Eastern Meadowlark

BirdNorth America

19 Aug

Sturnella magna

The eastern and western meadowlarks look nearly identical, behave in the same way, and share similar habitats — even overlapping in range in the central plains of North America — yet they are separate species that rarely interbreed. What keeps them apart are the different songs they sing.

The Meadowlarks

In the eastern half of North America, an unmistakable songbird flits through farmland and open country. It bears a black ‘V’ across its yellow chest, dark-streaked plumage, and white tail feathers. A familiar sight from fenceposts in Kansas to cornfields in Ohio.

In the western half of North America, a songbird that looks almost exactly alike — black ‘V’ on its chest and all — likewise lives in open country, in this case from the prairies of Alberta to the deserts of New Mexico.

These are the meadowlarks.¹

They look the same: mid-sized, with nearly identical plumage and long, thin beaks.²
They eat the same: foraging for an insectivorous fare of beetle grub and grasshoppers.
They make the same nests: shallow cups woven into the grass, often with roofs fashioned from bent stems.
They prefer the same habitat: open country with tall grasses and wide horizons — even overlapping where their ranges meet.

An eastern meadowlark (*Sturnella magna*).

A western meadowlark (*Sturnella neglecta*).

And yet, they are two separate species: the eastern meadowlark and the western meadowlark.

Species by Way of Song

The biological species concept is the most commonly used metric for defining species. It states that if two populations don’t interbreed, or very rarely do — their gene pools don’t mix, in other words — then they constitute separate species.

And so it is for the meadowlarks of North America. Even though they — eastern and western — share territory on the Great Plains of Nebraska and Kansas, and along the western edges of Iowa and Missouri, where grasslands and prairies blend, the two species do not. There is barely any interbreeding.

But how do they manage that, when even up close, telling one from the other is all but impossible?

We humans are a very visual species: you see it in our language, in our art, and in the way we go about choosing our partners. Whether shallow or profound in our personal character, the first thing we generally notice about someone else is their appearance. From that first glance, knowing nothing but how the stranger looks, it's possible to form an attraction to the person (although that can obviously change later, when we get to know them).

In terms of our sensory world, then, we are very similar to birds. They, too, are visual animals. Rock doves puff up their iridescent necks and strut around pompously; birds-of-paradise perform elaborate dances and shape-shifting displays that show off their most dazzling features; northern cardinals judge potential mates by the redness of their feathers and Inca terns do so by the plumpness of their “moustaches”; meanwhile bowerbirds build intricate stages and structures, decorated with various colourful objects — berries, shells, and plastic bits — to impress the eye of a female. But not every ave is a peacock.

An eastern meadowlark singing its clear, whistling song.

There’s an entire group of birds, well over 5,000 species, that we call songbirds. They’re not named so without reason, and the reason for their elaborate songs (mostly sung by males) is twofold: to protect their territory from intruders and to attract a mate. Meadowlarks are songbirds. And so, for one meadowlark to tell its own species from the other, all it has to do is listen.

The song of the eastern meadowlark is a clear, whistled melody; simple and flutelike, but varied, with a repertoire of 50–100 songs. The song of the western meadowlark, by contrast, is more complex and bubbly, a rich warble full of slurred, gurgling notes that sound almost like an improvised medley. To the distinguishing ear, they do not sound like the same species, and for birds that choose their mates by song, that singular difference really matters.

Nearly identical in every other respect, the songs of the eastern and western meadowlarks are distinct, and that is enough to separate them as species, even where they mix in space.

On the Origin of Species — and Their Persistence

In what is perhaps the most famous book in all of science, Charles Darwin’s On the Origin of Species lays out the foundation of evolutionary theory, but touches surprisingly little on the titular question: how do species form in the first place?

Why are there eight species of bears, instead of just one or two? Or over 400,000 beetle species — did we really need that many? And how or why did the eastern and western meadowlarks become two distinct species?

It is widely accepted that, in most cases, for two (or more) species to form from a single species, some kind of geographic separation is required. This most common type of speciation, known as allopatric speciation, occurs when a physical barrier — be it a mountain range, river, or ocean — splits a single population into two or more populations that can no longer exchange their genes.

The Congo River first formed between 1.5 and 2 million years ago, cutting through the centre of Africa. At around the same time, the divergence between two primate populations is believed to have begun; two populations that would eventually become bonobos (only found south of the river) and chimpanzees (only found to the north).

The Kaibab squirrel, a unique subspecies of tassel-eared squirrel, lives on the Kaibab Plateau and the North Rim of Grand Canyon National Park, while the rest of the subspecies live on the South Rim. Despite what may appear to be the obvious cause of the split, it wasn’t the gaping canyon that created the isolation, but climate change. During an ancient warming period, ponderosa pine forests, which the squirrels depend on, retreated to the cooler highlands of the Kaibab Plateau, cutting the population off from those to the south. When the climate later cooled and the pines returned to the lowlands, the squirrels on each side had already diverged into distinct subspecies.

The African Great Lakes — especially Victoria, Tanganyika, and Malawi — are home to a dazzling diversity of cichlid fish, shaped by climatic and tectonic events that periodically split the lakes into isolated basins before reuniting them. Across the Caribbean, various species of anole lizards evolved when their ancestors arrived on another island by natural raft and were thereupon stranded. Many of the Galapagos Islands, those famous laboratories of evolution, host their own unique finches, mockingbirds, and tortoises.
Isolation is what makes islands — and even lakes are islands in this sense — such great incubators for new species.

Chimpanzees (*Pan troglodytes*) above and bonobos (*Pan paniscus*) below.

A Kaibab squirrel — a subspecies of the tassel-eared squirrel.

Top to bottom; the cichlids of the African Great Lakes, some of the anoles of the Caribbean, and a few of the Galápagos finches.

What split the meadowlarks? While there’s been no in depth study, the most probable cause were the glacial cycles of the Pleistocene, which fragmented the grassland ecosystems into isolated refugia, separating meadowlark populations across eastern and western North America. Over a long period of isolation, different mutations arise and persist in the separated populations, either through adaptation to different conditions or random genetic drift. These gradual changes create what we may eventually judge to be distinct subspecies and, if enough differences accumulate, entirely separate species.

At some point, the geographic barrier that initially separated the populations into species may disappear, or the species themselves might migrate, leading to an overlap in ranges where they live side by side — as happened with the meadowlarks.

Wouldn’t they just interbreed again, then, eventually melding into a single species again? The meadowlarks don’t seem to. But what’s stopping them? Why is a different tune so repulsive? Why don’t they go in for some cross-species karaoke?

Hybrids: Vigour & Depression

When two different species that can interbreed do interbreed, their offspring can turn out less fit. And by fit, I mean that they’re less likely to survive and successfully reproduce. A mule — the offspring of a donkey father and a horse mother — is plenty “fit” in the regular sense of the word, known to combine some of the best elements of its two parent species (a phenomenon known as hybrid vigor). But it is completely unfit in the Darwinian sense, given that mules are almost always sterile.

A mule is a sterile hybrid between male donkey and female horse.

In this particular case, sterility is due to a mismatch in chromosome counts. When a horse, with 64 chromosomes, pairs up with a donkey, which has 62, their mule offspring inherits 32 chromosomes from one parent and 31 from the other, totaling to an odd number: 63 chromosomes. Because one chromosome lacks a proper partner during meiosis, the mule can’t form viable eggs or sperm, rendering it infertile.

Less extreme genetic incongruities can still lead to reduced fitness. Even when two species are close enough to produce fertile offspring, their hybrids might still suffer in other ways.

Known as outbreeding depression, it’s not a great sadness felt by a mixed-parent offspring, but the effects of two inherited genomes not working well together. Unlike mules and their hybrid vigor, a particular mix of genes could also result in a worse biology: a weaker immune system, poorly regulated growth, or a metabolism unfit for any particular environment. Indeed, outbreeding depression often occurs when the two parents come from different environments or fill different ecological niches. So their offspring, inheriting the traits that are intermediate between their parents — or a mix of traits that don’t complement each other — is neither fully adapted to exist as one species nor the other. A coat that’s too thin for the cold, but too thick for the heat. Teeth unsuited to either parent’s diet. A song too strange to attract mates from either species.³

The hybrid belongs to neither his mother's nor his father's kind; a worst-of-both-worlds scenario.

No wonder it's called a depression.

Attraction

But how to avoid such depression?

Sometimes, mechanisms to prevent interbreeding are in place when two closely related species collide for the first time (since their divergence). Perhaps some change in their anatomy makes the act of mating impossible, or maybe their breeding seasons have shifted and no longer overlap.

But what if the two species can interbreed, and their hybrid offspring pop out with a bad case of outbreeding depression?

It is in the interest of most animals to produce the fittest offspring possible. Often, then, it is also in their interest to prevent hybridisation. Of course, neither the individuals, nor the species as a whole, make a conscious decision — animals aren’t speciesist eugenicists trying to keep their bloodlines “pure”. But, over time, certain mechanisms would naturally evolve to prevent hybridisation.⁴

Let’s say that there are two closely related bird species that have recently begun to overlap in range. They can interbreed and produce offspring, but those hybrids are less fit than non-hybrid offspring. Individuals who mate indiscriminately between species waste time and energy producing less-fit hybrids who are unlikely to pass on their genes. Meanwhile, individuals who preferentially mate with their own species produce fitter offspring who outcompete hybrids and contribute more to the gene pool. Over generations, those with stronger species-specific mate preferences leave more descendants, reinforcing the preference for mating within the species.

A western meadowlark singing its more bubbly, warble-like song.

Discrimination can emerge via various mechanisms. Perhaps some visual distinction between the two species becomes a focus of courtship. Say, one species has red wing feathers and the other yellow. Both species evolve a preference for mates with their own feather colour and an aversion to the other. Over generations, that preference becomes increasingly important when choosing a mate, likely leading to a physical change in the species too — an exaggeration of feather colours that makes discrimination easier. Of course, to the birds themselves, this likely just manifests as a feeling of attraction to other individuals with clearly coloured red/yellow feathers.

But the distinguishing trait need not be visual. It could be a difference in a courtship display, a change in song. These, too, would follow the same pattern: even if all else about the two species stays the same, whatever traits prevent unwanted hybridisation are likely to diverge more and more as preferences strengthen.

Eventually, genes that code for that discrimination — given that discriminating individuals are more reproductively successful — will fill the gene pool of each of the two species, and the behavioural mechanisms that prevent interbreeding become ingrained. The genepools are separated, the species are isolated, even if they live side by side.

For the eastern and western meadowlarks, intermingling at their borders, that barrier is their song.

¹ There are eight species of meadowlark, split between two genera in the family Icteridae (New World blackbirds). The genus Sturnella comprises the three North American species (including the Chihuahuan meadowlark), while Leistes is made up of five species, mostly red-and-black in their plumage, and mostly from South America. They are all insect-eating grassland birds.

A Chihuahuan meadowlark (*Sturnella lilianae*).

² There are very slight physical differences between the two: the eastern has darker head stripes and a mostly white, instead of yellow, “moustache”. It also has more of a white tail when in flight, compared to the western.

There’s also the Chihuahuan meadowlark, which was split from the eastern species in 2022 because of genetic differences and its own distinctive song.

³ For example, in many places, feral domestic dogs breed with wild coyotes or wolves and produce hybrid offspring. Those offspring, taking on some traits of their domestic parent, often fare worse in wild environments: their abilities to hunt, their fear response, and sociability may all be diminished.

There’s also a flip side. When it comes to surviving in a more human-dominated environment, domestic-wild hybrids often fare better than their pure wild counterparts.

⁴ Those isolating mechanisms don’t always evolve or hold. When one species is super-abundant, and another is super-rare, there’s a high chance that the rare species will be “absorbed” into the abundant one through interbreeding and hybridisation. This genetic swamping can be seen in several cases: common coyotes hybridising with rare red wolves, domestic dogs with Ethiopian wolves, domestic cats with Scottish wildcats, and pied stilts with critically endangered black stilts.

**Where Does It Live?**

⛰️ Open country with tall grasses.
📍 Most of the eastern United States, through Central America, and northern South America.

‘Near Threatened’ as of 27 Aug, 2020.

IUCN

Size // Small
Length // 19 - 26 cm (7.5 - 10 in)
Weight // 90 - 150 g (3.2 - 5.3 oz)
Activity: Diurnal ☀️
Lifestyle: Social👥
Lifespan: Around 5 years
Diet: Omnivore (primarily insectivorous)
Favorite Food: Insects 🦗
Class: Aves
Order: Passeriformes
Family: Icteridae
Genus: Sturnella
Species: S. magna

The eastern and western meadowlarks live in open country with tall grasses and wide horizons, forage for insects like grasshoppers and beetle grubs, and make shallow-cup nests out of woven grass. In almost every way, they are identical.
The western species was first discovered in 1805 by explorer Meriwether Lewis, who thought (understandably) that it was the same species he’d seen in the east. It was only described as a distinct species some 40 years later, after a suggestion by John James Audubon — and it was given the specific name of neglecta.
Aside from (very) slight plumage differences, the main differentiator between species is their song. The song of the eastern meadowlark is a clear, whistled melody; simple and flutelike, but varied, with a repertoire of 50–100 songs. The song of the western meadowlark, by contrast, is more complex and bubbly, a rich warble full of slurred, gurgling notes that sound almost like an improvised medley. They sound like different species.
The two species share territory on the Great Plains of Nebraska and Kansas, and along the western edges of Iowa and Missouri. But where the grasslands and prairies blend, the two species do not. It’s likely that they’re kept from interbreeding by their different songs. But why are they so averse to a bit of cross-species karaoke?
When two different species that can interbreed do interbreed, their offspring can sometimes turn out less fit — less likely to survive and successfully reproduce — a phenomenon known as outbreeding depression. That may be due to some incompatibility in the parents' genomes or physiologies, or the fact that mixed offspring are simply not well adapted to survive or reproduce as either species. (The opposite phenomenon, known as hybrid vigour, occurs when hybrids happen to be more “fit” than non-hybrids).
What split the meadowlarks initially? While we don’t know for certain, the most probable cause was the glacial cycles of the Pleistocene, which fragmented the grassland ecosystems into isolated refugia, separating meadowlark populations across eastern and western North America.
Over a long period of isolation, different mutations arose and persisted in the separated populations — the meadowlarks evolved different songs that effectively isolated their genepools, and so, despite their similarities, they are considered separate species.