Division of Ecological Perplexities Presents:
NESTING PELAGIC BIRDS
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We do know, based on reports from early explorers, that before colonization there were enormous colonies of seabirds nesting on the island in various places, notably around Turtle Cove on the south end of the lagoon, and on the islands at the mouth of the lagoon. In addition, Green and Hawksbill sea turtles nested on the beaches. There are annecdotal reports of flightless birds and tortoises. However, following the usual pattern of settlement and exploitation, these birds and reptiles were viturally wiped out when men arrived with rats, cats, pigs, and hungry slaves. For example, Rickard and Bashall state that there were no large colonies of seabirds on the mainland of Diego Garcia in 1945. Here I would like to point out that this sort of extinction was not completely restricted to the arrival of Europeans, but is well known through the evidence available in Hawaii, Christmas Island, Easter Island, etc., and of course we should never forget the great Pleistocene Overkill Theory!
In the 1980s, there were plenty of sea birds too, which nested out on the Bird Shit Islands, which formed the "toes" of the DG foot. By 2006, the Royal Naval Birdwatching Society reported over 4,500 nests of boobys on Barton Point, proof that unmolested they will re-colonize the 'mainland' of the island.
Shearwaters are medium sized birds with long wings which get their name from their flight pattern over the ocean – they tend to fly right on the deck and ‘shear’ or skim along the uplifts created over the waves. The flight pattern is generally a rapid flap-flap-flap, followed by a long glide on down-curved wings. However, don’t rely on this as a positive indication of a particular species, since shearwaters vary their style depending on the winds and whether they are looking for food, migrating, etc.
Although the shearwaters in the IO are not believed to make epic migrations, traveling only to the Arabian Sea (which still seems pretty epic to me), some species migrate over 9,000 miles each way from their breeding islands in the Southern Hemisphere to wintering seas near the artic regions. They are also extremely long-lived, with one Manx Shearwater that summered in the North Atlantic known to have lived over 55 years.
Audubon’s Shearwater (Puffinus lherminieri)
Known to the Ilois as “Riga”.
This is the smaller of the two species on DG, with a length from the tip of the beak to the tip of the tail of about a foot, and a wingspan of about two feet. The Audubon’s Shearwater is primarily dark brown with white cheeks, a white throat and belly, and a long dark tail with white undertail coverts (the downy and/or small feathers covering the rectum and extending part way down the underside of the tail). It also has white under-wing feathers, which look like the bird has arms in long sleeves.
Shearwater (Puffinus pacifica)
This is the larger of the two Shearwaters that have historically nested on DG, being about a third bigger than the Audubon’s. It’s the size of a crow (but not crow shaped), and has dark gray feathers all over, a wedge-shaped tail (hence the name) and most importantly, flesh colored legs and feet. There is a pale morph, found mostly in the North Pacific, but which can occur south of the equator. The pale morph has a light grey feathers on the underside of the body and wings.
There are three species of Booby in the Chagos: Red-footed, Masked and Brown Boobies. The Masked and Brown Boobies nest on islands of the Great Chagos Bank. Although they are pelagic when not breeding, they do not wander very far from their nesting colonies, so you may not see Masked or Brown Boobies during your tour on Diego Garcia.
This is the largest of the boobies, with a wingspan of over five feet, and is found in tropical seas around the globe, except in the eastern Pacific and eastern Atlantic. This booby closely resembles the Red-Footed Booby, except that the bill and feet are yellowish in the male (greenish in the female), and the tail is black. It also has a black mask around the beak, which turns blue during the breeding season. Young birds are grayish brown, with white undersides, develop adult plumage after two years, and achieve sexual maturity at four years of age. Masked Boobies are known to live to be about 20 years old.
The Masked Booby nests in small colonies throughout its range, and Symens in 1996 estimated about 250 pairs breeding on islands of the Chagos bank. It appears that the breeding season occurs between February and August. However, its breeding practices are unlike those of the Red-Footed Booby. It nests on the ground in a simple, shallow depression, and generally lays two eggs, about a week apart. The incubation shifts are pretty short for Boobies, only about 10 hours. The chick of the first egg to hatch usually kicks the second hatchling out of the nest shortly after it hatches, and so only the oldest chick survives. It fledges after a long period (up to four months) and hangs out with the parents for another couple months, learning the ropes.
The Masked Booby isn’t vocal when over or on the ocean, but is heard to make a wide variety of whistles, hisses, and quacks in the nesting colonies.
There are four subspecies, which cannot be distinguished one from the other in flight. The subspecies that nests in the Chagos is S. d. melanops.
Brown Booby (Sula leucogaster).
The Brown Booby is mid-way in size between the Red-Footed and Masked Boobies, but in color is very different, and easy to tell apart. The adults have dark brown upper bodies, heads and necks, with a white belly, and under-wing coverts. The sexes have different beak and foot colors – yellow in the females, and greenish in the males. The juveniles are similar to the adults, but the belly and under-wing coverts are a light brown.
Like the Masked Booby, it nests on the ground, but makes a small mound of sticks and twigs for its nest (rather than a depression). Like the Masked Booby, it lays two eggs, and the first to hatch shoves the second out of the nest to die, and rarely are two young fledged. The average incubation shift is about 12 hours for the six weeks it takes for the egg to hatch, and the young birds leave the nest after about 13 weeks. Like other Boobies, the youngster hangs out with the parents for another couple months, learning how to be a Booby.
These birds are monogamous for several seasons at least, and have elaborate greeting rituals on the nesting grounds. They are reported to be silent, although some say they quack and grunt, so if you get up to the Chagos Bank and get to walk around the colonies there, listen closely and let me know.
There are five species of Frigatebirds, two of which, the Great Frigatebird (Fregata minor) and the Lesser Frigatebird (F. ariel), are found on and around Diego Garcia. In 1996, Symens estimated there were about 2,000 Great Frigatebirds in the Chagos, but only 85 nesting pairs, all on islands on the Chagos Bank. There were far fewer Lesser Frigatebirds, with only 72 nesting pairs found, also all on the Chagos Bank. Given the huge discrepancy between individual Great Frigatebirds and the number of nests, it is possible that rookeries remain to be found. They tend to nest in the same colony as Red-Footed Boobies, and as that species expands its colonies on Diego Garcia, it is probable that Frigatebirds are nesting there now, so keep your eyes peeled.
Frigatebirds attack other sea birds to steal their food, and are also called Man o’ War or Pirate birds. They are pelecaniforms, but unlike the pelican, they cannot swim, walk to speak of, or take off from the water or flat ground. But they do have the largest wing-loading to body weight ratio of any bird, and spend most of their lives in the air over the open ocean. They stay airborne for over a week at a time, and often ride along weather fronts, and are useful in predicting a change in the weather. They only land (always in trees) to roost or attend their nests and young.
They are lightweight birds though they look huge because of their wingspan, and are easy to spot overhead – they appear almost completely iridescent black, with long pointed wings and deep swallow tails. They never seem to flap their wings, and are expert at gliding and catching the slightest updraft.
The males have inflatable red-coloured throat pouches, which they inflate like a huge balloon to attract females during the mating season. They form colonies in trees, often within a colony of Red-Footed Boobies, and raise one chick. Both parents take turns feeding the chick until it fledges at about three months of age. At that point, the male takes off and attempts to attract a new mate to a new nest. The female Frigatebird cares for its chick for another eight to fifteen months, the longest adolescence of any bird species. It’s a common sight in Frigatebird rookeries to see a full-grown chick over a year old, larger than its mother, land on the nest and scream to be fed. It takes so long to rear her chick that female Frigatebirds breed only every other year.
Frigatebirds are pelagic feeders, and use their long, hooked beaks to snatch most of their food (primarily flying fish) from the ocean surface. They also rob other seabirds of their prey, even harrassing Boobies to the point where the Booby regurgitates, which the Frigatebird can catch in the air, or snatch off the ocean surface.
Frigatebird (Fregata minor)
The Great Frigatebird is a common sight overhead Diego Garcia, and several hundred live in the trees over on the east tip of the atoll and on the islands in the mouth of the lagoon. Although they can and do travel vast distances over the open ocean, they tend to spend most of their lives within 50 miles of their breeding colonies, and since they live to over 30 years old, it’s possible the birds you see overhead DG today are the same ones I saw there 25 years ago.
Great Frigatebirds weigh between 2-3 pounds, and have wingspans of seven and a half feet. Females are generally larger than the males. In coloration, the females are black with a white breast and throat. The males are iridescent black and the red throat pouch is often visible on flying birds. Young Great Frigatebirds have yellowish or whitish heads and throats.
Great Frigatebirds reach sexual maturity at nine years of age. Since the male participates only in the procreation, incubation, and first few months of chick rearing, they abandon their mates and young, and try to breed every year. After staking out a promising tree top, the males inflate the red throat pouch, spread their wings, and shake their heads from side to side at every female they see. After a female accepts his advances, it takes anywhere from a few days to a few weeks to construct a platform nest of sticks and beach junk, and after the egg is laid, the parents incubate the egg for about eight weeks in three to six day shifts, during which they loose 25-33% of their body weight.
In addition to the extraordinarily long period the mother cares for her chick, young Great Frigatebirds are known to play with other adolescents. One bird will pick up a stick or piece of flotsam and the other hoodlums will give chase until the stick is dropped, with the whole flock trying to catch the stick before it hits the water. They don’t seem to tire of this game, and it probably teaches the skills they will need to snag their food off the ocean surface, and to steal other birds’ food.
Great Frigatebirds will also hunt seabird chicks at their breeding colonies, taking mostly the chicks of tree-nesting terns and noddies.
Frigatebird (Fregata ariel)
The Lesser Frigatebird is like a smaller version of the Great Frigate, and indistinguishable in flight when at high altitude. However, when they are clearly visible to the naked eye or a powerful set of binoculars, it can be seen that Lesser Frigatebirds have white markings extending onto the undersides of each wing. The females also have a white breast. Like Great Frigates, juveniles have a white or pale buff head.
As noted earlier, there are far fewer Lesser Frigatebirds in the area, and all of those seen in 1996 were on the Chagos Bank. But keep a close watch for these overhead DG, as the environment on Diego improves for breeding colonies of seabirds.
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