Bullies of the Bird World
Some cuckoos apparently make an offer other birds can't refuse: Hatch my egg or else!
Organized crime was not what Manuel Soler expected to find on the Hoya de Guadix, a sparse, windswept plateau on the southern coast of Spain. Soler, an evolutionary biologist at the University of Granada, has spent every spring there for the past 14 years, climbing scattered stands of almond trees and peering into magpie nests.
Magpie eggs are not what interests him, however. The scientist is more concerned with an interloper: an egg surreptitiously laid by the great spotted cuckoo, an opportunistic animal that manipulates other birds into hatching and rearing its young along with their own.
How? Soler investigated that question by removing the cuckoo eggs from a number of magpie nests and rechecking the sites several days later. The results were dramatic. More than half of the nests had been completely destroyed. In several others, eggs were smashed and magpie chicks injured, seemingly as if cuckoos wanted to teach the adult magpies a lesson.
These observations strongly suggest that cuckoos punish hosts that remove cuckoo eggs from their nests, Soler wrote in a subsequent paper co-authored by University of Granada ecologists Juan Soler and Juan Martinez, and Anders Moller, an evolutionary biologist at Pierre and Marie Curie University in Paris. In effect, the scientists compared the parasitic creatures to the Mafia, because the birds enforce compliance through intimidation.
Cuckoos have a longstanding reputation as the thugs of the bird world, but even in that context the Avian Mafia theory is remarkable. Soler himself was taken aback by his findings, which suggest nothing less than an anthropomorphic enforcement racket as the means by which some cuckoos successfully reproduce. "We had never observed any predatory behavior in cuckoos," he says. "So when we found that most of the experimental nests had been preyed upon, we were absolutely surprised."
Other scientists searching for similar behavior in recent studies of great spotted cuckoos in Israel and common cuckoos in Japan were unable to find it. For the moment, then, the Avian Mafia theory remains just that--an intriguing idea. However, winged Mafioso are simply the latest answer to a question that has puzzled naturalists since Aristotle wrote about cuckoo behavior in the fourth century B.C.: Why would any species routinely accept and raise the young of another species, thereby decreasing its own chances for survival and reproductive success?
Cuckoos offer a splendid opportunity to study this question. The Cuculidae family is ancient, somewhere in the neighborhood of 40 to 60 million years old, giving its 130-plus different species ample time to have evolved interesting behaviors. About half of them are brood parasites, a trait they share with honeyguides, whydah finches, most cowbirds and some ducks and troupials. Brood parasites lay their eggs in the nests of other birds, known as hosts, then allow the host birds to incubate and feed their young. Apparently, cuckoos have evolved the ability to mimic the eggs of certain other bird species, and those are the species that they seek out when invading nests. Secreting pigment in their oviducts, the parasitic birds can closely replicate the host birds' eggs.
A cuckoo can dart into an unattended nest, snatch up an egg, lay a close copy and be gone within 10 seconds. After hatching, some cuckoo chicks (though not the great spotted) instinctively shove their foster siblings and remaining eggs out of the nest, so as to have all the food to themselves. "Horrible perfection" is how Charles Darwin described the cuckoo's evolutionary niche.
Precisely what pressures and adaptations led to such behavior remains unclear, though the advantages are obvious: If you can get someone else to incubate and rear your young, you can lay as many eggs as you like, and still have plenty of time and energy to attend to your own needs. "Once you get over the initial shock of how awful their behavior is, it's hard not to think of cuckoos as evolutionary marvels," says Scott Robinson, a wildlife ecologist with the Illinois Natural History Survey.
Cuckoos hardly look threatening. A slender bird about a foot in length, the common cuckoo has a long tail, small head and thin bill. The male's distinctive "cuck-oo" call is ubiquitous throughout Europe and Asia in the spring, and resounds through old world art and music, perhaps most famously in Beethoven's Pastoral Symphony.
Five cuckoo species are found in North America, though two just barely. Anis, tropical cuckoos that appear only in southern Florida, live in groups and lay all their eggs in a single nest. Mangrove cuckoos, about which little is known, are also found in southern Florida. Roadrunners are members of the cuckoo family, though not brood parasites. Large ground birds that can hit running speeds of 15 miles per hour or more, roadrunners live in arid, open areas of the West and Southwest, and apparently mate for life.
The most common American cuckoos are the yellow-billed, which ranges across the U.S. South to the Mexican border, and the black-billed, found east of the Rocky Mountains. These two species parasitize only occasionally--and then usually only each other's nests. Their calls are more of a "kow-kow" than "cuck-oo," and they tend to breed later than most birds, which may account for their sporadic parasitism: There simply are not that many nests to invade when they're ready.
Both the yellow- and black-billed cuckoos tend to follow outbreaks of food sources, such as singing cicadas. Like most adult cuckoos, they also love toxic and spiny caterpillars that other birds literally cannot stomach. Peculiar diets are in fact a common feature of parasitic birds and have been suggested as a possible reason for the practice of brood parasitism: Cuckoo chicks wouldn't last long on what their parents eat.
Most Americans probably know the cuckoo best through the eponymous clock, which contrary to popular belief was not invented in Switzerland. Cuckoo clocks originated in the Black Forest, an area in southwest Germany that during the seventeenth century developed a cottage industry in clockmaking. An entrepreneur by the name of Franz Anton Kettering is credited with producing the first cuckoo clock in 1730, capitalizing on the popularity of striking mechanisms and performing figures. Cuckoos were an inspired choice, mirroring their brief but vocal real-life appearances.
In 1979, a researcher proposed an idea about cuckoo behavior that seemed more akin to medieval legends than modern science. Amotz Zahavi, a behavioral ecologist at Tel Aviv University in Israel, published a brief paper in American Naturalist theorizing that cuckoos get other birds to accept their eggs not by stealth but by brute force, revisiting parasitized nests and exacting a terrible price for rejection. "I was not happy with the conventional story that parasites are cheating the hosts," he says. "The Mafia way--I am strong, and it is good for you to comply--is an honest approach."
Zahavi is a bright, original thinker whose unorthodox ideas are often greeted with skepticism. In 1975, he proposed the so-called handicap principle--the idea that apparent handicaps such as bright coloring in birds (an easy target for predators) are in fact an advertisement of superior genes ("I can survive even with this handicap"), and therefore an evolutionary advantage. "People thought it was crazy at first," says Peter Arcese, a wildlife ecologist at the University of Wisconsin, Madison. "But more recently, it's been well supported."
The Mafia proposal had a similarly inauspicious start. "I reviewed that paper for American Naturalist, and I really didn't think the hypothesis had much likelihood of being supported," says Stephen Rothstein, an evolutionary biologist at the University of California, Santa Barbara. "But it was a very clear, testable idea."
Soler has been intrigued by the question of how brood parasites manipulate their hosts since he began studying cuckoos in 1982. Initially, he shared Rothstein's pessimism. "I did not believe the Mafia hypothesis would work," he says. "But it was a fascinating idea nobody had tested, so I designed an experimental study."
The study was done over three nesting seasons (1990-92), during which Soler and his colleagues removed cuckoo eggs from a total of 29 parasitized magpie nests, and left them untouched in 28 others. Subsequently, 16 of the 29 experimental nests were completely destroyed, a fate suffered by only 3 of the 28 control nests. (Of the remaining experimental nests, 3 apparently were partially destroyed by cuckoos, 1 was ravaged by crows and 9 were left untouched.)
In spite of their findings, many scientists remain unconvinced. "A lot of people have been watching cuckoo nests for a long time and haven't seen this behavior," says Bob Payne, a noted cuckoo authority and ornithologist at the University of Michigan. "It could have been overlooked, but there's not much in the way of natural history observations suggesting that cuckoos are running a protection racket. I think the whole thing is an ethnic slur."
Rothstein, one of the world's foremost experts in brood parasitism, is taking a wait-and-see approach. "The results are not overwhelmingly strong, though they do point in the direction they're arguing," he says.
If not the threat of aggressive behavior, then what keeps birds from tossing out cuckoo eggs? Many birds do throw out parasitic eggs; American robins, orioles and bluejays, for example, will not tolerate an unfamiliar egg in their nests. Other birds simply abandon parasitized nests. If an indigo bunting finds a cowbird egg in its nest, it will fly off and start another one. Yellow warblers abandon the eggs but build a new nest on top of the old one.
Of the host birds that accept cuckoo eggs, some are apparently fooled by the cuckoo's mimicry skills. In Britain, for example, reed warblers have been shown to reject eggs that are different from their own, but will accept cuckoo copies. Other birds appear not to have developed sufficient powers of recognition. "Egg recognition sounds like a simple thing, but it's a totally new behavior," says Rothstein. "We don't know whether a species needs 500 years or several million to get the right mutations for that."
Experts also surmise that the costs of ejecting parasite eggs outweigh the benefits, making it better to keep the occasional odd egg. "If a bird sees a funny egg in its nest, it might gain if it throws it out and it's a cuckoo egg," says Payne. "On the other hand, it might be throwing out one of its own eggs that just happened to have a spot on it."
Host birds may also have a conditioned response to protect their own eggs. "They seem to play it safe," says Payne. "It usually works for a host bird to follow an internal rule: If an egg is in your nest, then it's yours."
There are other caveats to the so-called Avian Mafia theory. The fact that such aggressive behavior has so far been observed only on the Hoya de Guadix means that it could be site-specific, limited to southern Spain or certain geographical pockets where as-yet unknown forces come into play. Or the behavior could be restricted to only some parasites and some hosts. The magpies that Soler and his colleagues studied could also be victims of evolutionary lag, a situation in which one species--in this case, the parasite--has evolved a behavior against which the host has not yet had time to evolve a response.
Or such aggressive behavior could be a genuine show-stopper in the coevolutionary arms race, a strategy against which there is no defense. "It's an idea, just one possibility," concedes Zahavi, who nonetheless likes the symmetry such aggression lends the natural world. "That such behavior works with people, everybody knows. I think there is a need to collect more information. But I will not be surprised if here or there, people find more 'Mafia' among birds."
At this point, no one can say which view has more cachet--the idea itself or the mobster imagery, which in the polite, technical discourse of scientific journals resonates like the sound of submachine guns on a quiet suburban street. Zahavi seems amused by the terrible allure he has given the cuckoo--and not unaware of the publicity benefits.
"The Mafia is kind of an attraction," he admits. "Why do people go to see aggressive films, with the Mafia and shooting? I can't tell you. But they're not going to see movies about a nice family, where the mother and father take care of their young." Villains, perhaps, are somehow always more fascinating to humans.
To be safe, Ohio writer Frank Kuznik always gives a wide berth to cuckoos.