(Excerpted sections on boobies from Chapter 6 of Galapagos - A Natural History )
|A general section on Seabirds|
|A general section on boobies|
|A section on the blue-footed booby|
|A section on the masked booby|
|A section on the red-footed booby|
|A general section on seabird ecology with quite a bit of info about boobies|
Because the Galápagos Islands are surrounded by thousands of miles of open ocean, seabirds have a prominent place in the fauna of the islands. There are nineteen resident species, five of which are endemic, and most of which are likely to be seen by the visitor who spends a week in the islands, whatever the time of year. There may be as many as three-quarters of a million seabirds in the Galápagos, including 30 per cent of the world's blue-footed boobies, the world's largest red-footed booby colony and perhaps the largest concentration of masked boobies in the world (Harris 1984).
Birds must keep their plumage in good condition if they are to survive and breed. Most seabirds change or moult their feathers each year or so before they become too ragged. Between moults, birds spend much time preening their feathers, straightening them out and keeping them oiled and waterproof. The Galápagos penguin spends as much as three hours a day preening to keep even its reduced plumage in top condition (Boersma 1976). In virtually any seabird colony you visit, there will be some birds meticulously taking care of their plumage.
Visitors to the islands are able to retire to the shade during the heat of the middle of the day. Seabirds trying to raise their young must often spend hours, days, and even weeks, staying in the same spot as they incubate their egg(s) or brood their young. Nestlings must spend months in the same place with no shade. As you wander through seabird colonies in the Galápagos, look for ways in which the birds are trying to keep cool under the hot sun. Albatross chicks are often to be found sitting in the meagre shade of shrubs; boobies, pelicans, cormorants, and frigatebirds all use a form of panting, called "gular fluttering," to lose heat. The loose flaps of skin between the bill and neck are moved and the air currents generated evaporate moisture and cause evaporative cooling; boobies are frequently found with the feathers on their backs fluffed up and their bodies oriented so that the breeze comes from behind and cools the skin, and the feet are kept shaded. Red-footed boobies and frigatebirds may often be seen draped over their nest or a branch with their necks hanging down, wings drooped, and cloacal (anal) surfaces exposed to the air but in the shade of the body.
Many seabirds have a sunning behaviour. Frigatebirds and herons may be seen sitting or standing in a near vertical position with their wings turned to give a "palms up" appearance. Boobies are frequently seen with their wings outstretched behind them as they face their backs to the sun. The function of this type of behaviour is not well understood but may be connected with disturbing parasites or with Vitamin D production (Nelson 1979).
Seabirds must overcome the problem of salt accumulation. The Ancient Mariner was correct in saying "water, water everywhere, nor any drop to drink," as sea water contains about 3 per cent salt and is about three times as salty as a bird's body fluids. Most terrestrial vertebrates eliminate salts via their kidneys, but when faced with sea water they would have to excrete two litres of urine for every litre of sea water drunk. As birds conserve water by not excreting water but a paste composed of uric acid, seabirds have a special problem. In seabirds, as well as several terrestrial species, there are special glands located above the eyes which can secrete a 5 per cent salt solution. In most species, it dribbles out of the internal or external nostrils but with petrels this fluid is forcibly ejected through the tubular nostrils. Many birds have specially channelled bills so that the fluid flows to the end rather than getting into the eyes. If you watch a cormorant after it returns from the sea you will see a steady train of drips forming at the tip of its bill to drop off every ten seconds or so. One also often sees recently returned boobies shaking the drops from their bill.
The three species of boobies in the Galápagos are certainly the commonest and most frequently seen of the seabirds. Looking similar to Gannets, and in the same family (Sulidae), they are large birds 70 to 90 cm in length, with long (1-1.5 m) narrow wings. Their long pointed beaks give rise to their local name "piquero" (lancer), but no one seems certain where their English name "booby" came from. "Bobo" is a Spanish word for clown and may be the root of the word. It is thought that their name originated in the seafarer's term for them, for they appear stupid, often showing no concern on being approached and captured. Boobies frequently perch on ships at sea, sometimes using a vantage point near the bow from which to dive on flying fish skipping away from the bow wave. The word "booby-hatch" is almost certainly derived from their habit of perching at the bow.
The boobies all have a strong, direct flight with several powerful rapid wing beats followed by a glide. When in groups, they often fly in a ragged line. All species of boobies feed by spectacular plunge diving. When hunting, they will fly with their bill pointing downwards, fifteen metres or more above the water. On sighting their prey, they check their flight and hurtle in a headlong dive to the sea. After seeing them hit the water at break-neck speed and bob back up to the surface, one wonders how they survive the shock. Boobies and gannets are well built for penetrating air and water, with a pointed, tapered bill, torpedo-shaped body, and pointed tail. Air sacs in the skull cushion the impact of the dive and closed nostrils prevent water from being forced in.
All three species are colonial but, in the Galápagos, they range from the widely distributed small colonies of the blue-foot to larger and less frequent colonies of the masked to a few huge colonies of the red-foot. An unusual feature of the breeding biology of boobies is the way in which they brood their eggs. The developing embryo needs a constant temperature, and in most bird species this is provided via the bare skin of brood patches. These are areas with few feathers and a good supply of blood vessels so as both to monitor the temperature of, and to provide heat to, the eggs. boobies and gannets, together with cormorants, pelicans, and tropicbirds, incubate their eggs on their foot-webs which are well supplied with blood vessels. By either nestling down onto the eggs or raising the body so that cool air may circulate, boobies can maintain the temperature precisely around 39° C. Readers wishing to pursue studies of the boobies should consult Bryan Nelson's The Sulidae (1978).
No colour is more surprising amongst the drab rocks of these tropical desert islands than the intense bright blue of the blue-footed booby's feet (Plate 19). Shown off to their best in the dramatic landing "salute" and in the ritualised "parading" of courtship (Nelson 1968), the blue feet play an important part in the life of this species. How these birds got their bright blue feet continues to puzzle naturalists and perhaps the only safe answer to why is why not?
The blue-footed booby, Sula nebouxi, is the most commonly seen of the boobies as it nests near the coast in many places and also feeds close to shore. In flight, these birds have a characteristic shape. They can be told apart from juveniles of other species by the white nape patch. When visiting a colony, one can tell the female from the male of a pair as she appears to have a larger pupil and is also larger than the male. The difference in apparent pupil size is due to a dark pigment around the female's pupil and not to any real difference in size. If you haven't already been able to tell the difference between a pair of these birds, then the first vocalisation will clinch the matter. The male has a long drawn-out beseeching whistle while the female makes a nasal honk.
If you wait for a short time at any large colony of blue-foots, such as the ones on Española or Daphne Major, you will certainly be able to watch at least parts of the dramatic courtship display. Courtship will usually begin with a male advertising himself from a chosen site by "skypointing." (Plate 19). Once he has attracted a female, the two may parade to each other. The male in particular, waddles around with a cocked tail, alternately raising one blue foot then the other. While parading, the birds will alternately pose in the haughty-looking "bill-up-face-away" or shy-looking "pelican" postures. As courtship proceeds one or the other bird (usually the male) will skypoint; this becomes more frequent until both are skypointing to each other simultaneously or in turns. After mating, which occurs frequently , I have often seen the male pose in the bill-up-face-away posture, with his wings crossed behind his back, tail cocked, and head in the air. Frequently during the display, one or other member of the pair will pick up some twig or stone and with great ceremony place it on a non-existent nest. Ritual nest-building is a legacy of an evolutionary history in which ancestors constructed functional nests (red-footed boobies still do). This relict behaviour is important in strengthening the pair bond but has no practical value because they will scrape away all the twigs and stones when the female is ready to lay so that she may nest on the bare dusty ground. The blue-foots also have an endearing bill-touching ceremony in which they touch, or nearly do so, the pointed tips of their lance-like bills. Occasionally, this is done while holding a twig in the bill.
One to three eggs are laid on the bare ground and are incubated by both parents for a little over forty days. The eggs are laid three to five days apart, and this is usually reflected in the hatching sequence. When times are good, the parents may successfully fledge all three chicks, but, in harder times, they may still lay as many eggs yet only obtain enough food to raise one. The problem is usually solved by the somewhat callous-sounding system of "opportunistic sibling murder." The first-born chick is larger and stronger than its nest mate(s) as a result of hatching a few days earlier and also because the parents feed the larger chick first. If food is scarce, the first born will get more food than its nest mate(s) and will outcompete them, causing them to starve. The above system optimises the reproductive capacity of the blue-foot in an unpredictable environment. The system ensures that, if possible, at least one chick will survive a period of shortage rather than all three dying of starvation under a more "humane" system.
At the time of hatching, the nest area is surrounded by a ring of guano produced by squirting excrement in all directions while incubating. Once the nest is deserted, these rings have the appearance of being the landing pads of some extraterrestrial being. This ring often coincides with what is an imaginary line that delineates the nest site of the booby, within which parent-chick relationships are normal (Gould 1983). If a chick should get out of this area, it will not be treated as offspring nor be allowed back. This fact may be exploited by an older sibling in a situation where food is scarce. The older chick could force the younger one out of the "ring," whereupon it would be rejected. The ring itself is not necessary for this set of behaviours but is coincidental as it is frequently present. Aggression to chicks outside the imaginary line has probably evolved as a means of preventing the parents from raising another's young.
After hatching, the male plays a major role in bringing fish home. He has a longer tail than the female in relation to his body size, which makes him able to execute shallower dives and to feed closer to shore. He can thus bring many small feeds back at a time when the young are small. The female takes a greater part as time proceeds. Sooner or later, the need to feed the young becomes greater than the need to protect them and both adults must fish to provide enough. When newly hatched, the young are naked and unable to regulate their temperature or to protect themselves. One parent must therefore remain at the nest. When the young have grown a coat of white down and are able to pant or gular-flutter, the parents may leave the chick(s) but risk a predator snatching an opportunity for a meal. Occasionally, I have observed frigatebirds and hawks taking unguarded chicks, but soon the chicks are too large for these predators. Owls, too may take booby chicks. For the parents, there must be a fine balance between leaving their offspring to fend for themselves and finding enough food for them.
The blue-foots usually feed close to the coastline and are well adapted to do so by having a longer tail in relation to their body size than other species. This enables them to turn sharply in the water and execute shallow dives. The male is particularly specialised in this manner, as mentioned above, and is able to dive headlong into less than half a metre of water, though out of the breeding season he usually feeds further out. These birds may also feed in groups. When doing so, a group of hunting birds frequently dive in unison. Sometimes, especially in the cool rich waters of the western isles, blue-foots can be seen feeding in groups consisting of thousands of birds. It is an incredible spectacle to watch such a swarm circling, wheeling, and diving in an almost continual stream after a shoal of small fish (see back cover).
With a wingspan of 1.5 m, this species is the largest of the boobies. Its brilliant pure white body plumage contrasts with its almost black wing markings. Bryan Nelson (1968) has called it the "whitest bird imaginable." Its stout bill is yellow-orange, set against a blue-black face mask, which distinguishes this species from the white form of the red-foot. The young birds look similar to those of the blue-foot but the white of the underparts extends well up the neck.
Like the blue-foot, the masked booby, Sula dactylatra, nests on the ground, but being heavier and larger it has more trouble taking off. As a result, its colonies are more usually found near cliffs and on the steep outer slopes of tuff and cinder cones, where the upward air currents make it easier to take off.
This booby differs from the other two species in that it has a more or less fixed annual breeding cycle. On any particular island, breeding occurs at about the same time year after year, but from island to island this time of year varies considerably. On Genovesa, most eggs are laid from August to November, whilst on Española to the south, most are laid from November to February. In any one colony, there is a distinct synchrony of breeding, and, as a result of the nine-month breeding period, the colonies are usually empty for nearly three months of the year while the adults are leading oceanic lives.
The masked booby lays two eggs, and usually both hatch, but only one ever survives to maturity. Sibling murder is apparently obligatory in this species. This seems a perverse behaviour pattern, but it has been found that birds that lay clutches of two eggs are on average more successful in raising a chick than those that lay only one. This is because the second egg acts as an insurance policy in case the first egg is lost or if the first hatchling dies in the first few days of life. Sibling murder ensures that the parents never have to feed both chicks for any length of time and allows them to raise young as frequently as possible. This occurs by the older and more powerful chick attacking and forcing its sibling out of the nest, where it is likely to die from starvation, overheating by day, or cold by night. This system is different to that of the blue-foot, where food competition rather than direct aggression may lead to the death of the weaker chick.
Booby chicks that have survived the crucial first few weeks often seem too large and overstuffed in their downy coats. Young birds usually grow as large or larger than their parents before sprouting feathers. These come first at the tail and wings, then on the back, and gradually a set of flight feathers develops. Half feathered birds are truly comical, but fully feathered young birds are dull-looking and mainly grey or brown plumage and no colour to feet or bill. The young boobies will not become independent for some time. They must first develop flight muscles and then learn the complex skill of flying and plunge diving. They continue to be fed by their parents until they are able to look after themselves.
The behaviour of the masked booby is similar to that of the blue-foot, but in most respects it is less dramatic. The feet are a dull grey-green and there is little parading in the nesting territory. Sky-pointing is much less exaggerated and is only done by the male. The pelican posture and the bill-up-face-away are used in the same circumstances as are bill-touching and ritualised nest building. The masked booby seems more aggressive in many aspects of its behaviour than the blue-foot. Rival males jab bills and flail wings at each other in disputes. The territorial display is a "yes/no" head-shaking in which the head is nodded up and down and simultaneously turned from side to side in a rapid action. The blue-foot has a similar behaviour but leaves out the "no" part. There is also much mutual jabbing of bills between partners; this is only rarely seen with the blue and red-footed boobies. Like the blue-foot, there is a difference between the voices of the male and the female; the former has a weak whistle, while the latter has a loud raucous "quack." Unlike the blue-foot, there is no difference in apparent pupil size, but the female is again larger than the male.
The masked booby feeds further offshore than the blue-foot and is rarely seen fishing. They are occasionally seen taking off from the colony and diving immediately into the sea, after which they bathe themselves, perhaps to keep cool.
The red-footed booby, Sula sula, is the smallest and least often seen of the Galápagos boobies. It is, however, the most abundant of the three species, but its colonies occur at the edges of the archipelago. Except for the one on Genovesa Island, these are not often seen by visitors. It feeds well out to sea, where it catches flying fish or squid.
This booby's most distinctive feature, its bright red feet, are often hidden from view when perching or in flight, so the best identification feature is its blue bill and usually brown plumage (Plate 21). Identification is sometimes difficult because a small proportion of the population has a white plumage similar to that of the masked booby (Plate 21a). They can be told apart by the feet, if visible, or by the bill, Intermediates between the white and brown forms do occur. Why these two colour forms should occur is not understood. The juveniles of the red-footed are like the brown form adults, but lack the red feet and have a brown bill.
A major difference between this species and the other two is that the red-foot nests in trees and shrubs. They are well adapted to their arboreal life by having prehensile feet on short legs, but, as a result, shuffle awkwardly on the ground. Their territories are large, averaging 90 m2 on Genovesa Island. Even at this spacing, Genovesa Island harbours the world's largest "red-footed boobery" (as Dr. Nelson has called it) of possibly 140,000 pairs.
The red-foot has the least conspicuous social behaviour of the three species; this is because they are hidden by trees in which they build rather flimsy nests and also because, having such large territories, they have fewer interactions with each other. They perform head-waving as a site ownership display and also skypoint, but no more dramatically than the masked booby. There is little difference between the two sexes; the male has a higher-pitched and more nasal quacking sound than the gruffer female.
Because of its far-ranging habits, the red-foot is a slow breeder, taking well over twelve months to complete a breeding cycle . Eggs are laid in any month when conditions are suitable, but this seems to occur in bursts. These bursts may be related to food availability, but have no seasonal pattern. When the young of a breeding burst fledge, the bay at Genovesa seems to swarm with young red-foots. The Cryptocarpus bushes around the beaches are laden with these brown birds, as is the rigging of yachts that enter the bay at such times.
The Galápagos Islands harbour a diverse assemblage of seabirds. The seabirds are well enough known for it to be possible to look at and compare the various ways in which they use their environments and therefore coexist with little apparent competition. The nineteen species of seabirds in the islands use the resources of the sea and the space on land in different ways and so are able to occur together. Food is an important factor in determining tropical seabird numbers, so species that occur in the same areas have, through the course of evolution, arrived at different ways of obtaining their food. The main ways in which species may differ in this respect are by feeding in different places, at different times, using different methods, or catching different prey.
Ten of the fifteen Galápagos species that have been studied in detail have suffered periodic breeding failures which are probably related to a shortage of food, indicating the importance of food in determining the life strategies of these birds. Consequently, competition between species for food would be expected to be an important factor in the ecology of the seabirds. Dr. Michael Harris (1977) and others have collected data over many years on the feeding and breeding ecology of Galápagos seabirds and these data are summarised in Tables 12. From the first table, it can be seen that the various species do have feeding strategies that vary in such a way as to avoid competition. The table shows that squid and flying fish form an important part of many species' diets, as do crustaceans and other fish. Most of the range of food that is available is taken by one or other species of seabird. On the whole, the size of prey taken is related to the size of the bird except that the lava gull catches small fish from tide pools and the brown pelican filters small fish with its pouch as well as taking larger fish.
This is how Dr. Harris (1977) describes the ways in which different birds can take the same prey:
The various species are adapted to catch their fish in different ways, and the bills of these species reflect their feeding methods.
Only the swallow-tailed gull, the waved albatross, and the Galápagos storm petrel obtain most of their food at night. It is perhaps surprising that more species do not do so, as flying fish and squid are more abundant near the surface at night. Audubon's shearwater is diurnal in the Galápagos but largely nocturnal elsewhere. It is thought that other species may feed more at night than has been recorded, but, of course, this is difficult to observe.
In Galápagos waters most species have well-defined feeding areas. Eight species feed within a kilometre or so of shore, seven are pelagic, and four feed between the islands. The flightless cormorant is the closest feeder, requiring shallow rocky reefs to find its food. It rarely ventures more than 100 m from shore. The inshore feeders tend to breed in many small, well-dispersed, colonies which are close to the feeding areas, whereas the pelagic feeders have fewer, much larger and more separated colonies.
It seems that Galápagos seabird populations remain close to the ecological carrying capacity. This may explain why few migrant species overwinter in the islands as there would rarely be enough food to maintain extra populations of birds. Only the northern phalarope occurs in large numbers and this species takes smaller food than any of the resident species. Twenty-seven other species have been recorded at various times, showing that there is little problem for these migrants to get to the islands. Birds from a range of geographical origins reside and breed in the Galápagos. Twelve species are from tropical or subtropical waters, while four or five are more typical of cooler waters, such as are found in Humboldt Current off Peru.
The three species of booby make rewarding subjects for a study in evolutionary ecology (Nelson 1978). They are evolutionarily closely related and are similar in many respects; therefore, it is interesting to see what the differences are between them that allow for peaceful coexistence. All three species use more or less the same technique to catch their fish and the sizes caught overlap considerably. It seems that, in the course of evolution, they have adapted primarily to feed in different areas. The red-foot feeds mainly outside the archipelago, whereas the blue-foot's feeding habits are coastal. The masked booby feeds offshore but mainly within the archipelago. These basic differences in feeding strategy have led to complementary variations in behaviour and other aspects of their ecology. The blue-foot's feeding area is limited to a maximum of 1,350 km of coastline where the water is shallow and rich, while the red-foot has thousands of square kilometres of open ocean as its feeding domain, but the food is much more sparsely distributed and harder to get. Though the blue-foot is more commonly seen, it is actually less abundant than the other two species. The more easily available but more limited food of the blue-foot versus the harder to get but virtually unlimited food of the red-foot has led to the two species having markedly different breeding strategies. The blue-foot leads a "fast" life, raising as many as three young in two-thirds of the time needed by the red-foot to raise one. Though data are not available, the red-foot would be expected to be longer-lived than the blue-foot to compensate. The masked booby is in most respects intermediate between the red-footed and blue-footed boobies.
The behaviour of these species at the colony shows a range of variations related to the type of nesting area. The blue-foot has the most typically open and flat nesting area and has been able to incorporate more dramatic actions into its postures and displays than have its congeners. The masked booby nests principally on steeper slopes and cliff tops, and the red-foot nests in trees. In both the latter species' nesting habitats, life is more precariously balanced and, as a result, display behaviour can not be as active.