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.
Shearwaters are generally pelagic, meaning that they spend most of their lives over the open ocean, and return to land only to breed. They lay one egg in a burrow, and only return to the nest at night, to avoid predators. They make weird calls to their mates when they return at night, and on Diego Garcia, they have been heard in trees from about dusk to midnight near their burrows. In the “Plantation” days, the shearwaters seemed to always nest near the villages.
One way to identify Shearwaters is their nostrils. These are “tubenose” birds. Rather than having a cere with nostrils at the base of the upper beak, tubenose birds have tubes ending in nostrils coming about one-third of the way down their upper beaks. In Shearwaters, these tubes are not fused together, like they are with Petrels and Fulmars).
Historically, there have been two species of Shearwaters known on Diego Garcia - Audubon’s Shearwater, and the Wedge-tailed Shearwater – and these species breed on islands of the Chagos Bank and Peros Banhos today. About half the burrows were located under coconut palms, with the other half in broad-leafed forests. These species are commonly found breeding on the same islands throughout their range, which includes the Atlantic and Pacific Oceans. Symens (1996) reported that all the discovered Audubon’s Shearwater burrows were found intermixed in the Wedge-Tailed colonies on the Chagos Bank and Peros Banhos atolls.
You will probably have a hard time seeing a Shearwater. They only come ashore at night during the breeding season, and you’ll probably only know that if you hear their eerie calls in the trees. Shearwaters are well know for getting disoriented by artificial lighting at night – flying to them like moths - and you may see one on the flightline or downtown at night. However, your most likely way to sight these, or any of the pelagic birds, is to get out on the open ocean (fishing, or on one of the ships), where you may see these birds skimming low over the waves. People pay a lot of money to go on “Pelagic Bird Watching Tours” so if you get the chance, go on one there!
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.
There are four subspecies generally recognized to inhabit the Indian Ocean. You’ll only be able to tell the difference by detailed observation of captured birds, and/or their dissection (although you could compare DNA to other ‘collected’ specimens) – so don’t plan on going that route with the wildlife protection laws in the BIOT. The subspecies that most probably is found on DG is P. l. nicolae, since this is the subspecies found in the Seychelles, Maldives and the rest of the Chagos.
They reach sexual maturity after about eight years, and live to be 20 or more years old. On DG, they tend to nest in small colonies, probably beginning after March, indicating this is the northern hemisphere subspecies P. l. nicolae (southern hemisphere species usually start nesting in September). Huston (1975) heard them calling in the trees around the villages in April of 1971, from dusk to midnight. He didn’t state what their call sounded like, but the USF&WS says they make twittering and mewing calls to let their mates know they’re on the way to the burrow. Both parents taking turns incubating the eggs for up to ten days at a time, while the other hunts at sea. Eggs take seven weeks to hatch, and the young take about 10 weeks before they can leave the nest.
Adult Audubon’s Shearwaters do not wander far from the breeding islands, nor do they take long migrations. They also will feed in offshore, or, in the case of DG, inshore (i.e., the lagoon) so you have a better opportunity to see this species that the other shearwaters.
The Audubon’s Shearwater can dive below the surface from a swimming start, plunge down from above, and even snag-while-skimming for it’s food, which consists of the typical food of pelagic birds – small fish, squid and ‘crustaceans’ (krill). It doesn’t follow boats and ships looking for a handout, so if a bird is following your boat, its probably not an Audubon’s.
Wedge-Tailed Shearwater (Puffinus
Known to the Ilois as “Fouquets”. Also known at the “Moaning Bird.”
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.
In 1960, Wedge-Tailed Shearwaters were reported to be breeding on “Ile de Oiseaux” (Bird Island – one of the islands at the mouth of the lagoon), and it has been recorded in the seas around DG since. In 1996, Symens reported over 4,000 occupied burrows on the islands of the Chagos Bank and Peros Banhos. If it still breeds on DG, you should be able to tell it’s there by its moaning calls at night – it groans, and wails too. Typical calls are ‘ooooo-errrrr’, the oooo on the inhale, and the errrr on the exhale.
Wedge-tailed Shearwaters are pelagic, more so than the Audubon’s, and probably won’t be seen offshore or in the lagoon. They often dive for their food, and in 2001 Burger reported they can swim to a depth of over 200 feet to seek out their food, which is the usual small fish, squid and crustaceans.
The Wedge-tailed Shearwater breeds in colonies on small tropical islands, with Northern hemisphere birds starting around February and southern hemisphere birds around September. Since DG is marginally in the Southern Hemisphere (with nothing tropical to the south), these birds may breed either season. Wedge-tailed Shearwaters begin breeding at four years, nesting in burrows, or on the surface under some sort of cover. They are monogamous for years at a time, and only seek new mates if a breeding season ends in failure. Like the Audubon’s Shearwater, they incubate eggs in shifts, sometimes lasting up to two weeks. The eggs hatch after seven weeks, but fledging takes much longer than with Audubon’s, taking 16 – 18 weeks.
Red-tailed Tropic Bird (Phaethon rubricauda).
This is the rarest of the three species of Tropic Bird, but is found across the Indian and Pacific Oceans. It has been recorded from DG and the previously inhabited islands of the Chagos, as well as over the nearby seas. Before the construction of ‘downtown,’ it was reported to be common on the northwest tip of the atoll.
The Red-tailed Tropicbird looks like a husky tern, with white plumage, a bright red bill, black legs and feet, and long, quill-thin red tail feathers. The tail feathers are sometimes not easily visible in flight against the sky, and young Tropic Birds do not have the long tail feathers.
Red-tailed Tropicbirds tend to nest in large colonies on coral atolls and islands where they typically nest under the shrubs near the beach. Colonies are found from Mid-Pacific to the Mascerenes.
Because of the destruction of the ground-nesting seabirds on DG during the last century, they are still very rare, but perhaps have re-established themselves as breeders, since they wander widely when not breeding and may have ‘re-discovered’ DG. They are pelagic, but don’t congregate in flocks while feeding. Normally, they ‘plunge-dive’ for their prey, which is mostly fish and squid.
Peterson says the vocalizations are ‘a hoarse WOW’ and ‘a raucous RAT-CHET, RAT-CHET.’
White-tailed Tropic Bird (Phaethon lepturus).
This is the smallest of the three species of Tropic Birds, with a length of 15 inches and a wingspan of almost three feet. It has white plumage, a yellowish-orange beak, conspicuous black patches on the upper wings, yellowish legs and feet with black between-the-toe webbing, and clearly visible 15 inch tail feathers. Immature birds have no tail feathers, and black barring across the top of the wings, instead of the patches.
There are five subspecies, and P. l. lepturus is the local form in the IO. There is a golden-yellow morph (P. l. fulvus) found on Christmas Island (off the West Coast of Australia). Since White-Tailed Tropic Birds wander great distances when not breeding, you may see these yellow morphs on DG; they’ve been seen as far west as the Seychelles.
White-Tailed Tropic Birds are found in the Atlantic, Pacific and IO. Symens reported that in 1996 three pair were engaged in courting behaviour on Diego Garcia in 1996. Throughout their range, the White-Tailed Tropic Bird normally nests under brushy cover near the water, but also nests in trees when there are numerous ground dwelling predators (such as rats). It has also been found nesting on abandoned buildings.
They reach sexual maturity at four years, and nest year round throughout their range. Like many other sea birds, the adults take shifts incubating the single egg, with the average shift lasting up to a week. They have the throat pouch and large gullets like other pelecaniforms, but unlike pelicans and Boobies, they feed their young by regurgitating food, and shoving it down the chicks throat. When the chicks are fully fleged (at about 11 weeks) the parents abandon them, and the young birds get so hungry they leave the nest, head for the ocean, and teach themselves to fly and fish.
During courtship, the birds will fly in pairs, with streamers from the bird above turned downwards towards it mate. Otherwise, they are solitary feeders, who ‘plunge-dive’ to seize and swallow their prey, which, as usual, consists of fish and squid.
The call is a high-pitched scream: kee-kee-krrrt-krrt-krrt.
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.
Boobys are member of the order Pelecaniforms, and share many of the order’s characteristics. They are strong fliers, but they are real boobs when it comes to take-offs and landings; if they can, they use headwinds and high perches to assist on take-off. They have throat pouches and wide gullets to allow capturing and swallowing large fish, which they catch by diving vertically into the water, sometimes from heights of 50 feet or more. They also catch leaping fish while skimming over the surface, and when seen feeding in groups usually indicate a school of predatory fish like Tuna are below chasing feeder fish toward the surface.
Unlike most other birds, they do not have brood patches on their chests to keep their eggs warm, and instead incubate their eggs with their feet. They share incubation duties, but don’t stay at sea hunting for long periods. Their chicks feed by poking their beaks into the gullet where the adults store half-digested fish and squid.
It isn’t too hard to tell the three species apart when they are in flight. The Red-footed Booby is smaller and faster-flying than the other boobies, and of course has the red feet. Masked Boobies look like white morph Red-Footed Boobies, but have black tails, and the black mask around the beak. Adult Red-Footed Boobies (both morphs) have white tails. Immature Red-footed Boobies can be told from Brown Boobies, which have white bellies and under-wing coverts. See the picture above for a better idea.
Red-footed Booby (Sula sula)
Known to the Ilois as “Fou”.
Which reminds me of the joke about the foo-bird…
The Red-Footed Booby is the smallest of all boobies, with a wingspan of a little over four and a half feet (which is still a pretty big bird). This species exhibits a variety of color phases, although the “white morph” makes up the vast majority of the individuals on DG (the “dark morph” being predominant in the Galapagos). This morph is white, except for the black primary and secondary flight feathers on each wing. The bill is light blue while the skin around it is pink. Like the name implies, their feet and legs are red. Juveniles resemble the dark morph, and are wholly brown or dark gray, with a very dark bill, facial skin, and legs. The rump, coverts, and tails of both white and dark morphs are white.
Red-Footed Boobies normally feed alone, or sometimes with mixed flocks as they pursue the schools of fish and squid stirred up by tuna. They’ll feed day or night.
They are nest builders, and build these nest on the top of shore trees and scrub, like scaevola (which we all called scaveola when I was there). They’ve been know to build their nests in coconut trees, and in the same bush as a Frigate Bird.
Red-Footed Boobies live for more than 20 years, and are sexually mature at four years of age. They are monogamous, and perform elaborate greeting rituals, including harsh squawks and the male’s display of his blue throat pouch. They lay a single chalky white egg, which they incubate in 24-hour shifts for about six weeks, when the egg will hatch. Fledging occurs after about 14 weeks, and the parents will continue to care for the immature birds for up to four additional months. It takes up to three years before the young moult into their adult coloration, so what may appear to be dark morphs in a colonial area may just be immature white morphs.
There is a huge colony (members of the Royal Navy Birdwatching Society claimed 4,500+ nests in 2006) of Red-Footed Boobies on the east arm of the DG atoll, and more on the islands in the mouth of the lagoon. This species doesn’t necessarily follow the “northern winter” nesting season, and seems to spread it’s breeding and chick rearing over a long period, if not year round. It is possible to see the colony on the ‘mainland’ of DG, and it is worth the trip to do so if you can get the Brits to give you a permit. Here's a picture of a portion of the colony:
Masked Booby (Sula dactylatra)
Also called the “Blue-Faced Booby”.
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.
Great Frigatebird (Fregata minor)
Known to the Ilois as “Fregat”
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.
Lesser Frigatebird (Fregata ariel)
Known to the Ilois as "Pti Fregat".
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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This, and everything else I write and every photo I produce is copyrighted by Ted A. Morris, Jr.