Blue-footed booby
The blue-footed booby is a marine bird native to subtropical and tropical regions of the eastern Pacific Ocean. It is one of six species of the genus Sula – known as boobies. It is easily recognizable by its distinctive bright blue feet, which is a sexually selected trait and a product of their diet. Males display their feet in an elaborate mating ritual by lifting them up and down while strutting before the female. The female is slightly larger than the male and can measure up to long with a wingspan up to.
The natural breeding habitats of the blue-footed booby are the tropical and subtropical islands of the Pacific Ocean. It can be found from the Gulf of California south along the western coasts of Central and South America to Peru. About half of all breeding pairs nest on the Galápagos Islands. Its diet mainly consists of fish, which it obtains by diving and sometimes swimming underwater in search of its prey. It sometimes hunts alone, but usually hunts in groups.
The blue-footed booby usually lays one to three eggs at a time. The species practices asynchronous hatching, in contrast to many other species whereby incubation begins when the last egg is laid and all chicks hatch together. This results in a growth inequality and size disparity between siblings, leading to facultative siblicide in times of food scarcity. This makes the blue-footed booby an important model for studying parent–offspring conflict and sibling rivalry.
Taxonomy
The blue-footed booby was described by French naturalist Alphonse Milne-Edwards in 1882 under the current binomial name Sula nebouxii. The specific epithet was chosen to honor the surgeon, naturalist, and explorer Adolphe-Simon Neboux. The two recognized subspecies are:| Image | Subspecies | Distribution |
| S. n. nebouxii | islands near the Pacific coast from Mexico to Peru | |
| S. n. excisa Todd, 1948 | Galápagos Islands |
Its closest relative is the Peruvian booby. The two species likely split from each other recently due to their shared ecological and biological characteristics. A 2011 study of multiple genes calculated the two species diverged between 1.1 and 0.8 million years ago.
The name booby comes from the Spanish word bobo because blue-footed boobies, like other seabirds, are clumsy on land. They are also regarded as foolish for their apparent fearlessness of humans.
Description
The blue-footed booby is on average long and weighs, with the female being slightly larger than the male. Its wings are long, pointed, and brown in color. The neck and head of the blue-footed booby are light brown with white streaks, while the belly and underside exhibit pure white plumage. Its eyes are placed on either side of its bill and oriented towards the front, enabling excellent binocular vision. Its eyes are a distinctive yellow, with the male having more yellow in its irises than the female. Blue-footed booby chicks have black beaks and feet and are clad in a layer of soft white down. The subspecies S. n. excisa that breeds on the Galápagos Islands is larger than the nominate subspecies and has lighter plumage especially around the neck and head.The Peruvian booby is similar in appearance, but has grey feet, whiter head and neck, and white spots on its wing coverts. The ranges of the two species overlap in the waters of northern Peru and southern Ecuador.
Since the blue-footed booby preys on fish by diving headlong into the water, its nostrils are permanently closed, and it has to breathe through the corners of its mouth. Its most notable characteristic is its blue-colored feet, which can range in color from a pale turquoise to a deep aquamarine. Males and younger birds have lighter feet than females. Its blue feet play a key role in courtship rituals and breeding, with the male visually displaying his feet to attract mates during the breeding season.
Distribution and habitat
The blue-footed booby is distributed among the continental coasts of the eastern Pacific Ocean from California to the Galápagos Islands south into Peru. It is strictly a marine bird. Its only need for land is to breed and rear young, which it does along the rocky coasts of the eastern Pacific.A booby may use and defend two or three nesting sites, which consist of bare black lava in small divots in the ground, until they develop a preference for one a few weeks before the eggs are laid. These nests are created as parts of large colonies. While nesting, the female turns to face the sun throughout the day, so the nest is surrounded by excrement.
Natal dispersal
Females start breeding when they are 1 to 6 years old, while males start breeding when they are 2 to 6 years old. Very limited natal dispersal occurs, meaning that young pairs do not move far from their original natal nests for their own first reproduction, leading to the congregation of hundreds of boobies in dense colonies. The benefit of limited dispersal is that by staying close to their parents' nesting sites, the boobies are more likely to have a high-quality nest. Since their parents had successfully raised chicks to reproductive age, their nest site must have been effective, either by providing cover from predation and parasitism, or by its suitability for taking off and landing. Bigamy has been observed in the species, and cases are known where two females and one male all share a single nest.Foot pigmentation
The blue color of the blue-footed booby's webbed feet comes from structures of aligned collagens in the skin modified by carotenoid pigments obtained from its diet of fresh fish. The collagens are arranged in a manner that makes the skin appear blue. The underlying color is a "flat, purplish blue". That color is modified by carotenoids to aquamarine in healthy birds. Carotenoids also act as antioxidants and stimulants for the blue-footed booby's immune function, suggesting that carotenoid pigmentation is an indicator of an individual's immunological state. Blue feet also indicate the current health condition of a booby. Boobies that were experimentally food-deprived for 48 hours experienced a decrease in foot brightness due to a reduction in the amount of lipids and lipoproteins that are used to absorb and transport carotenoids. Thus, the feet are rapid and honest indicators of a booby's current level of nourishment. As blue feet are signals that reliably indicate the immunological and health condition of a booby, coloration is favored through sexual selection.Female selection
The brightness of the feet decreases with age, so females tend to mate with younger males with brighter feet, which have higher fertility and greater ability to provide paternal care than older males. In a cross-fostering experiment, foot color reflects paternal contribution to raising chicks; chicks raised by foster fathers with brighter feet grew faster than chicks raised by foster males with duller feet. Females continuously evaluate their partners' condition based on foot color. In one experiment, males whose partners had laid a first egg in the nest had their feet dulled by makeup. The female partners laid smaller second eggs a few days later. As duller feet usually indicate a decrease in health and possibly genetic quality, it is adaptive for these females to decrease their investment in the second egg. The smaller second eggs contained less yolk concentration, which could influence embryo development, hatching success, and subsequent chick growth and survival. In addition, they contained less yolk androgens. As androgen plays an important role in chick survival, the experiment suggested female blue-footed boobies use the attractiveness and perceived genetic quality of their mates to determine how much resources they should allocate to their eggs. This supports the differential allocation theory, which predicts that parents care more for their offspring when paired with attractive mates.Male selection
Males also assess their partner's reproductive value and adjust their investment in the brood according to their partner's condition. Females that lay larger and brighter eggs are in better condition and have greater reproductive value. Therefore, males tend to display higher attentiveness and parental care to larger eggs, since those eggs were produced by a female with apparent good genetic quality. Smaller, duller eggs garnered less paternal care. Female foot color is also observed as an indication of perceived female condition. In one experiment, the color of eggs was muted by researchers; males were willing to exercise similar care for both large eggs and small eggs if their mate had brightly colored feet, whereas males paired with dull-footed females only incubated larger eggs. Researchers also found that males did not increase their care when females exhibited both bright feet and high-quality offspring.Behavior and ecology
Hunting and feeding
The blue-footed booby is a specialized fish eater, feeding on small schooling fish such as sardines, anchovies, mackerel, and flying fish. It will also take squid and offal. The blue-footed booby hunts by diving into the ocean after prey, sometimes from a great height, and can also swim underwater in pursuit of its prey. It can hunt singly, in pairs, or in larger flocks. Boobies travel in parties of about 12 to areas of water with large schools of small fish. When the lead bird sees a fish shoal in the water, it signals to the rest of the group and they all dive in unison, pointing their bodies down like arrows.Plunge diving can be done from heights of and even up to. These birds hit the water around and can go to depths of below the water surface. Their skulls contain special air sacs that protect the brain from enormous pressure. Prey are usually eaten while the birds are still under water. Individuals prefer to eat on their own instead of with their hunting group, usually in the early morning or late afternoon. Males and females fish differently, which may contribute to why blue-foots, unlike other boobies, raise more than one young. The male is smaller and has a proportionally larger tail, which enables the male to fish in shallow areas and deep waters. The female is larger and can carry more food. Both the male and female feed the chicks through regurgitation.