on things like the region’s geographic area, any natural barriers within and around the region, climate variation, etc.

Evolution, immigration, and extinction are often acting in the present day, but the way these have unfolded over the region’s entire history is really what has shaped the biodiversity we find there.

As I said, complicated. But fascinating, if you ask me.

We’re on a slippery slope here, heading into a discussion of biogeography. I love me some biogeography, but let’s save that for another article.

Instead, let’s look at the way avian biodiversity is distributed around the globe.

The Global Distribution of Avian Biodiversity

Birds live on all seven continents, in a vast majority of the world’s ecosystems, including the open ocean and the frozen wastes of Antarctica.

We have our 10,000 bird species; we have all this spectacular diversity. But it isn’t distributed uniformly across the planet. Nor is it distributed randomly. Instead, bird diversity follows some interesting patterns that we can detect. 

One of the strongest patterns is the Latitudinal Diversity Gradient, also called the Global Diversity Gradient. This describes how the highest levels of biodiversity, the greatest number of species, live in the tropical regions of the world, and diversity decreases as you move away, north or south from the equator, towards the poles. That is the Latitudinal Diversity Gradient. It applies to birds and many other forms of life.

Why does this exist? Short answer: we don’t know. It’s not entirely understood.

It could be that since the tropics receive more sunlight, there is more energy to produce vegetation, and that ends up supporting more individual animals and, somehow, more species.

Or it could be that the mix of temperature and rainfall in the tropics is more agreeable to living things and that allows for more species to exist there.

Or, since tropical forests have been one of the most stable ecosystems for millions of years, extinctions might be relatively less frequent there, allowing more species to stack up over time.

It could be a combination of these and other factors. I think many scientists would agree that several forces are interacting to create this strong gradient.

There are several regions that stand out as major hotspots of bird diversity. It should be no surprise that these are in the tropics. 

If we look at a map of species richness across the planet, we can see that the Amazon basin just east of the Andes in Peru, Ecuador, and Colombia is a major hotspot for birds. And northern South America, in general, is super species rich for birds. 

But we can also look to East Africa, which has a high diversity of birds, as does Southeast Asia. Looking at the maps of these places, you’ll see that mountains seem to be associated with this high level of diversity. It may be that when there is topographic diversity in a region, more habitats are available for birds and therefore more species can live there.

Related to that idea is that besides the Latitudinal Diversity Gradient, there is in many places an Elevational Diversity Gradient. This isn’t as strong a pattern as the Latitudinal Gradient, and it’s even less understood. But in many places there are more bird species at lower elevations than at high elevations. In other places, there is a maximum of diversity at mid-elevations. Meaning that you have a certain number at lower elevations, then you find more and more species as you climb to mid elevations and then the number drops back down as you keep climbing to the mountain peak. Interesting, huh?

Another important concept here as we look at global bird diversity is endemism. A bird is endemic if it is found only in one region. 

For this term to make sense, we must first define the area we’re talking about: either a country, a continent, a region, an island, an island archipelago, etc. 

Once we’ve defined the area, we can say whether a species is endemic to it. Isolated islands have many, many examples of endemic birds. The Galapagos Islands are perhaps the most famous. Darwin’s finches, the Flightless Cormorant, the Galapagos Penguin… these are all endemic to the Galapagos. They’re found nowhere else in the world.

A variety of factors contribute to why a species might be found only in one place. It could be that the species was once very widespread and has gone extinct everywhere but that one place. Or maybe the species evolved in that place and never spread to anywhere else. That’s typically the case on isolated islands. Collectively, Islands around the world harbor lots of unique, endemic species. That’s another global pattern of avian diversity.

This Vanikoro Flycatcher is endemic to Fiji and the Solomon Islands. Photo by Ivan Phillipsen.

Why is the Biodiversity of Birds Important?

Why should human beings care about diversity in birds? Yes, this is one of those philosophical questions that we can get lost in for hours. Let’s not do that.

Let’s try to answer the question pragmatically and also from a self-centered perspective:

Birds serve us by performing important jobs in the world that benefit us. 

Birds act as pollinators and seed dispersers for many plants valuable to us, such as trees that produce building material, fruit and other foods, and medicine.

The more bird species there are in the world, the more plant species they can help propagate.

Birds also provide the service of regulating pests and diseases by eating insects and other invertebrates. Some species are scavengers, cleaning up carcasses that would otherwise pile up. 

The more bird species there are in the world, the more ways they can help reduce pests and disease.

These days, you have people like me and my tour guests who love to travel the world to see new and wonderful birds. Birdwatching is at the heart of a vibrant ecotourism industry that provides sustainable, nature-friendly jobs for many thousands of people across the globe. Bird diversity itself is the main attraction for many, many travelers. Those travelers support lots of economic growth.

Now, if we consider the good of more than just our own species, as we should, we see that bird diversity is also important because birds play integral roles in their ecosystems. They provide ecological balance. Many regulate the populations of the small animals they eat. In some places, like remote islands, birds are the top predators. Many bird species also serve as prey for other wildlife. They are connected to countless other organisms in these ways.

Some birds can be considered keystone species. One example is woodpeckers in a forest ecosystem. Woodpeckers have strong bills that they use to chisel out nest cavities in trees. Many other bird species and mammals depend on woodpecker cavities. And those cavities also help to start the decomposition of a dead tree, ultimately helping to recycle the nutrients in the tree back into the soil. So when woodpeckers aren’t present, a lot of things change in a forest, mostly for the worse.

I could go on and on about the importance of bird diversity. Besides all the things I just mentioned, there is the cultural, aesthetic, and spiritual importance of birds to us humans. The world has been filled with a sparkling, wild diversity of birds since our kind first stood up on two legs in eastern Africa. They have been our constant companions for hundreds of thousands of years. They are the voices of nature and they are the wild animals we can most easily connect with, anywhere on Earth.

We Need to Protect the Biodiversity of Birds

But as you probably know, birds are in trouble. Since the year 1500, humans have wiped out 182 species. We’ll never see the Great Auk again, or the Carolina Parakeet, or the Imperial Woodpecker.

You might think 182 species lost over 500 years isn’t so bad. But there are at least 1200 bird species that are likely to go extinct in the coming decades. Because of what humans are doing to them. That’s about 11% of all bird species. And that’s not counting the many species across the world whose populations are rapidly declining, even if they aren’t facing imminent extinction.

We are losing bird diversity much faster than it is being replaced by evolutionary and ecological mechanisms. 

You’re reading this, so perhaps I’m preaching to the choir here. We still have a chance to save the vast majority of our wonderful bird species. Each of us should do what we can to help these amazing creatures and the ecosystems they live in.

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