Blue-footed Booby

Blue-footed Booby
Conservation status
Least Concern (IUCN 3.1)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Suliformes
Family:	Sulidae
Genus:	Sula
Species:	S. nebouxii
Binomial name
Sula nebouxii Milne-Edwards, 1882
Range shown by red area

The Blue-footed Booby (Sula nebouxii) is a bird in the family Sulidae, which includes ten species of long-winged seabirds. It is of the genus Sula, which comprises six boobies. It is easily recognizable by its distinctive bright blue feet, which is a sexually selected trait. Males display their feet in an elaborate mating ritual by lifting one and then the other up, while strutting before the female. Both males and females prefer mates with brighter feet and adjust their parental investment based on the attractiveness of their mate.

The natural breeding habitat of the Blue-footed Booby is tropical and subtropical islands of the Pacific Ocean. Their range extends from the Gulf of California down along the western coasts of Central and South America to Peru. From about a third to a half of all breeding pairs nest on the Galápagos Islands.

The Blue-footed Booby usually lays one to three eggs at a time. The species practices asynchronous hatching, which means that eggs that are laid first are hatched before the consequent eggs, resulting in a growth inequality and size disparity between siblings. This could result in facultative siblicide in times of food scarcity. Therefore, the species is an effective model for studying parent-offspring conflict and sibling rivalry.

Taxonomy

The Blue-footed Booby was first studied extensively by Charles Darwin during his trip to the Galapagos Islands. There are two recognized subspecies, Sula nebouxii excisa (Todd, 1948) and Sula nebouxii nebouxii (Milne-Edwards, 1882). Its closest relative is the Peruvian Booby. The two boobies probably split off from each other in the recent past, because they still share a lot of ecological and biological aspects.

The name booby comes from the Spanish term bobo (which means 'stupid', 'fool', or 'clown') because the Blue-footed Booby is, like other seabirds, clumsy on land. They were also regarded as foolish for their apparent fearlessness of humans.

Description

The male (left) has a smaller pupil, has slightly lighter feet, and is smaller in size than the female.

The Blue-footed Booby is on average 81 cm (32 in) long and weighs 1.5 kg (3.3 lb), with females being slightly larger than males. The species can reach 70 to 90 cm in height, with a wingspan of up to 1.5 m. The wings are long and pointed, and are brown in color. The neck and head are brown with white streaks and the belly and underside are white. The booby's eyes are placed on either side of their bill and oriented towards the front, enabling excellent binocular vision. The Blue-footed Booby's eyes are yellow, with males having more yellow in their irises than the females. The Blue-footed Booby has permanently closed nostrils made for diving, necessitating breathing through the corners of their mouths. Their webbed feet range from a pale turquoise to a deep aquamarine in color. Males and younger birds are observed to have lighter feet than females.

Distribution and habitat

Blue-footed Boobies are distributed among the continental coasts of the eastern Pacific Ocean to the Galápagos Islands and California.

The Blue-footed Booby is strictly a marine bird. Their only need for land is to breed, which they do along rocky coasts.

Natal dispersal

Females start their own nest at 1 to 6 years, while males start from 2 to 6 years. There is very limited natal dispersal, meaning that young pairs do not move far from their original natal nests for their own first reproduction, which would explain the congregation of hundreds of pairs of boobies in dense colonies. The benefit of limited dispersal is that by staying close to their parents' nests, the boobies are ensured 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. It is interesting to note that bigamy has been observed in the species. There are known cases where two females and one male all share a single nest.

Foot pigmentation

The blue color of the webbed feet in Blue-footed Boobies comes from carotenoid pigments obtained from the diet. The pigments are antioxidants and stimulants for the immune system, so there is a trade-off between immune function and signaling, since any incorporation of the pigments into the feet is a direct detraction from those that could be used for immunity and detoxification. The blue feet are sexual signals that reliably indicate the condition of the male boobies, and coloration is favored through sexual selection. The brightness of the feet decreases with age, so females tend to choose males with brighter feet (younger males). Females prefer younger males because they have higher fertility and increased ability to provide paternal care than older males. In a cross-fostering experiment, it was shown that foot color really does reflect paternal contribution to raising chicks, because chicks raised by foster fathers with brighter feet grew faster than chicks raised by foster males with duller feet. Blue feet also indicate the current condition of males. Those that were experimentally food-deprived for forty-eight hours experienced a decrease in foot brightness. This is most likely because the deprivation of food reduces the amount of lipids and lipoproteins in the body which are used to absorb and transport carotenoids. Thus, the feet are rapid and honest indicators of a booby's current level of nourishment.

Researchers found that females continuously evaluate their partners' condition based on foot color. In an experiment, males whose partners had laid a first egg in the nest had their feet dulled by make-up. Consequently, the females laid smaller second eggs a few days later. Since duller feet usually indicate a decrease in health and possibly genetic quality, it is adaptive for females to decrease their investment in the second egg. The smaller second eggs contained not only less yolk concentration, which could in turn influence embryo development, hatching success, and subsequent chick growth and survival, but also contained less yolk androgens. Androgen plays an important role in chick survival, so this also shows that female Blue-footed Boobies use the attractiveness of their mates to determine how much resources they should allocate to their eggs. This possibly supports the "Differential Allocation Theory", which predicts that parents would invest more in the care of their offspring when paired with attractive mates.^{[citation needed]}

Females are not the only ones assessing their partner's reproductive value. Males, too, evaluate females and adjust their own investment in the brood according to their mate's condition. Like males, females also have a trade-off between signaling ornamentations and fecundity, because paler females actually produce more eggs in a brood. 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 was also an indication of female condition. In an experiment where the color of the eggs was muted by researchers, it was found that males were willing to exercise similar care for both large eggs and small eggs if his mate had brightly colored feet, whereas males paired with dull-footed females only incubated larger eggs. Interestingly, researchers also found that males did not increase their care when females exhibited both bright feet and high-quality offspring.

Behavior and ecology

Breeding

Displaying (Sky-pointing)

Another way of displaying by raising a foot

The courtship of the Blue-footed Booby consists of the male flaunting his blue feet and dancing to impress the female. First, the male presents his feet by strutting in front of the female. Then he will present nest materials to the female, and finally the courtship ends with a final display of feet to the female. The dance also includes "sky-pointing," which involves the male pointing his head and bill up to the sky while keeping the wings and tail raised. The Blue-footed Booby is a monogamous animal although they may have the potential to be bigamous. They reunite at their breeding grounds. The breeding cycle of the booby is every 8 to 9 months. The blue-footed booby is not a seasonally reproducing species. Instead, they are opportunistic breeders.

Hunting and feeding

The Blue-footed Booby's diet consists mainly of fish. Blue-footed Boobies are specialized fish eaters feeding on small school fish like sardines, anchovies, mackerel, and flying fish. They also feed on squid and offal. The Blue-foot dives into the ocean, sometimes from a great height, and can swim underwater in pursuit of its prey. It hunts singly, in pairs, or in larger flocks. Blue-foots 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 will signal the rest of the group and they will all dive together, pointing their bodies down like arrows and diving into the water in unison.

Plunge diving can be done from heights of 33–100 ft (10–30.5 m) and even up to 330 ft (100 m). These birds hit the water around 60 mph (97 km/h) and can go to depths of 82 ft (25 m) below the water surface. The specie's skulls contain special air sacs that protect the brain from the enormous pressure. The prey are usually eaten while the birds are still underwater. Surprisingly, individuals do not eat with the hunting group, preferring to eat on their own, 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 the tail is larger for its body, which enables the male to fish in shallow areas instead of just deep waters. The tail can flatten out, enabling him to change direction in the shallow water. The female is larger and can carry more food, which is regurgitated to the young at the nest. The males feed the young for the first part of the incubation period, because the males can bring back food more quickly than the female. When the demand for more food increases, the female begins to provide the food to the young.^{[citation needed]}

Rearing young

With egg and new young

The female Blue-footed Booby lays two or three eggs. Both male and female take turns incubating the eggs, while the non-sitting bird keeps watching. Since the Blue-footed Booby does not have a brooding patch (a patch of bare skin on the underbelly), it uses its feet to keep the eggs warm. The chicks cannot control their body temperature up until about one month old. Eggs are laid about four to five days apart. Blue-foots are one of the only two species of booby that raise more than one chick. This may be because of the male's specialized diving in shallow waters. They must be fed frequently, so the adults constantly hunt for fish. The chicks feed off the regurgitated fish in the adult's mouth. If the parent Blue-footed Booby does not have enough food for all of the chicks, it will only feed the biggest chick, ensuring that at least one will survive. Boobies may use and defend two or three nesting sites until they develop a preference a few weeks before the eggs are laid. Usually two to three eggs are laid, and one to two chicks are hatched. The incubation period is 41–45 days. They nest on bare black lava in small divots in the ground. The female will turn to face the sun throughout the day, so the nest is surrounded by excretion. These nests are created as parts of large colonies. The male and female share parental responsibilities. The male will provide food for the young in the first part of their life because of his specialized diving. The female will take over when the demand is higher.^{[citation needed]}

Sula nebouxii

Like other sexually size-dimorphic birds, female Blue-footed Boobies usually produce the smaller sex during times of food scarcity. During the breeding season, the female is about thirty-one percent heavier than the male. Booby chicks do not show clear differences in size based on sex, but females do grow faster than males, which means they require greater parental investment. Boobies are found to support the Flexible Investment Hypothesis, where a female will adjust the allocation of resources to maximize her lifetime reproductive success. This was shown in an experiment where female had their flight feathers trimmed, so that they had to expend more energy during flights to obtain food for their broods. Female chicks of such mothers were more strongly affected than their brothers, in that they had lower masses and shorter wing lengths.

Brood hierarchy due to asynchronous hatching

The Blue-footed Booby lays one to three eggs in one nest at a time, although 80% of nests only contain two eggs. The two eggs are laid five days apart. After the first egg is laid, it is immediately incubated, resulting in a difference in chick hatching times. The first chick is hatched four days before the other, so it gets a four-day head start in growth compared to its younger sibling. This asynchronous hatching serves many purposes. First, it spaces out the difficult time period in rearing where newborn chicks are too feeble to accept regurgitated food. In addition, it reduces the chance that a parent will suffer total brood loss to predators such as the milk snake.

Experiments have shown that asynchronous hatching may also reduce sibling rivalry. Experimentally manipulated synchronous broods produced more aggressive chicks, while in asynchronous broods chicks were less violent. This pattern of behavior arguably occurs through a more clearly established brood hierarchy in asynchronously hatched siblings. Although asynchronous hatching is not vital for the formation of brood hierarchies (the experimentally synchronous broods established them as well), it does aid in efficient brood reduction when food levels are low. The subordinate chicks in asynchronous broods died more quickly, thus relieving the parents of the burden of feeding both offspring when resources are insufficient to properly do so.

Facultative siblicide

Blue-footed Booby chicks practice facultative siblicide, which means opting to cause the death of a sibling based on environmental conditions. The chick that hatches first, often referred to as the A-chick, will kill the younger, B-chick, if there is a food shortage. The A-chicks grow faster than B-chicks and this initial size disparity is retained for at least the first two months of life. During lean times, the A-chick may attack the B-chick by pecking vigorously, or it may simply drag its younger sibling by the neck and oust it from the nest. Experiments where the necks of chicks were taped to inhibit food ingestion showed that sibling aggression increased sharply when the weight of the A-chicks dropped below 20-25% of their potential. There was a steep increase in pecking below that threshold, indicating that siblicide is in part triggered by the dominant chick's weight, not simply by the size difference between the siblings. It was also discovered that younger broods (those less than six weeks old) had three times the rate of pecking than older broods. This is perhaps due to the relative inability of younger-brood B-chicks to defend themselves against an A-chick attack.

The elder sibling also may harm the younger one by controlling access to the food delivered by the parents. A-chicks always receive food before B-chicks. This is not for lack of trying on the B-chick's part, since subordinate chicks beg just as much as dominant ones. However, the dominant chicks are able to divert the parents' attention to themselves, as their large size and conspicuousness serve as more effective stimuli.

However, another experiment showed that booby chicks do not operate exclusively by the "leftovers hypothesis", where younger chicks are fed only after the elder ones are completely satiated. The researchers identified a certain degree of tolerance of the younger sibling during short-term periods of food shortages. This tolerance hypothesis presents that the elder chicks will reduce food intake moderately, just enough so that the younger sibling does not starve. Although this system works during short-term food shortages, it is unsustainable during prolonged periods of dearth. In this latter case, the elder sibling will usually become aggressive and siblicidal.

Parental role in siblicide

In flight

Blue-footed Booby parents are passive spectators of this intrabrood conflict. They do not intervene in their offspring's struggles, even when they are upon the point of siblicide. Booby parents even appear to facilitate the demise of the younger sibling by creating and maintaining the inequality between the two chicks. They reinforce the brood hierarchy by feeding the dominant chick more often than the subordinate one. Thus, they respond to the brood hierarchy and not to the level of begging when deciding which chick to feed, as both chicks beg in equal amounts. This level of passivity towards the very possible death of their younger offspring may be an indication that brood reduction is advantageous for the parents. The Insurance egg hypothesis views the second egg and resulting chick as insurance for the parent in case the first egg does not hatch, or if food levels are higher than expected. The parents' behavior may make it seem like they are cooperating with the elder, dominant chick, and there appears to be very little evolutionary conflict between the parents and the A-chick.^{[citation needed]}

However, Booby parents may not be as indifferent as they seem. The parental behavior may in actuality be masking parent-offspring conflict, as the following experiments indicate. One team of researchers discovered that Blue-footed Booby parents make steep-sided nests that serve to deter the early ejection of the younger chick by the older one. This is in direct contrast to another species of Booby, the Masked Booby, where siblicide is obligate due to the ease in which older siblings can eject younger ones from their flat nests. When the blue-footed Booby nests were experimentally flattened, the parents restored them to their original steepness. In another experiment, Blue-footed Booby chicks were swapped into nests of the Masked Booby. These chicks were then more likely to engage in siblicide, which reveals that the type of parental care somehow affects the level of siblicide. Parents also appeared to respond more frequently to chicks that were in poorer body conditions during periods of food deprivation. Egg-mass analysis shows that in clutches produced at the beginning of the breeding season, the second egg in a nest were on average 1.5% heavier than the first. Heavier eggs give rise to heavier chicks that have greater fitness, which indicates that parents may have tried to level the playing field from the start to give the second chick a heightened chance of survival. Hormonal analysis of eggs also shows that there appears to be no parental favoritism in regards to androgen allocation, meaning that androgen levels are about the same in both eggs. However, the researchers note that this could be because the species have evolved more simple ways to manipulate asymmetries in order to maximize the parents' reproductive output. Thus, these experiments show that what may at first appear to be parental cooperation with the elder chick may in fact mask a genetic parent-offspring conflict.^{[citation needed]}

Long-term effects of hierarchies

There is always a dominance-subordination relationship between the chicks in a brood. Although the dominant A-chick grows faster and survives more often than the subordinate B-chick during infancy, there is actually no difference in reproductive success between the two types of siblings afterwards. In one longitudinal study, there appears to be no long-term effects of dominance hierarchies, and in fact the subordinate chicks were often observed to produce nests of their own before their dominate siblings. However, this finding may have been due to the weaker B-chicks being killed off more easily than more fit B-chicks when in the nest. This would inadvertently leave behind stronger, more resilient subordinate chicks in this particular experiment.^{[citation needed]}

Communication

Blue-footed Boobies will make raucous or polysyllabic grunts or shouts and thin whistle noise. The males of the species have been known to throw up their head and whistle at a passing, flying female. Their ritual displays are also a form of communication.

Mates can recognize each other by their calls. Researchers have analysed calls of Blue-footed Boobies and conducted playback experiments. Unique individual signatures were present although the calls differed between sexes. Both males and females can discriminate the calls of their mates from others. The acoustic cues used to identify individuals may differ between the sexes.

Further reading

• Nelson, Bryan. (1968) Galapagos Island of Birds. New York: William Morrow & Company.
• Perrins, Dr. Christopher M. and Dr. Alex L.A. Middleton. (1985) The Encyclopedia of Birds. New York: Facts of File Publications. ISBN 0-8160-1150-8
• Hutchins, Michael, Jerome A. Jackson, Walter J. Bock, and Donna Olendorf, eds. (2002) Grzimek's Animal Life Encyclopedia, Edition 2 - Volume 08 - Birds I, Farmington Hill, Michigan: Gale Group. ISBN 0-7876-5784-0