Blue-footed booby Galapagos Islands map
The blue-footed booby is a marine bird and only requires land to rear and breed young, as a result, it has evolved to become an excellent diver. With a pointed, tapered bill and a torpedo-shaped body, these birds were built for penetrating air and water. A light, long tail along with a short upper arm provides the booby with an excellent ability to dive and maneuver in shallow waters. They also have air sacs between their skin and muscles, and in the skull, which inflate to function as shock absorbers. These cushion the impact of their dive, specifically protecting their brain from the enormous amount of pressure it endures. Additionally, closed nostrils prevent any water from being forced into their nose.
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Watching these birds in action is awe-inspiring as they are gifted with a powerful and direct flight. The booby feeds by plunge diving into the water. This dive typically begins from heights of 10–30.5 m (33–100 ft.), although it can reach up to 100 m (330 ft.). They accelerate their dive by flying towards the water before folding their wings into their body and transforming into an arrow, hitting the water at approximately 97 km/h (60mph). Once in the water, they are also powerful swimmers and can dive to depths of 25 m (82 ft) below the water surface. It even has the ability to catch flying fish as they leap into the air, or dive into the water while in a swimming position. They have a formidable swim, which assists them in capturing their prey under water.
Although it occasionally hunts alone or in pairs, the booby typically fishes in groups of about 12 birds in areas of water with large schools of small fish. Intriguingly, each species of booby in the Galapagos Islands seems to have created its own ecological niche so as to avoid competition with the other species. While the blue-footed boobies fish close to shore, the Nazca booby fishes farther offshore and the fishing area of the red-footed booby is even farther from land. When fishing, the birds fly together in a ragged line and once one bird spots a fish shoal, it signals this to the rest of the group and they dive in unison, shooting towards the sea. Interestingly, males and females fish differently due to their slightly different body types. Because the male is smaller and therefore, has a proportionately larger tail, it is able to fish in shallow waters, as well as deep waters. On the other hand, the female is larger and is able to carry more food. The slightly distinct techniques of each sex may be what enables the birds to raise more than one young, unlike other boobies.
While it is a representation of grace and power in the air and water, the booby is not so well renowned for its movements on land. The name booby is derived from the Spanish word bobo, which can mean ‘stupid’, ‘fool,’ or clown. In part, this is due to their apparent fearlessness of humans, which today in time is unfortunately regarded as foolish. However, this name is more likely due to their behavior on land. This bird that evolved for the air and water has an almost clownish appearance as it clumsily wobbles about.
Courtship dances and blue feet
However, these awkward actions are perhaps best displayed during the famous blue-footed booby courtship dance. The dance centers around the blue feet of the male and so, to best display his most attractive trait, the male parades around the proposed nest site, strutting his blue feet for the female and lifting them high into the air while looking down with his beak. Then, after presenting nest materials to the female, he spreads his wings and stretches his beak upwards making a whistling noise. The ritual is then completed with yet another dazzling foot display.
Once two mates are paired, they interact through ‘sky-pointing.’ During this act the birds look up to the sky performing certain head and wing movements, the intensity of which determines the intensity of the union. In essence, the extent to which they extend their wings signifies the strength of the union, the determents of which are established further below.
The feet of a male blue-footed booby is an essential part of his dance, as well as in the females decision to choose a mate. Females choose their mate based on the size and luminosity of the feet of the male, and indeed it has been determined that males with brighter feet produce more offspring. The size of a male’s feet is important because the birds incubate their eggs using their feet, thus having big feet allows the birds to cover a greater circumference and incubate a larger area of the egg. On the other hand, the blue color has a fascinating relation with the health of a booby and its ability to rear its young. The brilliant blue is a result of carotenoid pigments, which are obtained from the birds’ diet of fresh fish. Carotenoid is an important antioxidant and also stimulates the immune system. Therefore, the pigmentation is understood as an indicator of the bird’s immunological state: only those birds with a strong immune system can afford to expend this pigment on the color of their feet.
Additionally, the color is an indicator of the condition of the body. In a recent study, birds were experimentally deprived of food for forty-eight hours, and as a result of the lack of nourishment, the brightness of their feet decreased. Likewise, young males have the brightest feet, while older birds have a much duller color. In other words, the bluer the feet, the healthier the bird and the more desirable he is. Foot color has been found to reflect the paternal contribution to raising young. A cross-fostering experiment determined that the chicks of foster fathers with brighter feet grew faster than those that were raised by foster fathers with duller feet. This is likely to be because the bird is healthier and is able to procure more food and a safer environment. Even more fascinating is that females physically and mentally adapt to the color of their mates feet. The bird’s foot coloration is extremely sensitive, so much so that females are able to perceive alterations in the coloring within 48 hours. As a result, the female is able to alter her investment in the young based on day-to-day assessments. During one study, the color of certain birds whose mate had already laid one egg was dulled with make-up. Following this, the second egg that the mate laid was smaller because duller feet are associated with a lower health and genetic quality, thus the females decreased their investment in the second egg. This reveals that females use the attractiveness of their mate to determine the resources they will put towards their eggs.
Likewise, the male determines his investment in the chick based on the condition of the female. Healthier females lay larger and brighter eggs, which in turn, have a greater reproductive value. As a result, the males are more attentive to larger eggs because they represent a female with greater genetic quality. Nevertheless, studies have shown that males are equally attentive to large and small eggs if the mate has brightly colored feet.
However, what makes blue-footed boobies perhaps even more unique is their unusual ability to enhance the color of their feet and thus, their sexual success, through abstinence, a trait which a remarkably few animals have. Studies have found that blue-footed birds have brighter feet when they do not reproduce for a year, whereas those that are successful in reproducing have drabber feet. As a result, those birds that were unsuccessful in mating are more successful during the following season. Scientists owe this to the energy that males exert during courtship and in rearing the young. Because of the efforts they exert, they have less carotenoid left over to divert to their feet. In several species, sexual displays are one of the most energetic acts that a male performs and this is certainly true for blue-footed boobies.
Rearing the young
Once a pair has been united, a nesting site must be established and secured. Blue-footed boobies are opportunistic breeders, which means that they will not begin breeding until the appropriate conditions are available. Thus, when an adequate amount of food is accessible, the breeding begins. Blue-footed boobies principally breed in three areas: on islands and headlands in the eastern Pacific Ocean (primarily between Ecuador and Peru), the Galapagos Islands and the islands in the Gulf of California. When it comes to choosing a nest, blue-footed boobies typically defend several nesting sites prior to laying the egg and do not actually choose a specific nest until just a few weeks before the eggs are laid. The nests are created in small divots of bare black lava in large colonies.
Once the eggs are laid, they must be kept at a constant temperature. Remarkably, the boobies are able to maintain a temperature at almost exactly 39C by nestling against the eggs or raising their body to allow cool air to circulate in the nest, and incubating the eggs with their feet. Additionally, as the bird is nesting it turns to face the sun throughout the day.
In this way, the nest is evenly surrounded by a wall of excretion, protecting it from external elements. Following the incubation period, which is approximately 41 to 45 days (taking place from May through August), the eggs begin to hatch. Blue-footed booby parents share the responsibility of rearing young until the chicks are old enough to be on their own. Initially, as the egg is incubating, the parents take turns sitting on the nest; while one mate incubates the eggs, the other stands on guard to protect the nest. Furthermore, once the eggs hatch, this repartition of responsibility continues. During the first part of the chicks’ life, the male provides food to the young, using his specialized diving skills. Later, when the demand for food is higher, the females procure the food owing to their ability to carry more food at once.
Once the chicks are old enough to leave the nest, they rarely travel far. There is fairly limited natal dispersal among blue-footed boobies or in other words, mates do not move far from their original natal nests. This results in extremely dense colonies of hundreds of boobies. However, it is beneficial for the birds because, being close to their natal nest (which was successful for their own growth and protection), it is likely to be a suitable area for rearing young and will have a high nest quality.
As was previously mentioned, the blue-footed booby typically (80% of the time) raises two chicks, sometimes as many as three, and in fact it is one of only two species of booby that raises more than once chick during a breeding cycle. However, the eggs are not laid at the same time, but approximately five days apart. This has many benefits for the blue-footed population. This asynchronous hatching allows the parent to provide more attentive care to the chicks in the first few days of their life, while they are too feeble to consume regurgitated food. Because the chicks are born at different times, they do not undergo this stage during the same period and thus, the parent is not forced to tend to both at once. This method also greatly increases the chance that at least one of the brood will survive. Furthermore, research has determined that asynchronous hatching reduces sibling rivalry. Synchronous broods that were produced experimentally resulted in more aggressive chicks in comparison with asynchronous broods. As a result of being less aggressive, subordinate chicks in asynchronous broods die more quickly, which as a result, relieves the parents of the burden to feed the chick when resources do not permit this.
However, this hatching method creates a pecking order among the chicks. The moment the first egg is laid, it is incubated and thus, the first chick will hatch roughly 5 days before the second, giving it five additional days of growth. As a result, the first-born dominates the second chick in order to receive a greater share of the food provided by the parents. By diverting the parents’ attention from the younger chick using its size, it is able to acquire food first and ensure that it receives its fill. In the event that the parents are not able to procure enough food for the entire brood, this often leads to the death of the subordinate chick either because the older chick does not leave it a sufficient amount of food or because the parents merely stop feeding it. If the food shortage becomes extremely severe, it is likely that the dominant chick will kill the weaker one in an act of siblicide.
While this sounds harsh, sibling rivalry and siblicide are vital to the survival of the blue-footed boobies as they allows the parents to focus their minimal resources on one chick. During prolonged shortages, the elder chick will become increasingly aggressive until it either pecks its sibling to death or ousts it from the nest. However, it is important to mention that booby chicks do not exclusively apply the Leftover Hypothesis (in which the younger chicks are fed only after the elder ones are completely satisfied). During short-term food shortages, studies have shown that the older chick demonstrates a certain tolerance towards the younger one. The first-born will moderately reduce its food intake just enough so that the younger one(s) does not starve. In fact, the occurrence of siblicide is directly connected to the weight of the dominant chick. If its weight falls below 20-25% of its potential, aggression dramatically increases, eventually leading to the death of the subordinate chick. However, if this is not the case, it is unlikely that the older sibling will harm the younger one. Interestingly, this behavior does not appear to affect the chicks later in life and on the contrary, subordinate chicks have been observed to produce their own nest before their dominant sibling.
Nevertheless, this is not to say that parents play an entirely passive role throughout this process. Experiments have discovered that parents will allocate their resources to ensure their reproductive success during times of food scarcity. In other words, if the parents do not have enough resources to feed all of the chicks, they will favor the smaller individual, which requires a smaller investment. The initial size disparity between the chicks only exists for about two months and following this period, their size is based on sex instead of age. Because females grow faster than males, they tend to be larger, representing a greater investment for the parent. As a result, the parents’ attention is turned from the unfortunate female towards the male (if this be the case). However, regarding the event of siblicide, not only do the parents fail to aid the struggling chick, but rather they encourage the inequality, feeding the dominant chick more often than the subordinate one. This leads experts to believe that siblicide is beneficial to the parent and that the second egg is more a type of insurance in the event that the first one does not hatch.
On the other hand, experiments have discovered that the parents do attempt to avoid siblicide in itself. Blue-footed boobies, in comparison with masked boobies for example, design steep-sided nests to deter the dominant chick from ousting the subordinate one and when the nests were experimentally flattened, the parents rebuilt them. Additionally, although it has not been determined whether this is intentional, it is possible that females produce a larger second egg in order to increase the survival probability of the second chick. Analyses have shown that the second egg in a nest at the beginning of the breeding season is, on average, 1.5% heavier than the first, ultimately leading to a stronger chick. This suggests that the parents do place a stronger effort on the second egg. However, this may simply be due to the evolution of the species, which favors maximizing reproductive output.
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Threats to the blue-footed population
Unfortunately, over recent years an island-wide decline in population has provoked concern regarding the blue-footed population, generating the need for a comprehensive study to accurately assess the current population size and understand the reasons behind their decline. The most recent study, published in the online journal Avian Conservation and Ecology in April 2014, confirms the decline and additionally, suggests that it may be a result of the decline in clupid fish, particularly in sardines.
According to this study, although the adult population is reaching normal life expectancies, the birds are having difficulties breeding, thus resulting in a gradual decrease in population. Experts cannot be certain whether this decrease is permanent or simply a normal fluctuation, but current findings suggest this to be long term.
Rather then unsuccessful rearing of the young, it appears that many of the birds are not even trying to breed due to a shortage in food. Previous studies on Española determined that blue-footed boobies breed successfully when there is plentiful access to sardines. However, the waters around Española Island has been experiencing extreme shortages of sardines since 1997. This is clearly displayed through the populations of Nazca boobies living on the island that, while they prefer sardines, are able to breed while consuming other prey. This data indicates that blue-footed boobies are unable to acquire the sufficient amount of prey in order to breed, although they have enough to satisfy their own survival needs. It is possible that other factors are involved in this decline; however, no evidence has been found to suggest that this is resulting from introduced predators, disease and/or persecution by humans.
With exception to the cause for recent declines, the blue-footed population faces relatively few threats; indeed one of the largest is siblicide. However, they are occasionally hunted by short-eared owls and suffer the rare shark attack. In addition, blue-footed boobies have shown to be significantly impacted by major and harmful environmental alterations. For example, in 1992 a colony on Isabela Island failed entirely to reproduce as a result of El Niño. The following year, once El Niño had passed, reproductive success was once again high and the population was thriving. Although scientists cannot be certain of why this occurred, it may be related to significantly decreased concentrations of the hormone corticosterone. Corticosterone has a crucial rule in regulating energy, immune reactions and stress responses. It is likely the birds suffered from acute stress and weakened immunological systems, but luckily, this was not long term.
In coming years, experts hope to find an answer to the problems faced by the blue-footed boobies in order to deter any significant losses and keep these magnificent birds on their marvellous blue feet.