Birds of Summer

Far be it from me to argue with Alexander the Great’s favorite tutor, but I think Aristotle got it wrong.

As a schoolboy growing up along the soggy shores of Fountain Grove Conservation Area (CA), my summers were bookended by the comings and goings of swallows. Without benefit of a calendar, I learned that the arrival of April’s first barn swallow, gliding across our lawn like a figure skater on ice, meant I would soon be free of classroom captivity. And in August, when utility lines along the blacktop began to sag with gregarious flocks of southbound swallows, I knew my idyllic and carefree days were ticking to an end.

A Survey of Swallows

The swallows of Aristotle’s Classical Greece, separated by thousands of miles and hundreds of generations from those of my Missouri childhood are, incredibly, the same species, Hirundo rustica, the ubiquitous barn swallow. But you wouldn’t know it by looking at them.

Greek barn swallows, like those found across Europe and western Asia, have a pale, often snow-white belly with a steely blue band across the chest. North American barn swallows sport buffier bellies — as if they were dusted with cinnamon — and lack a complete chest band.

Whatever form they take, barn swallows are one of the world’s most cosmopolitan birds. They chase summer from one hemisphere to the other, nesting throughout North America, Europe, and Asia, and wintering in South America, Africa, and northern Australia. You’re as likely to spot one wheeling over a Midwest pasture as you are to see one swooping under the Ponte Vecchio in Florence, Italy, or swirling over herds of zebras on the plains of Tanzania. They’ve even been spotted in Antarctica, likely blown far off course by storms.

Worldwide, more than 90 species of swallows fly the skies. Twenty-three species occur in North America, but many of them are confined to Central America or the Caribbean. About eight kinds of swallows regularly turn up in the United States, and six of them nest in the Show-Me State.

Missouri’s swallows range in size from the purple martin, which is slightly smaller than a robin, to the bank swallow, which is barely bigger than a sparrow. Some are dazzlingly colored, like the tree swallow, whose iridescent blue feathers shimmer like an oil slick in the sun. Others, like the northern rough-winged swallow, wear an understated cloak of dusty brown feathers. Some nest in birdhouses, some in barns, some burrow into stream banks. Although each species has its own idiosyncrasies, they all make a living the same way — by catching insects on the wing.

Billion Bug Highway

If you were to lie outside on a summer day and gaze into the clear blue sky, you probably wouldn’t see much. Maybe a few wispy clouds. Or a bird fluttering past. But there are literally billions of creatures suspended in the air above our heads. We just can’t see them.

To find out how much life might be up there, English entomologists used precision radar to calculate the number of insects flying over south-central England. The radar was extraordinarily sensitive, capable of detecting anything more massive than 2 milligrams. (To put that in perspective, a mosquito is about 2.5 milligrams.) For more minute creatures — like midges or fruit flies — the biologists dangled insect nets from balloons floating at various heights.

The results of their study, published in a 2016 issue of the journal Science, were staggering. More than 3.5 trillion insects, spiders, and other arthropods — equivalent to 3,200 tons of biomass — pass over cold, damp southern England each year. In warmer, sunnier places (like Missouri), nearly 6 billion insects may pass overhead in a single summer month.

Biologists call this buzzing, fluttering, drifting swarm of life “aerial plankton.” And swallows, like whales of the skies, are perfectly adapted to use it for food.

Whales of the Skies

With its beak snapped shut, a tree swallow resembles a button-nosed Hollywood ingenue. But the swallow’s dainty beak disguises a truly cavernous mouth. Wide open, the bird’s gaping maw conjures up images of a yawning hippopotamus or a hungry humpback whale. The oversized mouth serves an important purpose. Like a catcher’s mitt, it offers an extra-large target for bagging fast-flying objects.

But to bag bugs, you first must find them. The atmosphere is enormous, most insects are tiny, and the pesky critters don’t stay put for long. Swallows must spend hours on the wing to catch enough bugs to fill their bellies. They accomplish this with two energy-saving adaptations — pointed wingtips and low wing loading.

At each wingtip, air spills out from the high-pressure area under the wing to the low-pressure area above it. This forms a vortex that creates drag, slowing a bird down. Wings that taper to knife-edged points — like those of nearly all swallows — reduce the surface area where vortices form, enabling faster, more efficient flight.

Biologists use the term “wing loading” to compare the mass of a bird with the area of its wings. Chunky birds like wild turkeys — which can weigh over 20 pounds yet have relatively stubby, 50-inch wingspans — have high wing loading. Purple martins, Missouri’s largest swallow, weigh less than 2 ounces (about as much as 10 nickels) yet have wingspans that stretch nearly 16 inches. As you might expect, swallows have exceptionally low wing loading and require comparatively little energy to stay aloft. In other words, comparing a wild turkey to a purple martin would be like comparing the fuel efficiency of an Airbus 380 to a hang glider.

Swallows are so efficient at flight that they spend the majority of their lives aloft. Barn swallows were found to spend 75 percent of their waking hours airborne, and tree swallows spend up to 80 percent.

Swallows eat on the wing and even drink and bathe while aloft, skimming the surface of a pond or marsh so they can dip their beaks or bodies in the water. But there’s one activity that brings these virtuosos of the sky back down to earth, raising babies.

Master Builders

In spring, the entrance road to Eagle Bluffs CA is the only dry stretch for miles. Marshes and mudflats abound, and if you park beside one of the puddles, you’ll soon witness the animal equivalent of a Saturday morning mob of DIYers descending upon a home improvement store.

Hundreds of cliff swallows swarm the mud holes, touching down for a few moments and then streaking away in a flurry of feathers. Using their beaks like trowels, the birds scoop up mouthfuls of mud and fly back to the construction site, in this case, the I-beam of a nearby bridge. Mouthful by mouthful, the birds construct dozens of gourd-shaped nests, all jumbled together higgledy-piggledy like barnacles on a ship.

Both males and females share nest construction duties. Some of the birds — older ones who arrive earlier in the season — repair existing nests. Other birds start from scratch, mortaring mouthfuls of mud to the wall of the bridge with a shaking motion of their beaks. The shaking is a key requirement in the swallow building code. It causes the mud to release some of its liquid, allowing the glop to seep into cracks and crevices. This results in a more solid foundation once the mud dries. The feathered bricklayers first construct a narrow, semicircular ledge and then stand atop this muddy scaffolding to finish the walls and roof of the nest. On average, it takes about seven days and 900 to 1,200 mouthfuls of mud to build a nest from scratch.

Once construction is complete, nest owners sit in the tubular entrance and lunge at neighbors who stray too close. Birds steal wet mud from untended nests, and squatters often try to usurp existing residents. When that happens, invaders are met with vicious resistance, getting pecked and bludgeoned by the nest’s rightful owners. Feathers are plucked out, and sometimes the scuffle tumbles out of the nest and into the river below.

Watching a cliff swallow colony is a bit like watching a soap opera — in more ways than you might think.

The Affairs of Swallows

To the casual observer, cliff swallow couples seem monogamous. One male and one female tend to a nest. But numerous trips to the mudflat offer numerous opportunities for extramarital hanky-panky. In a Nebraska study, 43 percent of cliff swallow nests contained a nestling that wasn’t related to one of its parents.

For many swallow species, infidelity is the rule rather than the exception. In male purple martins, the younger you are, the more likely it is your mate will stray. In one study, nearly half of yearling males had offspring in their nests that were unrelated to them, but less than 10 percent of older males did. Male bank swallows are more likely to chase heavier females because females weigh most right before laying an egg. And barn swallows sometimes kill another male’s nestlings in an attempt to break up the pair and mate with the female. Often, it works.

But the epitome of avian infidelity is the tree swallow. Using DNA fingerprinting, researchers found that in some populations, nearly 90 percent of nests contain offspring not descended from the male tending the young. Females may be seeking insurance that their eggs will be fertilized. But stronger evidence supports the idea that tree swallows stray simply to ensure their offspring are genetically diverse. In the game of natural selection, a poker hand containing a mixture of suits is more likely to win against one containing only diamonds.

Migration Maelstrom

By August, a new generation has fledged, and the affairs of summer are over. North America’s swallows then turn their attention to migration.

In a farm field north of Swan Lake National Wildlife Refuge, the show starts at dusk. The western horizon, orange as a ripe mango, begins to darken, but not from the sun winking out. A massive cloud billows closer, swelling like a desert sandstorm. Soon, a whirlpool of swallows churns in the heavens overhead. The murmuration shrinks and expands, swoops and sails, and a finger of birds snakes to the ground, as if a funnel cloud were spawning a tornado. Once the scouts declare that the surface is safe, thousands upon thousands of swallows swirl down, and the sky empties in a torrential downpour of birds.

Unlike many songbirds, swallows migrate during the day, presumably so they can forage for insects as they put miles behind them. At night they gather in enormous, mixed-species flocks to roost and rest. The flocks can grow so large — hundreds of thousands of birds — that they show up on weather radar like a blossoming storm.

Tree swallows and northern rough-winged swallows winter along the Gulf Coast and Central America. Missouri’s other species push farther south into Amazonia, Chile, and Argentina. These long-distance migrants spend half of their lives in transit, arriving on wintering grounds in November and migrating north again in February.

Hard to Swallow

A few years ago, I was on the road back to my parents’ house when I noticed something strange. I’d been driving for miles through humid farm country, but my windshield, usually splattered with so many bugs that it resembled a Jackson Pollock painting, was alarmingly clean.

Around the same time, newspaper headlines began popping up, warning of an “insect apocalypse.” Fanning the media frenzy was an analysis of population data in the journal Biological Conservation. The authors reported 40 percent of the world’s insect species were declining, and a third were endangered. Insects, the study led one to believe, were hurtling toward extinction, threatening a “catastrophic collapse of nature’s ecosystems.”

Since then, the Threatcon level has been dialed down as subsequent studies offer evidence that insect losses aren’t so precipitous. But that doesn’t mean biologists aren’t alarmed. Insect numbers are declining. And their losses will ripple through ecosystems. It’s probably already happening.

Biologists believe nearly a third of North America’s birds — a net loss of about 3 billion individuals — have disappeared since 1970. And aerial insectivores like swallows are among the most steeply declining groups. Bank swallow numbers in North America have crashed by nearly 90 percent since 1970, and six other swallow species in the United States have declined, though not so alarmingly.

While biologists work to tease out causes for swallow losses — as with most ecological problems, they’re likely numerous, complex, and intertwined — I can’t help but remember my clean windshield and suspect the dots connect to problems in the food chain. When insects disappear, so do the birds that eat them.

Perhaps this notion has clouded my memory. After all, Partners in Flight lists Missouri’s swallows — with the notable exception of bank swallows — as being of “low conservation concern.” Yet over the past several summers, the utility lines at my parents’ home have seemed to sag a little less. Maybe rose-tinted glasses have caused me to remember more birds in the 1980s than there truly were. Or maybe not. Maybe the swallows of my youth are disappearing, year by year, bird by bird. And if so, maybe Aristotle was right.

A single swallow, perched alone on a utility line, would make for a poor summer indeed.

Matt Seek is the editor of Xplor, MDC’s magazine for kids.