It’s the first week of May, a warm spring, and you’re walking in Central Park in New York, maybe on the way to the American Museum of Natural History, because of the whale, and there’s an oak tree, and maybe you stop for a moment because the sun is warm and you want to let it soak into the nape of your neck. You are looking up at the oak tree, and something catches your eye, several somethings, and it becomes clear there are numerous small birds on the lowest branch of the tree. You stop, and maybe your eyesight is good enough to appreciate the kaleidoscopic array of colors on these minute creatures, and marvel for a moment that they (and you) exist. You walk on.

Noticing all those birds is a bit out of the ordinary, perhaps, but it’s a plausible variation on what is likely our most regular and easily dismissed encounter with the sublime. We see birds almost every day of our lives, and if we’re paying attention it’s hard not to summon a little wonder at them, their seemingly effortless flight and articulated musculature and weightless skeletons and piping song, sweet or raucous.

Wonder may be a common reaction, but how we channel it tends to split between two dominant modes of birdwatching, which I’ll call the aesthetic and the statistical. Birdwatching is a classic source of aesthetic pleasure and inspiration, the topic of countless poems and paintings and documentaries with soaring string soundtracks. Taught minimalist Chinese poetry. People love watching birds pastime for aesthetic reasons, because observing these beautiful little animated creatures is enjoyable and rewarding and because the practice belongs to our shared cultural heritage. There’s a large literature¹ of people marveling at birds in refined ways, celebrations of form and flight and grace that present themselves as the purest way of appreciating nature.

However lovely and true that may be, I’m more interested in the aesthetic mode’s baser twin, the statistical mode, which revolves around the practice of “listing.”

Illustration by Antonia Stringer

Listing is birdwatching at its most quantitative. It involves dividing that cloud of small birds in Central Park into discrete species, and tallying them. And like many quantitative pursuits, listing has a competitive side: 670 species seen in North America (that’s good). 12 warblers in a single spring morning in a New England orchard grazed by faint spring morning light (also good). 90 species in a year (you're slacking). In Britain, there is a wonderfully apt word for hobbyist birdwatchers of this inclination, “twitchers,” though I suspect it’s one of those social labels—like “hipster”—that no one actually claims for herself. Who would cop to appreciating birds only as digits? Twitchers are wont to do things like travel from London to the Hebrides to see a Yank (a vagrant North American species) and make their species list one point larger. Because of things like this, and the sense that twitchers are somehow missing the finer aspects of their hobby, trivializing birds into some sort of game, they are often viewed with mild derision and mistrust by outsiders. Yet despite this stigma, the practice flourishes, even inspiring a bad Jack Black movie (The Big Year). People like to count birds.

Unfortunately, the raw metric of achievement here lies on intellectually shaky ground, because “species” is a surprisingly problematic concept. Scientists, it turns out, like to count birds, too, but struggle with it more than the average hobbyist, and not necessarily for fear of overlooking their aesthetic attributes. As a unit, the species appears to be self-evident, something we can all easily recognize. But when we attempt to define it, we begin to flounder. Are the Chihuahua and the wolf, for instance, really the same species? Darwin held that species are not philosophically valid entities, but merely the result of the human tendency to perceive discontinuity along a continuous gradient. Science requires delimitations, though, and most people agree that the idea of the species is useful and compelling, and so we continue to try to make it work, over and over again. A recent review listed (count ‘em) 24 distinct concepts, many more with alternate definitions and addenda.

The most widely accepted of this array is the “biological species concept” (BSC), which defines a “species” as the members of a population that actively or potentially interbreed in nature. It’s a powerful and intuitive idea (who better to define a given species than the members of that species themselves?), and it remains the concept most frequently taught to fledgling biologists. But it has its limitations. Asexual organisms, for instance, never interbreed, though they clearly share genetic, morphological, and ecological similarities. Then there are hybrids (e.g., the liger): two apparently distinct species can occasionally interbreed and produce viable offspring, but remain, by and large, separate and distinct. Perhaps most frustrating to those who draw lines between species for a living (taxonomists and systematists) is when two similar-seeming populations are geographically distant from one another, and there’s no way to tell whether they would naturally interbreed—think the Chihuahua and the wolf again, in suburban Texas and the Great White North. In these cases, the BSC fails to offer satisfying conclusions.

These shortcomings have been seized upon by proponents of its primary theoretical competitor, the “phylogenetic species concept” (PSC). The PSC draws finer lines, defining a species as the smallest set of organisms that share a common ancestor and can be distinguished from other sets of organisms—the “tips” of a phylogenetic tree, or “tree of life.” In this framing, what’s important is not the ability to interbreed, but being on a unique evolutionary trajectory. Here, the Chihuahua and the wolf would be considered distinct species, as they are subject to dramatically different selective pressures. Since potential or actual reproductive isolation is not required, the PSC presents a more dynamic view of speciation, one which allows species to emerge and dissolve very rapidly. It’s also a decidedly more modern approach to evolution, both in outlook (everything is relative and subject to change) and methodology (rooted in DNA sequence data). But the PSC avoids answering hard, basic questions about where to draw those lines and provides no useful rule of thumb. It’s a species concept for people who are uncomfortable with the idea of species, or at least with the tidiness of the BSC.

The debate over which concept is “correct” has proved surprisingly acrimonious. The BSC tends to be favored by the old guard of evolutionary biology: usually elderly men at Ivy League schools and their immediate disciples, blessed with a background in traditional natural history and a gift for subtle putdowns. PSC advocates tend to be male (okay, it’s a male-dominated field), a bit younger, more outwardly aggressive, and work at the premier public research universities, usually outside of the Northeast and California (don’t take these stereotypes too seriously). The acrimony may make for a tense and unpleasant atmosphere at times, but it has had the unexpected benefit of enlivening the proceedings (and writing) of academic biologists. After a period during which the PSCers were in ascendency, flaunting in journal articles the claim that their concept was widely used outside of the tweedy (tweety?²) cloisters of ornithology, one gray-haired and an eminent member of the old guard—a formidable, grey-haired Berkeley professor—shot back in print that “Flocks may follow errant shepherds.” Poetry is never far below the surface in the study of life.

In substance, the debate ranges from case studies, which demonstrate the applicability of either concept to a given group of organisms (does the BSC work with plants? what level of divergence makes a species in the PSC and is it different in birds than in snakes?), to more strictly philosophical arguments. These often involve ontological or metaphysical questions, like whether “species” designates a class (a “natural kind,” possessing some essential unifying quality) or an individual (spatiotemporally located, cohesive at any one time, consistent through time), and whether a given concept mistakes grouping (the characters uniting members of a taxon) and ranking (what delimits species?).

The arguments for and against these concepts are passionate and varied because in some ways they present dramatically opposed theories of knowledge and nature. I suspect that, to classical taxonomists, the PSC is troubling because it allows for an almost infinite division of populations into ever-smaller and always unstable groupings. Similarly, I suspect the BSC troubles PSCers in its convenience, its promises of resolution and clear limits in defiance of the dominant messiness of biology as a science.

There have been peacemakers in the fray, notably adherents of “pluralism,” who advocate keeping an open mind, considering the variety of organisms and their evolutionary histories, and who generally deny the preeminence of any one concept in favor of a holistic approach. But even pluralists are subject to bitter criticism (they've been called lazy, insufficiently curious, incompetent, and generally not engaged with science). Because of this, the majority of biologists, even those whose work is intimately concerned with species definitions, try to stay clear of the debate, lest they attract stray vitriol.

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With no disrespect meant to the advocates of the PSC, who are mostly exceptional scientists, I think the hegemony of the BSC is due in large part to the charisma, talent, and experience of its greatest champion and co-formulator, Ernst Mayr. Ernst Mayr was born in Germany in 1904 and died 100 years later in Cambridge, Massachusetts, having become the most important biologist of 20th century and generally earning adulation from all parties in his obituaries. “Despite not visiting an English-speaking country until his twenties, Mayr mastered English as a second language to the point where his English prose style was widely admired for its clarity” wrote Jared Diamond on the occasion of his his passing. Mayr was hugely but carefully prolific; his final book, What Makes Biology Unique?, was published the year of his death. Mayr's talents included an incredible memory, an eye for detail, and the ability to synthesize common elements from disparate fields into elegant, uniform hypotheses. He was also an excellent writer.³ Other than the BSC, Mayr is best known for his contributions to the modern evolutionary synthesis, a mid-20th century fusion of Mendelian genetics and gradual evolution by natural selection. In his work Systematics and the Origin of Species, he clearly articulates the ideas of his contemporaries into a coherent whole. It became a classic in the field.

Arguably the most fascinating period of Mayr’s life came before this reputation had developed, as Mayr was among the last of the classical biologist-explorers. From the mid-1800s until around 1930, the Western scientific establishment exploded outward by sail and rail and steam and mule and canoe and foot into the remotest⁴ corners of the world in a frantic, glorious, and often tragicomic quest to catalogue the diversity of life on earth. Darwin’s Voyage of the Beagle is the best-known record of such efforts, but the genre is remarkably diverse—from formal expeditions by Royal Societies and National Scientific Foundations to ego-driven jaunts by unscrupulous private collectors. Malaria, dried orchids, and native mistresses resulted.

As a young man, Mayr participated in the last of these great adventures of collection and discovery, the Whitney South Seas Expedition. The Whitney Expedition, funded by the American Museum of Natural History, plied the waters of the tropical Pacific. These islands contained what was (and remains) among the poorest-known biota on Earth, and in the 1920s and 30s it was a blank on the map indeed. A 75-ton schooner, the France, served as a roving base for the expedition, and over the course of a dozen years a dozen scientists collected 40,000 specimens from the islands in the region.

Mayr was a participant for two years during a later stage of the project, replacing intrepid but lesser-known collectors Rollo Beck and Hannibal Hamlin. Even in this relatively short span, Mayr demonstrated that he was both tough as nails and an extraordinary field biologist:

During these expeditions, forlorn, at times given up for dead, exposed to tropical diseases and the danger of headhunters, he collected the skins of thousands of specimens, eating the flesh of many. Mayr was not only the ornithologist who probably tasted the largest number of different species of birds, but he also named 26 new species and over 400 new subspecies, more than any other taxonomist.

Naming 426 taxa is, in case you had any doubts, a remarkable achievement (on par with successfully avoiding headhunters), and following the Whitney Expedition this wealth of knowledge fell directly back into his hands when AMNH hired him to curate the expedition’s fruits. For the next half century, he put his skills to use describing, delimiting, listing, and otherwise quantifying the bird species of the Pacific. All of which is to say that (as you might expect of the BSC’s originator) Mayr liked to count birds as much as everyone else, while still being a talented natural historian with a great depth of knowledge in exactly the small details you’d expect someone preoccupied with numbers to overlook.

A preoccupation with numbers seems to come with the territory when you’re interested in birds, whether you’re a scientist or hobbyist. The clearest difference is that scientists, as experts, enjoy certain privileges over the amateur: while one website suggests that bird-listing can “consume lives, ruin marriages, relationships, friendships work/career prospects and decimate any financial savings,” Mayr probably never got a similar warning. Ken Kauffman, in his ambivalent essay on the value of listing, offers by way of explanation for this phenomenon that intention, “as in so many other realms of human endeavor,” separates methodical survey efforts from gratuitous “big days” to log as many species as possible. Here again is a discomfort with listing rooted in the sense that it trivializes something important, that twitchers play games while taxonomists are engaged in serious work. But are the fundamental motivations and rewards of these groups all that dissimilar? I don’t think so.

Birdwatching is external, its rewards intellectual. People like to count birds because tallies give us a more tangible reflection of the activity, a record to return to, bragging rights (in, um, certain circles). But the fact is that those numbers still mean something, no matter how reductive they seem. They represent a huge diversity of form and behavior and ecology, the end points of myriad intertwining and diverging evolutionary arcs. And even as your garden-variety twitcher strives for a numerical milestone, I find it hard to shake the feeling that their paper sheets full of checkboxes and Latin names in italicized ten-point font record something more than just statistics—they're trying, in a way, to articulate what it means to be caught up in a common story of life on Earth, the limits of ourselves and our evolutionary companions.

I suspect there’s something else, too. When I was a young, bird-oriented teenager in Vermont, the spring migration would generate in me a strong desire to list things—think those warblers in that sun-dappled orchard, shooting for double digits. Frost on the grass, up at dawn, my attention honed to something small and trying to comprehend the significance behind differences both subtle and glaring.

Whether you’re a twitcher or a scientist, the pursuit itself feels pretty nice.

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¹ See “An Exhilaration of Wings: The Literature of Birdwatching” for a collection of essays by such luminaries as Muir, Audubon, and Wordsworth.

² Sorry.

³ “Despite not visiting an English-speaking country until his twenties, Mayr mastered English as a second language to the point where his English prose style was widely admired for its clarity” wrote Jared Diamond on the occasion of his his passing.

⁴ My perspective here is necessarily Eurocentric.

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