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The Strange Social Life of Swallows
A pair of Yale ornithologists has discovered that the way one species of birds gets together provides some clues to the ways the rest of us cohabit.
Summer 1993
by Bruce Fellman
Every spring, as the famous cliff swallows return to San Juan Capistrano, a less-renowned flock of the same birds
makes its way to Keith County in southwestern Nebraska. And, just as
predictably, there to greet them on the windswept grasslands and sandhills are
two Yale biologists. Since 1982, the husband-and-wife research team of Charles
and Mary Brown has studied the day-to-day activities of a bird whose behavior
has some disturbing similarities to that of humans.
“Cliff swallows are sinners,”
says Mary Brown, only half facetiously, as she lists what among humans would
qualify as major misdeeds—cuckoldry, rape, philandering, grand theft,
voyeurism, and infanticide. “Observing these birds is like watching a soap
opera,” says Charles.
But these birds also have their
redeeming features, including a talent for cooperation, and that combination of
traits, notes Mary, a research associate at the Peabody Museum of Natural
History, is precisely what makes them so intriguing a species for study. Like
many biologists, the Browns are interested in the evolution of group living,
and cliff swallows, who live in colonies made up of as many as 3,700 nests,
have revealed much about the benefits and liabilities of membership in nature’s
societies—including our own. “There are advantages and disadvantages to living
in a group, and not all animals receive the same payoffs,” says Charles, who
might almost be discussing the relative merits of human lifestyles. “Some
individuals do better, while others do worse.”
To illustrate his case, Mary
describes the behavior of one swallow she and her husband have been
studying. “Yellow-orange”—to help in identification, the scientists paint the
birds’ normally white foreheads with different color combinations—was
constantly under attack by another, “fiendish,” bird, which regularly—and for
reasons the Browns discovered—threw Yellow-orange’s eggs out of the nest.
Yellow-orange seemed powerless against the assaults, demonstrating, according
to the researchers, the behavior of “the classic victim.”
But if there are winners and
losers in swallow society, there are middle-grounders as well, like
“Blue-silver-blue,” a member of the same colony, but one who played neither
victim nor victimizer. “He just looked after his nest,” says Mary.
Such up-close analysis can only
be accomplished by getting out into the field and getting one’s hands dirty,
and so almost every day from mid-April to mid-August, the Browns and their
assistants, many of them Yale undergraduates, spend hours broiling under the
prairie sun or getting drenched by thunderstorms. But their efforts, says Scott
K. Robinson, an ornithologist with the Illinois Natural History Survey who has
also investigated the evolution of colonial living, have paid off handsomely.
“They’ve tied together the costs and benefits of living in groups,” says
Robinson. “The Browns have pulled off one of the best and most detailed
large-scale, experimental studies of a colonial bird that anyone has ever
accomplished.”
It has been a daunting task. Part
of it involves banding the birds—more than 54,000 so far—and marking
individuals for long-term monitoring. Some of the studies have required that
large numbers of the swallows’ cantaloupe-sized mud nests be fumigated to kill
the parasites (chiefly fleas and a relative of the bedbug) that can wreak havoc
on the occupants. There are also swallow songs to record, insect-foraging trips
to document, eggs to count and number, blood to draw and analyze, and nestlings
and adults to weigh and measure.
For the most part though, the
Browns and their colleagues watch swallows and record their daily lives in minute detail.
As naturalists from Aristotle to Darwin to the modern masters have learned,
look long enough, and a pattern may emerge.
Charles Brown has been looking at
birds for most of his 34 years. The Texas native grew up in the Dallas area,
and by the time he’d turned 15, he was publishing scientific papers in
ornithology journals. His interest in avian biology continued at Austin College
in Sherman, Texas, and when he entered graduate school at Princeton, he
decided to focus on why birds live in different-sized groups.
Brown’s mentors suggested that he
examine the cliff swallow colonies around Cedar Point, Nebraska, which were
known to vary from tiny groups that had only several nests to gatherings of
several thousand. In the course of his explorations in 1982, the biologist met
Nebraska native Mary Bomberger, who was also doing bird research. “I became
Charles’s first field assistant,” explains the 36-year-old scientist, “and I
guess I was just the most persistent.”
They were married and came to
Yale three years later—after, they hasten to add, the 1985 research season had
ended. Their wedding cake was topped with a life-sized cliff swallow made of
frosting.
The Browns’ work quickly revealed that these birds live in colonies of connivers. “They’re constantly trying to
get at each other’s eggs—and at each other,” says Charles. They steal the fresh
mud used to build nests, and at the stream banks where they flock to gather
building materials, the males often try to force themselves sexually on females
who are not their mates (the females, for their part, may turn the tables,
soliciting sex from males who, for reasons known only to cliff swallows, are
considered better than their existing mates). At the colonies, the birds make
off with the grass lining of nests, and often a swallow will take over a
neighbor’s abode for a while, simply, it would seem, out of spite.
But malice, the Browns suspect, doesn’t
guide behavior. Indeed, behaving in a Machiavellian—or moralistic—manner is not
within a swallow’s ability. More likely, the driving force is simply
self-interest. What’s in it for the bird that temporarily commandeers a nest or
merely eavesdrops on a nearby family is information.
One item of potential interest
seems to be the number of parasites in residence at a neighbor’s domicile. Some
nests are literally crawling with bedbugs and other bloodsucking insects, which
take a fearsome toll on young birds. In a series of telling experiments, the
scientists sprayed parts of colonies with pesticides (the swallows suffered no
ill effects) and then compared the growth rates of nestlings with those in
unsprayed nests. Ten days after hatching, parasite-free young birds weighed an
average of 20 percent more than their bug-and-flea-infested counterparts, whose
rate of survival to adulthood was half that of swallows in sprayed nests. In
colonies where the parasites were particularly abundant, none of the hatchlings
survived.
Parasites and other unwelcome
guests are among the costs of living so close together. The Browns discovered
that as any colony grew in size, the nests frequently wound up closer together,
which made it easier for parasites to find new victims. In some cases,
parasites increased so much that the swallows, who tended to return to the same
colony every year, abandoned their old homes for new ones. Several years later,
when the bedbugs and fleas have starved, the birds often return to set up housekeeping
in their former haunts.
So the interloping swallow,
hunkered down in another bird’s nest, is in one sense the fastidious visitor
who gives the house the white-glove treatment. But there’s more going on than a
hygiene check.
With the advent of chemical
techniques to determine parentage, scientists have learned something
unsettling: Among many bird species, a sizeable proportion of the eggs in each
nest do not belong to the parents who provide care. Such parasites as cuckoos
and cowbirds have long been known to foist off their own eggs on unsuspecting
mothers and fathers of a different species, but it turns out that within a
species, individual birds play a similar game.
Many biologists believe that
“intraspecific brood parasitism” persists because the birds that practice
it are usually related to those they deceive. Since both parasite and victim
share many of the same genes, there’s a genetic self-interest at work that
apparently outweighs the extra effort required by the involuntary adoption.
The Browns learned, however, that
among cliff swallows the explanation doesn’t hold. When they did the blood
analysis that established family ties in the nest, the researchers found that
the deceivers were most often totally unrelated to the parents who were prevailed
upon. Both victim and oppressor, however, usually knew each other. From
previous bird-watching experience, the would-be parasite learned both when the
resident mother and father would be away and the state of their nest.
Surreptitiously and in less than half a minute, the intruder tossed out one
egg. Within a few days, a new egg—and one laid by or fathered by the
invader—often took its place. The host family generally never knew the
difference.
The Browns found that between 20
and 40 percent of the nests they monitored wound up parasitized, a high
percentage among birds. But in about 6 percent of the victimized nests,
something almost unheard of occurred. In certain colonies, the Browns wrote
numbers on each egg to keep tabs on population changes, and to their amazement,
they saw instances when an egg was jettisoned and the substitute was a numbered egg from a neighbor’s nest (the
scientists have evidence that swallows also move recently hatched chicks).
Carrying such a load takes a
remarkable degree of dexterity (the birds, usually males, transport the eggs in
their mouths), and the task also appears to require foresight, for the tossing
is done as much as several days in advance of the transfer. And since all the
eggs in the surrogate’s nest hatch at about the same time, the parasite must
have known when the bird it coopted started incubating its clutch, and timed
the subterfuge accordingly. “A cliff swallow is surprisingly wise for a
creature that weighs 20 grams,” notes Mary.
Wise as they may be, the plotters
don’t get off scot-free. “The birds that do the parasitizing are commonly
parasitized themselves,” Charles explains, adding that a bird is vulnerable any
time it’s away from home and not guarding its abode.
In a colony of self-interest, tit
for tat is the rule.
This strange game of “musical
nests” evolved, according to Charles, as a kind of “don’t put all your eggs in
one basket” strategy. “Nests often fail,” he says. “The cliff face may fall
down, there are swallow bug infestations, and there may be attacks by predators
like bull snakes. By putting an egg in another nest, you spread out the risks.”
So cliff swallows live in a
society held together by neighbors taking advantage of each other. And it
doesn’t stop at the nest.
The highly maneuverable birds
feed on equally agile flying insects, many of which are no bigger than
mosquitoes. And when a swallow finds a swarm of bugs, its return to the nest
with a beak stuffed with food does not go unnoticed. “Word” gets out, for the
colony functions as an information center, and other birds, duly tipped off,
follow the successful hunter on its next foray.
“That swallow’s good luck is
being parasitized by others in the colony,” says Charles, “but if they may be
raising some of your kids, the cost of being exploited is reduced, and you’re
less likely to need to come up with counterstrategies to avoid sharing your
food.”
Not only does nature permit
exploitation, but in one case, cliff swallows actually encourage other
birds to come-and-get-it. This unusual act of largesse takes place away from
the colony and is heralded by a sound that resembles the “high-pitched creaking
of a rusty gate,” says Charles.
This “squeak call,” the Browns
and their research assistant, Martin Shaffer, ’88, discovered, is given only
when a bird finds a swarm of flying insects on the cool, dreary days that are
common early in the nesting season. The likelihood of finding prey is slender
under such circumstances, for bad weather tends to curb the activity of most
bugs prominent in a swallow’s diet. But when one of these jaunty hunters
manages to locate a decent meal, the bird does something uncharacteristic in a
natural world dominated by selfishness. The swallow quickly squeaks its good
fortune to any other members of its species who happen to be in the vicinity,
even those just passing through the area on migration.
Scientists call this unusual
behavior—which is about as close as nature comes to a random act of
kindness—“pseudo-reciprocity.” The bird that gives the call might someday be on
the receiving end of another swallow’s hunting fortune, but there are no
guarantees that they’ll ever meet again under circumstances where the favor can
be returned.
Appearances aside, however, the
practice is not altruistic. Regardless of what the future might hold in terms
of paybacks, there is an immediate reward for cooperating with strangers, says
Charles, for many eyes make for the most efficient way to track an insect
swarm. “It’s to the advantage of the caller to attract other birds, and there’s
no real cost to sharing,” he says. “The food supply is very dense, and it’s not
there for very long anyway.” Call it enlightened self-interest.
Indeed, looking out for number
one seems to explain most of creation, and among cliff swallows, selfishness
figures heavily in the evolutionary path that brought these birds together.
Self-interest even shapes group size, the Browns have shown. In small colonies,
there may be fewer parasites and parasitized nests, but there are also fewer
eyes watching for insects. In huge colonies, there’s the bedbug factor, an
increase in nest interference, and, counterbalancing the greater likelihood
that someone will find food, there’s the prospect that too many hunters in any
given area will actually eliminate the prey supply.
So for swallows, neither rural
isolation nor “urban sprawl” is the best lifestyle alternative—there aren’t
enough benefits in either arrangement to overcome the costs.
“Basically, social life is a
mixed blessing,” says Charles, and among the mix of advantages and
disadvantages, the “suburbs”—in swallow colonies, if not always in human
ones—turn out to be just right.  |
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