Thistle & Finch

Against the rubber tongues of cows and the hoeing hands of men
Thistles spike the summer air
And crackle open under a blue-black pressure.

Every one a revengeful burst
Of resurrection, a grasped fistful
Of splintered weapons and Icelandic frost thrust up

From the underground stain of a decayed Viking.
They are like pale hair and the gutturals of dialects.
Every one manages a plume of blood.

Then they grow grey like men.
Mown down, it is a feud. Their sons appear
Stiff with weapons, fighting back over the same ground.

– Ted Hughes, “Thistles”

August. A meadow spreads out before you on a golden, listless evening. You’re somewhere in Ohio – or perhaps it is Oregon, or Vermont, or Alberta. You run your fingers through the grass. Two finches settle, twittering, into a clump of flowers; the flash of bold yellow against the soft purple is picture-perfect. A half-hearted breeze starts up, and a few bits of white fluff drift towards the horizon. The buzzing of a hundred bees puts you in mind of a giant, purring cat. The earth is content, stretching out into the plenteous bliss of late summer. 

A scene like this is surely among the most visceral memories of nature for many Americans. And yet its peacefulness is a mirage. The community described here is a short-term arrangement, an artifact of disturbance, and the writing is already on the wall: the saplings poking up here and there from the meadow foretell its end. If it is not mowed or burned, it will soon enough be swallowed by forest. In the meantime, its inhabitants will jostle amongst themselves to find who can most effectively live the hard, fast, violent, profligate life of the weed. 

At their forefront will be an imposing character that could fairly be dubbed the king of the weeds: a plant whose prowess at this unforgiving lifestyle has won it a vast range across the Northern Hemisphere, as well as a faithful devotee in one of North America’s most beloved animals.

Of the bird species that are familiar to the average backyard enthusiast, most can be characterized in ecological terms as generalists. This is no coincidence. Take the chickadee (that is, any one of the several species of chickadee) as an example. Its modest bill is something like the compact car of eating devices, the utilitarian average of all the many specialized tools wielded by other songbirds: decent enough for snatching caterpillars, decent enough for plucking fruits, decent enough for cracking seeds. Thanks to its versatility the chickadee finds ways to get by throughout the year, when many of its cousins must migrate to meet their needs; thanks to its opportunism, it has happily adapted to the strange new habitats and food sources offered by human civilization. Small wonder that it’s endeared itself to us so thoroughly. 

The American goldfinch – similarly loved, to the point of being tapped as the official bird of three states – breaks this mold in striking fashion. We learn that it is somehow different when we learn that to attract it, we should put out a special feeder with thistle seed. This only hints at the depth of its oddness, as we shall see. But why should this bird have such a particular taste? And how is it able to be so successful, so abundant, in spite of it? What is it about thistle?

The Scots know. As the story goes, an invading Norse army was once staging a surprise attack under the cover of darkness, when one of the attackers stepped on a thistle, howled out in pain, and the jig was up. The plucky herb was promptly enshrined as the national emblem, and to this day the highest chivalric honor a Scot can receive is the Order of the Thistle. Nemo me impune lacessit, goes the motto of the Order, and it captures well the attitude of the plant as well as the people it represents: No one provokes me with impunity.

A weed – that is, a plant that stubbornly grows where it “shouldn’t” – must indeed be able to handle provocation. On top of this, there are a few other crucial problems an organism must solve in order to flourish in this lifestyle. In the topsy-turvy new world of the Anthropocene, where disturbance is the rule instead of the exception, a species that solves these problems well will be poised for a kind of success that would have been unimaginable to its ancestors. The thistle belongs to a group of plants that has solved several of them in unparalleled style, and it adds an innovation or two of its own to round out the list. 

Though the name “thistle” is applied casually to all sorts of unrelated plants, thistles properly speaking make up a taxonomic tribe, the Cardueae, within the sprawling Asteraceae family. This is the family of the ubiquitous daisies and sunflowers, goldenrods and dandelions, and a great many of their domesticated kin that fill our plates and brighten our gardens: lettuce, artichoke, chicory, chrysanthemum, dahlia, zinnia. What they all share is a revolutionary new take on the flower. 

The blossom of the sunflower, or what looks like its blossom, is in fact an elaborate illusion. It is not a flower but a delicately arranged constellation of flowers (florets) of different types, which presents itself to the eye as a unit. Around the edge are the bright yellow ray florets, mimicking the petals of an ordinary flower, but each in fact its own sterile bloom, serving no purpose but to attract attention. And in the center are the many tiny disc florets – as many as a thousand, in some species – which bear the actual machinery of sex. In these species that involves both male and female parts, stamens and pistils, sharing very close quarters within each individual flower. Freed of their obligation to look good, these tiny, ultrafunctional blossoms have made another use of their decorative parts. The petals of each are fused into a miniature tube, to guide pollen out and in with the aid of the pollinator’s tongue; the sepals, ordinarily pressed into service to prop up the petals, are radically reimagined as threadlike bristles that surround the developing embryo.

This bold arrangement has advantages. The sunflower head with its thousand blooms can offer the bee or butterfly a staggering amount of nectar, and can produce a staggering amount of seed. And when it comes to dispersing that seed, while the sunflower itself favors a different approach, many of its relatives unveil a final great trick. The fine bristles around each seed unfurl into a featherlight parachute, a perfect flotation device with which the precious embryo can ride the wind to virtually anywhere. 

The thistles in particular have made a fine art of this wind-riding business. It is no accident that our language boasts the very specific word thistledown. Around each floret of the typical thistle – and these are disc florets only, clustered pinkish or purplish tubes that go for quantity over style – a parachute, or botanically speaking a pappus, forms that is of exceptionally fine design. In the genus Cirsium the pappus is distinguished by the plumelike branching of its threads. This produces an almost netlike parachute that presents a greater effective surface area to the breeze. The simple enhancement is a profound one. The triumphant portfolio of Cirsium includes our field thistle and over a hundred other species, with multiple cosmopolitan superweeds among them.

Each one of the thistle’s tiny wind-riders is an incomparably well-equipped voyager. But it is their sheer numbers that truly set them apart. In certain species, each single thistle plant has been estimated to produce as many as 120,000 seeds. This is a nearly inconceivable number, and in the world of weeds, where the size of the army is all-important, it is a game-changer. How does the plant manage it?

In part, by going all-in on flower density: packing in the disc florets and abandoning the costly rays, while counting on the collective effect of the bright, crowded floral tubes to do the job of catching the eye. (It doesn’t hurt that the purplish hues favored by thistles fall right in the sweet spot in terms of spectral sensitivity for bees, the dominant pollinators across much of the continent.) But in part, too, by taking that very simplest route to the top: being bigger, literally, than all the competition. 

As you know if you’ve ever walked amongst them, the tallest field thistles can easily grow taller than you or me, dwarfing the vast majority of herbaceous plants that share their habitats. The perks of this kind of stature are not hard to grasp. You have all the sunlight you could want; you are stupidly easy for your pollinators to find; and you have a nicely elevated launching pad for your barrage of airborne progeny. The fact that the thistles stick out, that size like this is rare, suggests that attaining it must not be easy. 

When it comes to scheduling their lives and orchestrating the business of reproduction, plants generally follow one of two broad strategies. To simplify things crudely: annuals, like most of our food crops, prioritize making babies over personal growth. Each individual completes its life over the course of a single growing season, building up its body only as much as is necessary to go out with a fruit-laden bang at the end. In unpredictable environments like our successional meadow, where next season can’t be counted on to resemble this one, this strategy often tends to predominate. Perennials by contrast go the slow-and-steady route, investing in vegetative structures as support for the long haul, and going dormant during the off-season. In choosing one of these pathways a plant species can prioritize size or fecundity, but typically not both.

The thistle, like many other successful herbs of temperate climates, has found a third way. Its strategy is neither annual nor perennial but biennial, a compromise, playing out over a period of two years. The first growing season is dedicated to building a strong vegetative foundation, and ends with a stout taproot (up to two feet deep) and a robust “rosette” of leaves at ground level. As soon as its second spring arrives it begins to bolt, using last year’s resources to shoot towards the sky like a magic beanstalk. The first year’s prudence gives the plant a vital leg up over its neighbors in the second. The prevalence of thistle is a testament to how often the gambit works. 

Of course, being monstrously large is not without its downside. If you are highly visible to pollinators you also tend to be highly visible to herbivores, and long leaves and thick stems are irresistible to a grazer looking for an easy meal. But again the thistle has an ace up its sleeve, and we’ve already mentioned it: spines. Those along its leaves and stems serve to arm the plant against deer and other grazers, not to mention the odd clumsy Norseman. Meanwhile, the smaller spines along the involucre (the green “cup” holding the flower head) apparently go some way towards deterring a more insidious class of herbivore: ants and other insects who might clamber up to poach seeds before they’re fully ripe. 

(As a sidenote, for those who are willing to brave the prickly gauntlet, the reward is rich. If you’ve got the time, I suggest gathering thistle stems in spring – with the help of some sturdy gloves – and carefully de-spining and peeling them. Boiled for a few minutes and buttered, they’re among the tastiest of weeds.)

It all adds up to a plant that is, ironically enough, something like the medieval Norseman of herbs (or at least his stereotype). Hulking, armored, virile, the thistle invades relentlessly and is well-built to conquer and lord over newly discovered lands. It demands tribute from the many creatures who depend on it, and ruthlessly repels those who would come at it with violence. 

And yet there is one creature who laughs in the face of all this – who takes what it wants from the thistle, who loves the prickly weed to the point of arranging its whole life around it. This is the goldfinch, and if that last statement sounds like hyperbole, consider the facts. The goldfinch uses copious amounts of thistledown to line its nests, making them pillow-soft. While other songbirds, even those that identify as seed-eaters, feed their nestlings a largely insect-based diet, the goldfinch regurgitates cropful after cropful of thistle seeds. No doubt much evolutionary work has gone into retooling its gut to extract the necessary protein, although the details of this remain mysterious. 

And so committed is the bird to this routine, so faithful is it to its plant of choice, that it makes an enormous sacrifice. Instead of launching into the breeding process as soon as the days get warm, instead of racing to squeeze in two or three broods in a season, the goldfinch waits to reproduce until long after its neighbors are in full swing. More often than not it is late June before a pair bothers to start scouting for a nest site, and it may be well into August before any eggs are laid. 

Is it truly waiting for the thistles? One researcher sought to get into the finch’s head, as it were, by putting individual birds in temperature-controlled rooms with views of flowerpots containing different plants. Though neither factor did the trick on its own, the right combination of climatic and visual stimuli had a stunning effect. Once the rooms were warmed up to around eighty degrees, finches exposed to the sight of blooming thistle doubled their testosterone. Time to get in the mood – the days of plenty are coming fast! To see such a dramatic physiological response in an animal, to what is in effect only a secondary cue (since the blossoms per se are no use to the finch), is unusual to say the least. The study bears powerful witness to what has surely been many, many millennia of coevolution between these two partners. 

But more questions rear their heads, and the mention of “coevolution” suggests one of them. Is this really a one-way street, as I presented it originally, with the finch taking what it wants from the plant and giving nothing in return? Perhaps this would be to underestimate the wiles of the thistle. Though I’m not aware of any work that attempts to quantify this, the birds are likely to play at least some role in helping the plant to distribute its seeds. Finches gathering fluff are sure to drop some en route, while disintegrating nests at season’s end return a few more to the earth. And it is possible that some seeds survive the fraught passage through the bird’s gut, and remain viable at the other end. Such an adventure could extend the thistle plant’s reach, in theory, much further than the average gust of wind. 

Another question: is this unconventional lifestyle, full of sacrifice, really worth it for the goldfinch? The opportunity cost of forgoing an extra breeding cycle is surely massive – and for what? So that the parents might stuff their kids full of their own favorite food? What else might be going on here? 

One answer to that question turns out to involve another bird species entirely, a notorious character that will appear again later in this book. The brown-headed cowbird is, as many a bird lover knows, a rogue: it lays its eggs in other birds’ nests, tricking its hosts into raising new cowbirds, often at the expense of their own young. The cowbird is not picky about whom it takes advantage of, as over two hundred bird species are on record as falling victim. Thanks to its skulduggery some of these species have reportedly had their overall nesting success slashed by as much as a third. Yet its attempts to trick the goldfinch – often an abundant neighbor, and potential patsy, in its preferred habitats – are invariably doomed. The baby cowbird, it seems, cannot abide a diet of regurgitated thistle seed. 

This doesn’t stop them from trying. In one study of eight hundred goldfinch nests in Ontario, no fewer than 47 were parasitized by cowbirds. Yet of the eggs that hatched, only twelve sad cowbird chicks survived past the third day, and only one grew old enough to leave the nest – but was too weak to do so. It seems that by embracing its eccentric palate, the goldfinch has managed to dodge one of the likeliest causes of breeding failure for the birds of North America.

Spend enough time with any bird, and it will thwart your expectations. Outside of the summer and fall, the goldfinch menu becomes considerably more eclectic. Finches will gladly feast on sweetgum balls, alder catkins, or elderberries when given the chance; you might spot them slurping maple sap, plucking aphids from stems, or even grazing on green algae at the water’s edge. A species does not survive across this continent for millions of years without some degree of flexibility. But the thistle bird has undoubtedly earned its nickname. In the process, it has shown what can be gained by hitching one’s wagon to a plant – provided that plant is as reliable, and remarkable, as the king of weeds.

Image (c) Thomas & Dianne Jones, CC BY 2.0

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