How Do We Define Bird ‘Species?’

What exactly is a species? The answer isn’t as simple as you might think.

November 29, 2020
Ivan Phillipsen

Reasons we categorize organisms into species

Probably ever since humans started to use language we’ve had a desire to name things and put them into categories. We might have started with the kinds of fruits we could eat and the different animals we hunted. Then there were types of trees to distinguish, based on their utility for making tools and shelter. There were good reasons to name different kinds of rocks, medicinal plants, dangerous insects, and so on.

This primitive impulse combined with our more recent motivation to classify “all the things” brought us to the monumental task of identifying every unique form of life on Earth. Birds being the most important, of course. This is what countless scientists around the world are hard at work on.

There are several practical reasons, from the perspective of science. First off, the species is considered a fundamental unit of taxonomy and biological diversity. When we assign a genus and a species name to a bird, we’re placing that bird within the greater framework of the great tree of life. A species’ classification is our best guess about its evolutionary history and relationships to other critters.

The species as a taxonomic unit is the focus of much evolutionary theory and ecological research. If we’re going to study these aspects of biology, it’s pretty helpful to have names for our species. A lot of what we’ve learned about life on our planet has come from viewing it at the level of the species.

Having identities for each of our planet’s species is also super important for effective communication about them—communication among scientists, for sure, but also educators, conservation workers, government officials, and nature enthusiasts like birders.

Naturally, we can best communicate about a species when we all agree on its distinctiveness and on what to call it. Before we came up with any kind of standardized names for species, we had only local or “common” names to work with. This could be messy.

The more widespread a bird or other animal, the more local names it's likely to have. Swainson’s Hawk, for example, has a range that spans vast portions of North and South America. It’s scientific name is Buteo swainsoni, but it goes by several other common names, depending on which country or culture you’re talking about: busardo chapulinero, aguilucho langostero, or gavilán de Swainson.

Local or common names for birds are still used in many places. And that’s all fine and dandy. But thankfully we also have our scientific names for birds. At least in principle, scientific names are standardized and recognized across the boundaries of countries, languages, or cultures.

Another big reason we identify and name species has to do with conservation. How we classify a bird is often what determines whether or not it can be protected by conservation laws. Laws like the Birds Directive in the European Union and the Endangered Species Act in the United States. Sometimes a bird or other organism will get legal protection only if it’s considered a full species, rather than just a subspecies.

Take, for example, the Socorro Parakeet (listen to call), Psittacara brevipes; it is found only on its eponymous island, 200 miles off the west coast of Mexico, in the Pacific Ocean. This small, isolated landmass is the peak of an old volcano, reaching almost 4,000 feet above the surrounding sea.  In other words, it’s an endemic species on the island.

Socorro Parakeet (Psittacara brevipes). Photo by Ignacio March Mifsut.

When it was first found it was considered a subspecies of the similar looking Green Parakeet, Psittacara holochlorus. There are many thousands of Green Parakeets across Mexico and Central America. It’s not considered a threatened species, at least not at the Global Level. As I said, The Socorro Parakeet is now considered a full species by some authorities. Surveys suggest that only a few hundred Socorro Parakeets are left. Because this species has such a small population size and a tiny geographic range, ornithologists are making the case that it needs protection.

So, if the Socorro Parakeet was classified as a subspecies, some people might think it doesn’t need any special protection, legal or otherwise. These cold-hearted people could argue that since the parakeets of Socorro Island are just a small population of the abundant Green Parakeet, it would be no big deal if they got wiped out.

What is a species?

A species comprises ‘groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.’ Phrased another way: a species is a group of organisms that can successfully interbreed and produce fertile offspring.’ This is just one way to define a species. It’s called the ‘Biological Species Concept,’ and it’s the definition you’re most likely to find in a biology textbook. There’s a good chance you’ve heard it before.

The Biological Species Concept was formally introduced in 1942 by Ernst Mayr, one of the 20th Century’s most famous evolutionary biologists. He also happened to be an ornithologist. At the heart of Mayr’s species concept is this idea of reproductive isolation.

Simplistically, it works like this: if two organisms can mate to produce healthy, fertile babies, then they are members of the same species. But if their offspring aren’t fertile, that means the original parents are not of the same species.

The classic example is a female horse mating with a male donkey to produce a mule. Mules are almost always infertile. So, if we apply the Biological Species Concept, we must conclude that horses and donkeys are separate species. Mules, then, are hybrids between the two.

For a couple hundred years leading up to the introduction of the Biological Species Concept in 1942, biologists and naturalists had been swarming across the planet, collecting, identifying, and naming species.

Those early naturalists looked mostly at the physical, the anatomical differences between organisms when deciding which were different enough to be treated as separate species. And since nature is positively bristling with diversity, they found lots of opportunities to divide and subdivide groups of organisms into more and more species.

With respect to birds, the naming frenzy brought the global number of species to a staggering maximum of 18,937 by 1909. I say staggering because remember that today we recognize only ten or eleven thousand bird species.

Only a few years after the scientific community embraced Mayr’s idea, enthusiastic taxonomists had rearranged the world’s birds into a mere 8,616 species. The trend had become to lump variable geographic forms of birds into single species, reassigning many former species to the subspecies level. The main criterion behind these decisions was, “Reproductive isolation, yes or no?” If “no,” lump ‘em and call ‘em subspecies.

In recent decades the number of bird species has been creeping upward again. This is happening in part because we’ve recognized some limitations of the Biological Species Concept. Some species previously lumped under that concept have been split once again.

Also, the data we can collect from birds has become more varied and rich. A lot of this is genetic data. These days it’s relatively routine to sequence whole genomes from multiple birds to work out their evolutionary relationships.

And biologists have come up with other species concepts. The Biological Species Concept is no longer the only game in town.

For more than a hundred years, our lists of bird species have been expanding and contracting, accordion-like, as we lump species together or split them apart. There’s a chance ornithologists will someday recognize many more bird species than they do right now. But if this happens it will be a slow grind. Scientists are conservative and skeptical, as they should be. They don’t want to just name species all over the place, willy nilly.

The Species Problem

You and I aren’t alone in feeling like there has to be more to identifying bird species. More than just determining—or making assumptions about—which birds are reproductively isolated. Biologists have struggled to define species for a long, long time. Over 80 years before there was a Biological Species Concept, Charles Darwin wrote, “No one definition of species has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species.”

This uncertainty became known as “The Species Problem.” In 2001, the evolutionary biologist Jody Hey wrote, “The species problem is the long-standing failure of biologists to agree on how we should identify species and how we should define the word 'species'.”

The Biological Species Concept was a helpful step forward in resolving the Species Problem. But it does have some limitations. Let’s give those a quick look, shall we?

Problems with Biological Species Concept

The test of specieshood here is whether or not two birds or whatever can reproduce successfully, right? It might not be too hard for ornithologists to apply this test to birds that have overlapping distributions and that share the same habitat. Careful observation alone might be enough to figure out which birds can or can’t interbreed.

But there are many, many scenarios, where applying the test of reproductive isolation is either: (A) super challenging, (B) probably unethical, or (C) just plain impossible.

Let’s say you have two populations of birds, each living on a separate island. As far as you can tell, all these birds are identical except that their tail feathers are blue on one island and green on the other island. The islands are close enough that the birds might be able to fly back and forth between them, even if only rarely.

Now you want to figure out if the two populations represent two species or just one. How do you determine if a bird from one island can—at least potentially—breed with a bird from the other island to produce fertile offspring?

There are several ways you might confirm this, most of which will take lots of time, hard work, and probably money. More often than not, ornithologists have opted to just make educated guesses about the reproductive isolation of birds in situations like this.

Another problem has to do with hybrids. Back in the day, zealous supporters of the Biological Species Concept would lump together any two types of bird into a single species if those birds could produce fertile hybrid babies. Even the tiniest amount of successful hybridization, they argued, was enough to prove that two bird populations belong to the same species.

Is that approach reasonable? I mean, to consider two birds as separate species, how much hybridization between them is too much? In recent decades, we’ve learned a lot about natural hybridization in wild birds. First off, it turns out that about 10% off all bird species hybridize with other species. This is way more common than we once thought.

We now have more data and better tools, like genetic analysis, to look closely at how hybridization really plays out in birds. It turns out there are a lot of nuances to hybridization and we just don’t have a “black-and-white” way of looking at hybrids when applying the Biological Species Concept.

There’s another scenario, called a cline by Julian Huxley, where this species concept fails to be helpful. Say we have a widespread bird whose populations freely and happily interbreed with their neighboring populations. Except that this isn’t true for populations at the far ends of the bird’s geographic distribution. Birds from each end don’t interbreed with each other.

Do birds living at the two ends of this vast, continuous distribution belong to one species or two? We can’t easily answer that question with the Biological Species Concept. Granted a scenario like this may be uncommon in nature, but it does happen.

So those are some big limitations of the Biological Species Concept when applied to birds. But you should know that it also fails when we try to use it with prehistoric animals or to asexual critters like bacteria.

Other Species Concepts

Very smart biologists have been trying, for years and years, to come up with a better species concept. Maybe one that can be applied to every living thing, unlike the Biological Species Concept.

We now have literally dozens of different species concepts to choose from.

Thankfully, most ornithologists these days rely on just a few well-supported species concepts as they go about the business of identifying and naming bird species. The Biological Species Concept is still one of the most widely used, despite the problems we just discussed. It works pretty well in many situations. Before applying this or any other species concept, a biologist will probably have reasons to suspect that two seemingly similar organisms might be different species.

Despite all our modern progress, the starting point for identifying bird species is generally the same as it’s been for hundreds of years. An ornithologist looks at birds from two populations and notices that they seem different somehow. Maybe they have some subtle differences in plumage. Or their songs sound a bit different, or their feeding behavior. Intrigued, our ornithologist might bust out a measuring tape and some calipers to collect hard data from birds in both populations.

If the two populations appear to be identical across all the measured dimensions, that might be where the investigation ends. Conclusion: they’re the same species.

But, if there are measurable, quantifiable differences... well then, it might be worth a closer look. Once upon a time, back in the 1800s, finding a few morphological or behavioral differences was all you needed to distinguish new species.

In those days, biologists and naturalists were applying what we would call a Morphological Species Concept. This approach is still helpful in some cases, such as identifying prehistoric species, since the only data we have for them comes from fossils.

As for identifying living species these days, our standards are more rigorous than they were in the 1800s. Morphological and behavioral data often need to be scrutinized in combination with genetic data, geographic distributions, and evidence of breeding compatibility, or the lack thereof.

So you’ve got a bunch of data like this and it suggests that your bird of interest is distinct. This is the point where you’d need to decide on which species concept to apply.

If you don’t want to use or for some reason can’t use the Biological Species Concept, your best alternative is probably the Phylogenetic Species Concept. This one was introduced in the 1980s. Some ornithologists would argue that we should always use the Phylogenetic Species Concept and do away with the crusty old Biological Species Concept.

A phylogenetic species is a single evolutionary lineage that stays cohesive, close-knit, in both space and time. A phylogenetic species is one small branch on the Tree of Life. Its members all share at least one unique, inherited feature that separates them from members of other groups. That trait might be a plumage pattern or eye color, it might be a song phrase, or it might be a sequence of nucleotides in a gene.

The Phylogenetic Species Concept has a strong focus on the history of a species. It’s about how much genetic connection there was in the past. The Biological Species Concept, by contrast, is more about what’s going on today, with respect to who can successfully breed with whom. Reproductive isolation in the present is mostly irrelevant in the Phylogenetic Species Concept.

I should point out that these two species concepts aren’t always at odds with each other. They’re not necessarily mutually exclusive and they frequently point to the same answer.

The Phylogenetic Species Concept has its pros and cons, which shouldn’t surprise you. One interesting challenge is the question of which unique, inherited traits should we use to define species this way? If we’re using genetic data from DNA sequences, for example, what is the minimum amount of genetic difference we’re willing to use to separate species? These aren’t easy questions to answer.

Under the Phylogenetic Species Concept, if a population of birds has a unique trait, it’s a species. Many of what we call subspecies right now might be reclassified as full species if ornithologists around the world suddenly embraced the Phylogenetic Species Concept.

In such a hypothetical situation, the Dark-eyed Junco here in North America could go from being one species to 15 overnight. Likewise, the European Robin could get split up into 8 species and the Superb Fairywren in Australia could become 6.

Naming Committees and Such

As I mentioned earlier, there are several official checklists of the world’s bird species. Four to be exact. All of the lists have between 10 and 11 thousand species. The biggest difference in length between any two of these lists is 956 species. Not only do the absolute numbers of species differ, but the actual identities and names of the species are sometimes different from one list to the next. Why? It’s because this is science, and science can be messy. What we know about nature is constantly changing and getting updated as we get more data; as we improve our methods of analysis. But that’s okay, you know, because they’re all working hard to get our birds sorted out. Through this slow and sometimes painful process we’re moving closer to understanding how things really are.

Each of the four major bird checklists has a scientific committee or group of editors that makes decisions about what species to add or remove with each update. They consider peer-reviewed published research and other information as they make these decisions. Each committee has codified guidelines for their process. Some lean more heavily on one particular species concept than another. Some are more conservative with splitting species, others are more liberal.

Most if not all of these committees are moving towards a multidisciplinary approach to identifying and classifying species. It’s called ‘integrative taxonomy.’ Multiple sources of data are brought together to make decisions about species. Morphology, genetics, behavior, ecology, and geography can provide complementary perspectives in the integrative taxonomy approach.


Remember the Socorro Parakeet? Well, in a 2017 study published in the Journal of Ornithology researchers used genetic data to construct a tree of evolutionary relationships between that bird and it’s closest relatives, the other so-called subspecies of Green Parakeet. This tree—this phylogeny—showed that the Socorro birds all belong to one branch of that tree, to the exclusion of the mainland birds.

The authors of this study make a case for the Socorro Parakeet getting full species status. The genetic data supports this. These island birds also have longer tails and lack the little orange blotches in their plumage that other Green Parakeets have. So there’s some morphological evidence there too.

The authors suggest that by applying the Phylogenetic Species Concept, the genetic and morphological data show that the Socorro Parakeet should be treated as a species, not a subspecies. They also remind us that this bird is geographically isolated on its far-flung island. So in effect, it’s reproductively isolated.

For another example, take the wren. There’s a tiny brown ball with feathers known as the Eurasian Wren. It has an astonishingly loud and complex song for such a small bird.

Eurasian Wren (Troglodytes troglodytes). Photo by Szymon Bartosz.

Here in North America, we have two wrens that are very much like the one in Europe. We have the Pacific Wren, which I have in my backyard, and the Winter Wren, which lives east of the Rocky Mountains.

These three species look so much alike that, not long ago, they were thought to be only one species: Troglodytes troglodytes, the Winter Wren. But back in the early 2000s, a couple studies (1st publication) were published (2nd publication) on the genetic relationships among populations of the bird formerly known as the Winter Wren. It turned out that there were deep genetic divisions between some of this wren’s populations, which were spread across the northern hemisphere.

One of these studies focused on a field site in British Columbia, Canada. There, the western and eastern subspecies of North American wrens came into contact, sharing the same coniferous forest habitat. The researchers in that study measured the genetic differences of multiple birds in this contact zone. But they also measured differences in the birds’ songs. The results revealed major differences between the two subspecies, in both their genetics and their songs.

So even where these subspecies lived side by side, their songs were so different that females of one subspecies probably had little interest in the males of the other subspecies. Interbreeding, it seemed, was rare.

Long story short: in about 2010, the checklist committee overlords elevated the two American subspecies to full species status. They became the Pacific Wren (Troglodytes pacificus) and Winter Wren (Troglodytes hiemalis). Birds in Eurasia kept the original scientific name and their updated common name is either Eurasian Wren or Northern Wren. The Biological and Phylogenetic species concepts both came into play in the decision to split that one species into three.

These little brown birds provide good examples of what we call cryptic species. At least to human eyes, these wrens are indistinguishable. We thought they were all one species, but good data and careful analysis allowed us to throw back the curtain, revealing the hidden diversity that was there all along.

And there might even be several more cryptic species masquerading as the Eurasian Wren. There’s published genetic data that supports this. So in the future our checklists might split this wren into four or more species.

With cryptic bird species, we have a single form or morphology that is in reality multiple species, multiple genetic lineages. But sometimes we have the reverse situation: birds with strikingly different appearances that apparently belong to one species—a single genetic lineage.

A great example of this “multiple appearances, one lineage” scenario is seen in a couple of cheery little songbirds from North America: the Blue-winged Warbler and the Golden-winged Warbler.  These two species are the same size and shape, but their colors are dramatically different and they sing different songs.

But unlike the wrens we just talked about, these two warblers hybridize frequently where they come in contact. Their hybrid offspring are fertile. The genomes of the Golden-winged and Blue-winged warblers show evidence that their interspecies trysts have been going on for thousands of years. Hybridization is so common between these little dudes that two of their hybrid forms have their own names: Brewster’s Warbler and Lawence’s Warbler.

Given all this interbreeding, shouldn’t we be treating the Blue-winged and Golden-winged warblers as one species? The answer is “yes” if we were to adhere to the old-school version of the Biological Species Concept. But, as far as I know, these birds have always been treated as two species.

Now here’s where this gets really interesting: Initial genetic studies of these birds couldn’t find any real differences between the two species. Perhaps not surprising given how much they hybridize. However, there had to be some differences, because we know plumage colors are controlled by genes.

Blue-winged warbler (Vermivora cyanoptera). Photo by Wwcsig.

Finally, a 2016 study of these birds published in the journal Current Biology compared their entire genomes. That’s an enormous amount of data. This study found that there are indeed some genetic traits unique to each species. But this amounts to only a teensy weensy bit of difference across the two genomes. The Golden-winged and Blue-winged warblers are 99.97 percent the same at the DNA level. It seems the few regions of their genomes that are different are, as we might expect, associated with feather pigmentation.

So before this recent study using genome data, the available genetic data combined with all that freewheeling hybridization might have impelled us to lump these birds as one species. But now that scientists have found a few genetic traits which are unique to each of the two warblers, the Phylogenetic Species Concept can come to the rescue and provide justification for keeping them separated.

That’s a good thing for the Golden-winged Warbler because it’s populations are declining rapidly due to habitat loss. Continuing to recognize this bird as its own species will help it keep its legal protections in the US and Canada.

Get email updates about new podcast episodes, articles, and other bird-related goodies!