On Galápagos, Revealing the Blue-Footed Booby’s True Colors
With no real predators, the birds live proud, public lives. That accessibility has proved a bonanza for scientists, casting light on their mating habits and even why the shade of their feet matters.
A booby doing its mating dance. Credit Wolfgang Kaehler/LightRocket, via Getty Images
NORTH SEYMOUR ISLAND, Galápagos — The birds move with comic grace, like Fred Astaire and Judy Garland as hobo swells in oversize shoes. The male faces the female and slowly, slowly lifts up one foot, sets it down and lifts the other. Check out my feet! They’re blue. Really, really blue.
The female mirrors his ponderous moves. Mine are blue, too. Is this ground sticky, or what?
He leans over, spreads his wings wide, points his bill at the sky and whistles breathily, as if blowing on a toy flute. She grunts and totters up to him, and they clack bills. He grabs a pebble, and they clack bills again; he drops the pebble and spears a twig. Clack, whistle, grunt, whistle. And suddenly, she backs away.
Desperate, the male solemnly starts high-stepping again, displaying his beautiful teal-blue feet. But the courtship has fizzled, and when the female again lifts up a foot in response, it looks as if she’s waving goodbye.
It’s dating time here for the blue-footed booby. Everywhere, dozens of times a day, the large, handsome seabirds are making their highly ritualized courtship display — one reason the boobies are among the most celebrated and beloved residents of this archipelago.
Blue-Footed Booby Bird Mating Dance | Love in The Animal Kingdom | Nature on PBS 3:08
They are also feeding voraciously and spectacularly, circling high over the water, alert for the slightest flicker of fish, and then freezing in midair for a fraction of a second before dropping headfirst onto their targets, like missiles falling from a plane.
They squabble with one another over territory and nesting sites. They dodge parasitic frigate birds with red-balloon crops that pluck at the boobies’ tails and try to force them to regurgitate freshly caught fish but mostly fail.
On the Galápagos and on Isla Isabel, a Mexican national park south of the coast of Baja, blue-footed boobies have no real predators to fear or human hunters to shun, and as a result they live proud, public lives. That openness and accessibility, beyond captivating tourists, have proved a bonanza for scientists, too.
Blue-footed boobies on the Galápagos archipelago in Ecuador. The color of their feet is an important factor in mating. Credit The Asahi Shimbun Premium, via Getty Images
Research teams from Mexico and the United States have followed populations of the long-lived birds for years, even decades, and they have gathered a wealth of insights into the deep nature of being Sula nebouxii: how boobies choose and lose mates, the shifting allure of fidelity versus adultery, the measured brutality of older siblings, the contingency of parental love and the reason behind the boobies’ fetish for feet.
“They’re superfascinating animals and such a good research model,” said David J. Anderson of Wake Forest University in Winston-Salem, N.C., who studies both the blue-footed booby and the related Nazca booby. “They let you move among them without minding too much. You try to do that with a continental bird or mammal — forget about it. But with these guys you see it all.”
In one discovery that subverted expectations, Oscar Sánchez Macouzet and Hugh Drummond of the National Autonomous University of Mexico and their colleagues determined that boobies subjected to severe bullying and abuse as nestlings suffered few consequences as adults.
No matter how relentlessly the birds had been pecked at and bitten by older siblings, no matter how often food had been snatched from their beaks or how slowly they had grown, on reaching maturity the once-persecuted birds proved surprisingly confident, capable — unflappable. They were able to attract partners, repel rivals and raise families as successfully as their domineering peers.
Joe Burgess/The New York Times
“Bullying in infancy does not make wimpy adults,” Dr. Sánchez Macouzet said.
While many boobies change partners from season to season, there are great benefits of long-term fidelity, researchers have found. Comparing the breeding success of pairs that had been together for years with that of similarly mature boobies that had recently repartnered, scientists determined that the established pairs reared 35 percent more offspring to fledglinghood compared with the new mates.
And in new findings that will be published soon — and are enough to turn this working mother’s feet cerulean — scientists have discovered that the key to a successful long-term booby partnership is the equitable sharing of nest duties year after year.
Biparental care is the rule among boobies, but longtime mates have perfected the art of symmetry and turn-taking. They spend the same time brooding and feeding the young, and expend the same physical effort as seen in measures of blood cells and body mass.
Such egalitarian couples, said Dr. Sánchez Macouzet, “have reached the sweet spot of cooperation, compatibility and a willingness to avoid the exploitation of your partner.”
Dr. Drummond and his colleagues have also identified another booby mating pattern that seems to work nearly as well as a stable long-term partnership and in some ways contradicts it: the May-December effect.
For reasons that remain mysterious, Dr. Drummond said, booby couples in which one bird is young and the other old often have greater breeding success compared with pairs of the same age.
Analyzing the outcomes of 3,361 booby offspring, the researchers found that the chicks of age-mismatched parents were significantly more likely to later become parents themselves compared with the progeny of similarly aged pairs. It didn’t matter whether the mother was younger than the father or vice versa.
“The advantage to the chicks was the same either way,” Dr. Drummond said.
“Life is complicated,” he added, “and every year boobies are conjuring with several different variables when making their choice among partners.”
Blue-footed boobies, which hit the water at 60 miles per hour, hunting in Elizabeth Bay, at the western side of the Galápagos archipelago. Credit Tui De Roy/Minden Pictures
Someone familiar? Someone new? Nonnegotiable: The feet must be blue.
Blue-footed boobies are members of the family Sulidae, a group that includes about 10 species of gannets and boobies and is, by some analyses, part of the larger pelican order. The name booby is thought to come from bobo, the Spanish word for stupid or clown, a reference to the bird’s awkward waddle.
Blue-footed boobies can be found throughout the tropics and subtropics of the eastern Pacific. Though their overall population is not considered endangered, their numbers on the Galápagos have fallen since the 1990s, the result, scientists believe, of a local decline in the sardine stocks that the boobies need to breed.
The birds are about the size of large sea gulls, with wingspans up to five feet. Adult females are about 20 percent to 30 percent heavier and stronger than males. Boobies stay close to home, and if given the chance, most will live and breed within a few dozen feet of where they were born.
They often hunt small, schooling fish in flocks, each hitting the water at 60 miles per hour, its brain protected by specialized air sacs in the skull.
In many birds, males are the fancy ones, their exaggerated, colorful features the result of generations of females’ expressing their whimsical tastes in mates. Among blue-footed boobies, by contrast, males and females are both choosy about their partners, and one of the traits they fixate on is the relative blue-ness of a partner’s feet.
The optimal color, it turns out, is more of a turquoise.
Analyzing the booby’s blue feet, researchers have determined that the color is a result of both structural and pigmental components. The basic tone, a flat, purplish-blue, is set by an alignment of proteins stacked in the skin like spaghetti in a box that preferentially encage and emphasize blue light.
But the birds’ bodies modify that basic blue through diet, extracting bright yellow pigment from carotenoids in the fish they consume and concentrating it in their feet to create a dazzling aquamarine.
Researchers have found that booby eyes are keenly sensitive to blue-green light, and for good reason: Foot tone turns out to be a revealing sign of health and hardiness.
In a series of experiments, Roxana Torres of National Autonomous University and Alberto Velando of the University of Vigo in Spain and their colleagues showed that male boobies’ feet turned drab after just 48 hours of food deprivation and brightened again when feeding resumed. The injection of diphtheria and other vaccines would also affect a booby’s blue coloration, by setting up a competition between the skin of the feet and the newly taxed immune system for precious dietary carotenoids.
A Blue-footed booby and its two chicks. Credit Tui De Roy/Minden Pictures
Males avoid mating with females whose feet has been dulled with paint, the researchers also discovered. And when a male’s feet are artificially dulled after his mate has laid one egg, the female responds to the apparent decline in his condition through a downsizing of her own, making her second egg smaller than the first.
The calculus of daily life never stops. Boobies generally lay two eggs several days apart, and the older chick ends up with an enormous advantage over the second-born. In some booby species, like the Nazca, the difference is fatal.
“The big one up and kills the little one, pushing it right out of the nest,” Dr. Anderson said. The only hope for the second-born is if its sibling dies.
Among blue-footed boobies, though, sibling violence is provisional. As long as the parents can regurgitate enough food for both, the older, bigger sibling tolerates the second hatchling.
It will peck at the younger bird and demand chronic displays of submissive behavior, like facing away with its bill tucked down, but in good times it will let its sibling live.
Should the body mass of the elder nestling decline to 80 percent of normal, however, “it will increase the daily pecking of its sibling by 500 percent,” Dr. Drummond said. The heightened abuse can be fatal.
Just make it through your miserable childhood, beta booby, and you’ll soon be high-stepping in style.