Blue-footed booby

Name and taxonomy

The scientific name of the blue-footed booby is Sula nebouxii. The French naturalist Alphonse Milne-Edwards described the species in 1882. He named it after Adolphe-Simon Neboux, a naval surgeon and naturalist. Two subspecies are recognised, and the Galapagos birds are the larger and paler of the two. The English word booby probably comes from the Spanish bobo, which means foolish. Sailors used it because the birds move awkwardly on land and show no fear of people. The name stuck, though the bird is probably as precise as any seabird in the air. Spanish speakers along the coast may still call the group piqueros.

Evolution and relatives

The bird belongs to Sulidae, the family of gannets and boobies. Six species share the genus Sula. The Peruvian booby is the closest relative, and a study of many genes dated the split at 1.1 to 0.8 million years ago. The two ranges overlap off northern Peru and southern Ecuador. A Peruvian booby has grey feet, a whiter head and white spots on the wing coverts. The two species share a way of life, which is probably why they look so alike.

Like other members of the family, the booby has a torpedo-shaped body and a dagger bill. Because it enters the water head first, its nostrils are sealed shut for good. The bird breathes through the corners of its mouth instead. Air sacs in the skull spread the shock of impact away from the brain. These traits are typical of a family that feeds by plunging.

Appearance

The average adult is 81 cm long and weighs about 1.5 kilograms. Males average 1.283 kg and females average 1.801 kg. The wingspan measures about 152 cm, and the wings are long, pointed and brown. The head and neck are pale brown with heavy white streaks, and the belly is clean white. A white patch marks the back of the neck where it joins the mantle.

The eyes are yellow and sit on either side of the bill, facing forward. That placement probably gives the bird the binocular vision a dive needs. Males carry more yellow in the iris than females. Foot colour runs from pale turquoise to deep aquamarine, and males and young birds are the palest. Chicks hatch with black bills and feet under a coat of white down. The blue bill has serrated edges that grip a slippery fish.

Habitat and range

The species ranges down the eastern Pacific from the Gulf of California to Peru. It breeds on dry islands off the coasts of Mexico, Central America and South America. The Galapagos Islands hold about half of all breeding pairs. In some years a few birds wander north to the Salton Sea in California. The booby is a marine bird and comes ashore only to breed and raise young.

A nest is no more than a scrape in bare black lava. A pair may hold two or three possible sites and settle on one shortly before laying. Nests sit inside large colonies, but each pair keeps a gap around its own patch. The sitting female turns to face the sun through the day, so droppings ring the nest. Young birds settle close to where they hatched, which packs the colonies tightly.

Diet and feeding

The booby lives on small fish that shoal, above all sardines, anchovies, mackerel and flying fish. It takes squid and scraps thrown from boats as well. Parties of about 12 birds search for a shoal together. The first bird to spot fish signals, and the whole group drops at once. Each bird folds its wings back and enters the sea like an arrow.

Dives usually start between 10 and 30.5 metres up, and some begin as high as 100 m. The bird hits the surface at about 97 kilometres an hour. It can then swim on down to 25 m below the surface. Most prey is swallowed before the bird comes back up. It can also dive from a floating position, which the other boobies cannot do. Birds prefer to eat alone in the early morning or late afternoon. A booby may travel far from the colony to find a shoal worth diving on. Fish supply therefore appears to set how many chicks a colony can raise.

Behaviour and social life

The booby is colonial on land and often social at sea. It hunts alone, in pairs or in flocks, depending on where the fish are. Males and females feed in different water, which spreads the effort of a brood. The smaller male has a proportionally larger tail and works both shallow and deep water. The heavier female dives deeper and farther offshore and carries more food back.

On land the bird walks in an ungainly high step and shows little fear of people. That tameness gave the species its name and once made it easy to catch. Pairs defend a small circle of ground rather than a wide territory. Bigamy happens, and two females sometimes share one nest with a single male. Colonies are noisy, and a landing bird may have to cross ground that other pairs guard. Young birds seem to gather at the edge of a colony before they start to breed.

Communication and senses

Boobies make harsh grunts, shouts and thin whistles. A male will throw his head back and whistle at a female passing overhead. Females answer with a groaning call rather than a whistle. Calls differ between the sexes and also between individuals. Both partners can pick their mate's voice out of the colony.

Display appears to carry as much meaning as sound. Sky-pointing raises the bill, wings and tail together and marks the end of a courtship run. Foot colour is a signal in itself, and it fades within 48 hours if a bird goes hungry. Forward-facing eyes probably give the depth judgement a plunge diver needs.

Reproduction and life cycle

The booby is an opportunistic breeder and starts a cycle every 8 to 9 months. Females first breed at 1 to 6 years old and males at 2 to 6. Courtship begins when the male struts before the female with his feet raised. He offers scraps of nest material, then finishes with another display of the feet. Pairs are usually monogamous for the cycle.

The female lays one to three eggs, and about 80% of nests hold exactly two. Eggs come four to five days apart, and the first is incubated at once. Neither parent has a brood patch, so both wrap the eggs in their warm webbed feet. Incubation runs 41 to 45 days, and one bird sits while the other watches. Chicks stand on a parent's feet for their first month. Both adults feed them regurgitated fish taken from inside the bill.

Because laying is staggered, the first chick starts life four days ahead of the second. In a lean season the older chick pecks the younger one or drags it out of the scrape. Aggression rises sharply once the older chick falls 20 to 25% below its target weight. Parents do not step in, and they feed the larger chick first. They do build a steep-sided nest, which makes it harder to push a chick out. The oldest bird on record reached 18 years.

Predators and threats

Milk snakes raid nests for eggs and small chicks. Staggered hatching lowers the chance that a raider takes a whole brood at once. Adults face few predators on a bare island, which may explain their tameness. Egg collectors still take a small number of clutches outside the Galapagos. Ticks infest nestlings, and broods with parents of different ages carry the fewest. A chick on open lava may also suffer heat and thirst on a still day.

Food supply seems to be the real threat. Sardines have been largely missing from the waters around Espanola Island since 1997. In 2012 and 2013 only about half the prey items in the diet were sardines. Birds short of sardines often fail to breed at all rather than raise fewer young. Researchers found no sign that people, introduced predators or disease drive the decline.

Population and conservation

The IUCN lists the blue-footed booby as Least Concern. Breeding pairs number under 40,000, and the world total is near 90,000 birds. Roughly half of the breeding population sits in the Galapagos Islands. Galapagos law protects the species where it is most concentrated. Mexico lists the bird under special protection in its national wildlife rules.

A study finished in April 2014 confirmed a fall in the Galapagos population. The birds appear to be breeding poorly rather than dying in unusual numbers. Researchers linked the trouble to the decline of sardines in local waters. Annual counts are still needed to separate a long slide from a normal swing. A colonial seabird tied to one prey fish is probably vulnerable to any change in the sea.

Relationship with humans

Boobies once settled on the decks of sailing ships, and crews caught and ate them without effort. William Bligh and other shipwrecked sailors lived on them for a time. That behaviour probably built the bird's reputation for stupidity. Today the same tameness seems to make it a favourite of visitors to the Galapagos. Tourists may stand close to a dancing male without disturbing the display.

The species appears to be a mainstay of Galapagos wildlife tourism. Its courtship dance appears in nature films and in guidebooks worldwide. Egg collecting is now the only direct human take of any size. Fishing that removes sardines probably hurts the bird more than any hunting ever did. The booby is also a study animal for work on sibling rivalry and parental care.

In culture and myth

In Spanish tradition the name bobo marks a person who is slow or clownish. Sailors carried that word to the bird, and English kept it as booby. The name probably says more about sailors than about the animal. A bird that never met a land predator may have had no reason to flee a boat.

In modern Galapagos tourism the booby appears as a mascot of the islands. Its dance appears on shirts, posters and stamps sold to visitors. In British sailing tradition, boobies were remembered as food of last resort. The bird probably serves best as a teaching example of sexual selection. Few animals seem to show a signal of health as plainly as a foot that fades in two days.