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. 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. The
courtship of the blue-footed booby consists of the male flaunting his blue feet and
dancing to impress the female. The male begins by showing his feet, strutting in front of the female. Then, he presents nest materials and finishes the mating ritual with a final display of his feet. 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. File:Blue-footed Booby (Sula nebouxii) -displaying.jpg|Displaying (sky-pointing) File:Blue-footed Booby (Sula nebouxii) -one leg raised.jpg|Another way of displaying by raising a foot
Rearing young The blue-footed booby is one of only two species of booby that raises more than one chick in a breeding cycle. The female blue-footed booby lays two or three eggs, about four to five days apart. Both male and female take turns incubating the eggs, while the nonsitting bird keeps watch. Since the blue-footed booby does not have a
brooding patch, it uses its feet to keep the eggs warm. The
incubation period is 41–45 days. Usually, one or two chicks are hatched from the two or three eggs originally laid. The male and female share parental
responsibilities. The male provides food for the young in the first part of their lives because of his specialized diving. The female takes over when the demand is higher. Fledglings are more likely to become reproductive adults when one parent is old and the other young. The reason for this is unknown, but nestlings with different aged parents are least infected by ticks. File:Blue-footed booby with chick and egg.jpg|With egg and chick File:Blue-footed Booby with young.jpg|With egg and new young File:Sula nebouxii - 03.jpg|Chick
Brood hierarchy due to asynchronous hatching Although the blue-footed booby lays one to three eggs in one nest at a time, 80% of nests contain exactly two eggs. Eggs are laid five days apart. After the first egg is laid, it is immediately incubated, which results in a difference in chick hatching times. The first chick hatches four days before the other, so it receives 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 during which newborn chicks are too feeble to accept
regurgitated food. In addition, it reduces the chance that parents will suffer total brood loss to predators such as the
milk snake. Asynchronous hatching may also reduce
sibling rivalry. Experimentally manipulated synchronous broods produced more aggressive chicks; chicks in asynchronous broods were less violent. This pattern of behavior arguably occurs through a 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. Subordinate chicks in asynchronous broods die 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, opting to cause the death of a sibling based on environmental conditions. The A-chick, which hatches first, will kill the younger B-chick if a food shortage exists. The initial size disparity between the A-chick and B-chick is retained for at least the first two months of life. 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. Although subordinate chicks beg just as much as their dominant siblings, the older chicks are able to divert the parents' attention to themselves, as their large size and conspicuousness serve as more effective stimuli.
Parental role in siblicide Blue-footed booby parents are passive spectators of this intrabrood conflict. They do not intervene in their offspring's struggles, even at 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. Blue-footed booby chicks that were placed in masked booby nests were more likely to engage in siblicide, which reveals that parental care somehow affects the level of siblicide. Parents also appear to respond more frequently to chicks that are 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. Since heavier eggs give rise to heavier chicks that have greater fitness, this evidence indicates that parents may try to rectify any disadvantages that accompany a late hatching date by investing more into the second egg. Hormonal analysis of eggs also shows that no parental favoritism seems to exist in regards to
androgen allocation. This may simply be because the species has evolved simpler ways to manipulate asymmetries and maximize parental reproductive output. What may at first appear to be parental cooperation with the elder chick may in fact mask a genetic parent-offspring conflict.
Communication Blue-footed boobies make raucous or polysyllabic grunts or shouts and thin whistling noises. The males of the species have been known to throw up their heads and whistle at a passing, flying female. These ritual displays are also a form of communication. Mates can recognize each other by their calls. Although calls differed between sexes, unique individual signatures were present. Both males and females can discriminate the calls of their mates from others.
Population decline Concerns of a decline in the booby population of the Galápagos Islands prompted a research project in its cause. The project, completed in April 2014, confirmed the population decline. The blue-footed booby population appears to be having trouble breeding, thus is slowly declining. The decline is feared to be long-term, but annual data collection is needed for a firm conclusion that this is not a normal fluctuation. Food problems may be the cause of an observed failure of the birds to even try to breed. This is related to a decline in sardines (
Sardinops sagax), an important part of the boobies' diet. Earlier work on
Española Island had successful breeding in blue-footed boobies that have access to sardines, in which case their diet consists essentially wholly of sardines. However, sardines have been largely absent from the Española area since 1997, as has been shown from
Nazca boobies there, which also prefer sardines, but can breed using other prey. In 2012–13, roughly only half of prey items in the boobies' diet were sardines. No evidence was seen of other possible causes for the decline, such as effects of humans, introduced predators, or disease. ==References==