August 2020

“Nature abhors a vacuum,” is how the famous phrase goes. Whenever I see it, though, I always think it’s missing a follow-up sentence: “And evolution is the process that allows nature fill that vacuum.”

As I research topics for this blog, I always marvel at how every imaginable ecological niche is filled with living things beautifully suited to its demands and rewards. Animals and plants that have evolved in size, shape, and behavior specifically to be able to survive, to thrive, there.

With all their diversity, birds provide endlessly vivid examples of this phenomenon. Just in our area, a few moments’ looking around might reveal a Ruby-throated Hummingbird hovering as it drinks nectar (wing beats per second: 50); a Turkey Vulture soaring effortlessly on thermals as it searches for carrion (wing beats per second: 0); an American Robin yanking up worms (that it’s heard wriggling beneath the earth) from a manicured lawn; and a Barn Swallow darting and twisting with stunning agility as it gobbles up tiny flying insects.

Four members of the same group of living things, each species so different and so perfectly adapted to the niche it fills. And since they are all so commonly seen, so familiar, it’s easy to forget how distinctive their adaptations are.

To me, though, among the most extraordinary niche-filling feat in birds is less visible: The life cycle of shorebirds, the sandpipers and plovers that move through our region each spring and fall.

Or maybe that should read spring and “fall.” Because not only is southward shorebird migration happening right now, it’s already been going on for several weeks. Along with reminding us of how imprecise our definitions of nature’s timelines can be, these little birds also tell a vivid story of adaptation and survival.

Not all shorebird species we see are passage migrants. The Killdeer (a plover), for example, nests in this area at such locations as Croton Landing and Croton Point Park. Some individuals actually spend the entire year here without migrating at all.

But the vast majority of sandpipers and plovers we’re spotting these days on local riverbanks, beaches, and mudflats are merely stopping to rest and refuel before heading on. They’re following an unusual—and unusually challenging—schedule, one demanded by the characteristics of the ecological niche they have evolved to fill.

The story of one shorebird species, the Semipalmated Sandpiper, is both dramatic and characteristic. These tiny birds—6 or so inches long and weighing less than two ounces—nest on the Arctic tundra of North America. They spend the winters, however, in South America, sometimes as far south as Tierra del Fuego. (Roughly a 9,000-mile trip each way!)

As if that isn’t demanding enough, some Semipalmateds take off from the New England coast and don’t land again till they reach South America. This nonstop hop, completed without food or rest, can cover 2,500 miles.

The challenges don’t end there. Naturally, summers on the tundra are extremely brief, as is the Semipalmated Sandpipers’ access to the food sources (insects, crustaceans, marine worms) they depend on to survive and raise their young.

Therefore, while the birds don’t arrive till May, nesting season has to be finished by early July. (If their first nest fails, they do not try again.) Unlike more temperate species, neither adults nor the newly fledged young can take weeks to build their energy before heading south. They have to start their journey almost at once.

That’s why, by early August—when some less itinerant local nesters are still raising their second broods of the season—Semipalmateds start to show up here and in migration stopovers across the U.S. (Those whose nests failed up north may start appearing even earlier.)

And they’re far from the only species on a tight schedule. The Least Sandpiper—the smallest shorebird in the world, less than 6 inches in length and weighing in at a single ounce—is another marathon flyer, nesting on the tundra and wintering as far south as Chile. And, while both Greater and Lesser Yellowlegs don’t have as far to travel—they nest in boreal forests and wetlands and some winter over within the U.S.—they have also started to pass through our region.

It’s always a little bit of a shock to see the first shorebirds showing up in midsummer, when the last thing I want to think about is the changing seasons. But it also reminds me that migration (and the avian evolution that underlies it) is a fascinating—and remarkably complex—spectacle, filled with surprises and moments of enlightenment. And that we get to witness it, in all its complexity, every spring and fall.

By which, of course, I really mean “fall.”

Copyright © 2020 by Joseph Wallace

Ever since the first flowering plants emerged about 130–140 million years ago, they and those that eat them have been engaged in a kind of evolutionary arms race. As plant-eaters chase them down, plants, which obviously can’t run away, have developed other ways to defend themselves and the flowers and seeds that sustain their species.  

Plants play defense via such obvious physical attributes as tough leaves, thorns and spines, and the sheer abundance of the seeds they produce. (One vivid local example of this last survival strategy is the cottonwood, which blankets some neighborhoods in “snow” every spring.)

Another widespread adaptation is also effective: Many plants have a bitter taste, and some are poisonous. They’ve developed an astonishing array of chemical compounds that serve to teach anyone who dares to eat them not to do so again. Scientists have identified more than 3000 separate alkaloids (one group of such compounds), and a single plant can produce 30 or more.

Humans being human, we’ve learned to use some plant toxins for our own purposes: For remedies (e.g., digitalis derived from foxglove plants and used to treat heart problems), in “recreational” drugs (the psychoactive ingredients in cocaine), and as seasonings for our foods. (Capsaicin in chili peppers is meant to deter you from eating them.)

Other animals have also learned to put these toxic chemicals to use. In one remarkable example, small tropical insects (most likely ants), far from being harmed, store plant alkaloids in their own bodies to use as a defense system. But it doesn’t always work: Certain frogs still gobble the ants down, having evolved special glands to sequester the same toxins in their skin. These are the Dendobatid frogs, commonly known as poison or poison-dart frogs, and some of them are so toxic that merely holding one can cause serious neurological, respiratory, and other effects in humans.  

The never-ending battle between plant and plant-eater has also led to the development of microhabitats. These are tiny ecosystems populated by insect species that share one central trait: the ability to eat a plant that most species find toxic. We can find one vivid example of this phenomenon in our own gardens and fields right now.

Not many years ago, milkweeds were considered merely denizens of vacant lots and untended roadsides, no more or less remarkable than a hundred other weeds. What changed their status, of course, was the besieged Monarch butterfly, whose caterpillars eat milkweed…and only milkweed.

Milkweed sap contains chemicals known as cardiac glycosides, which are toxic to most insects and animals and can cause serious symptoms in humans if consumed in large quantities. The sap is also thick, sticky, milky, and hard to clean off your hands, and it can burn if it gets in your eyes. Very few species will dine on a milkweed plant more than once. 

The Monarch caterpillar and some other species, however, find it delicious. These species benefit from their specialized tastes for a simple reason: When most plant-eaters find your chosen food toxic, you don’t encounter much competition for resources.

Yet the benefits of an all-milkweed diet run far deeper. Like the rainforest ants and poison frogs, Monarch caterpillars and butterflies store the plant toxins in their own cells, which makes them as poisonous as the plants they depend on. In a further evolutionary development (also shared with many poison frogs), they’re boldly colored. Predators of all kinds seem to recognize their yellow- or orange-and-black markings as caution flags: Avoid!

Another common milkweed resident, The Milkweed Tussock (or Tiger) Moth, whose caterpillar has bright orange-and-black tufts and adult a tiger-striped abdomen, shares both the Monarch’s toxicity and warning colors. Fascinatingly, though this color palette warns predators away from the caterpillars, it’s useless against the bats that prey on the night-flying adult Tussock moths. So the moths have evolved a secondary defense: They have special organs that emit clicking sounds, alerting the bats to the moths’ bitter taste.

Similarly, both Large and Small Milkweed Bugs boast bold orange- or red-and-black patterns. So does the Red Milkweed Beetle, a member of the longhorn beetle family. (Remarkably, in this species the base of the long antennae actually bisects the insect’s eye, which gives it its genus name: Tetraopes, meaning “four-eyes.”)

Not every insect that takes advantage of the milkweed microhabitat needs such evolutionary adaptations, however. Some just like to spend time there.

Unlike the sap, milkweed nectar contains no cardiac glycosides, and the abundant, long-lasting flowers are popular with a wide variety of butterflies, moths, bees, and other pollinators. Inevitably, the presence of such insects also brings creatures that prey on them.

The Golden (or Goldenrod) Crab Spider, for example, can frequently be found amid milkweed flowers. There it sits quite still, waiting to ambush an unwary bee, butterfly, or other visitor.

As with so many inhabitants of this microhabitat, the spider’s life story contains some unexpected details: It is one of only two spider species in North America that can change color to match its background. It can be white with green or purple patches (good milkweed camouflage), or bright yellow (preferred in another favorite habitat, goldenrods.)

Researching this essay, I was amazed by how much I hadn’t known about the milkweed’s world. But the truth is that microhabitats exist everywhere—in leaf litter, the soil beneath your feet, the trees on your street—and they’re all just as intriguing. I’m looking forward to exploring more of them soon…and telling their stories here.

Joseph Wallace