Revealing the blue-footed booby's true colours

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 is dating time at North Seymour Island, Galapagos for the blue-footed booby.

On the Galapagos 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.

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 super fascinating animals and such a good research model,” said David J Anderson of Wake Forest University, USA, who studies both the blue-footed booby and the related Nazca booby.

Mating pattern
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 or how slowly they had grown, on reaching maturity the once-persecuted birds proved surprisingly confident. They were able to attract partners, repel rivals and raise families as successfully as their domineering peers.

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% more offspring to fledglinghood compared with the new mates. And in new findings that will be published soon,  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.

Hugh 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, Hugh 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. Analysing 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.

In many birds, males are the fancy ones, their exaggerated, colourful 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 blueness of a partner’s feet. The optimal colour, it turns out, is more of a turquoise. Analysing the booby’s blue feet, researchers have determined that the colour 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 emphasise 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. 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,” David 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 behaviour but in good times it will let its sibling live. Should the body mass of the elder nestling decline to 80% of normal, however, “it will increase the daily pecking of its sibling by 500%,” Hugh 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.