ARTICLES ON HAWAIIAN
BIRDS AND BIRDWATCHING AND OTHER
PACIFIC WILDLIFE
The Boobies of Hawai'i
The term Sula is derived from the old Scandinavian word for Solan Goose, or Gannet. Spanish sailors believed that they looked or acted stupid and called them bobo, which means "clown" or "dunce".
Three species of Booby (Sula)
reside in Hawai'i - Masked Booby, Red-footed Booby and Brown Booby. All three species nest on islands in the chain from O'ahu westwards to Kure Atoll. The Red-footed
Booby is the most abundant of the three species in the Islands, and large colonies occur on O'ahu, Kaua'i, and
the Islands of the northwest chain, including Lehua Rock off Ni'ihau, Nihoa, Laysan and Midway. The Masked Booby,
the largest of the Boobies occurs in small numbers in the Main Islands, on O'ahu as well as several of the northwest
Islands. Brown Boobies nest on O'ahu, probably Kaua'i, Lehua Rock off Ni'ihau and in the northwest chain.
Red-footed Booby
This pantropical species is the smallest of the World's Boobies and is a colonial nester, with colonies of several
thousand pairs occurring regularly. The species builds its nest from twigs, grass and various vegetation off the
ground in trees and bushes, where they usually lay a single egg, although two are occasionally laid only one chick
will usually survive. The species has been successful in colonising new areas, and has suffered less predation
than some species due to its arboreal nesting habits. Only Abbott's Booby (Sula
abbotti) shares the habit of tree-nesting in the rest of the sulid family.
Several colour morphs of the species can be found. The most common
plumage type is white with black secondaries and primaries, although birds exhibiting brown plumage are regularly
seen in predominantly white-plumaged colonies. Birds may show brown bodies with white heads, brown bodies with
white tails, brown bodies with white head and tail or all brown, although the brown is quite pale and never reaches
the dark brown of Brown Booby. All adult Red-footed Boobies show pale blue-grey bills with a pink and blue base
and of course, red feet. Juveniles are various shades of brown-grey all over, but the plumage is more mottled,
less warm-toned and uneven in colour than in brown morph adults. The legs are pale pink or greyish and the bill
is grey.
The species is quite vocal, but has a limited vocabulary which consists of loud screeching squawks and guttural
noises, mostly consisting of low "krauks" and "Aaks". The Hawaiian name for this species is
'A, presumably named after the species call.
Three subspecies are recognised, Sula sula sula
(Caribbean and Atlantic), S.s. rubripes (Hawaii,
China Sea and Australia) and S.s. websteri (Tropical
Pacific and Galapagos), although the classification as sub-species appears to be on range alone, rather than plumage
variation, although small size differences exist in individuals from different regions and as many as five sub-species
may actually exist.
In Hawai'i the species nests
on Kure Atoll, Midway Atoll, Peal and Hermes Reef, Laysan, Lisianski, Gardner Pinnacles, French Frigates Shoals,
Necker, Nihoa, Kaua'i, O'ahu and Johnston Atoll. Elsewhere it nests in the Pacific Ocean, the Caribbean and Atlantic
Ocean and the Indian Ocean. During the non-nesting season the species tends to remain near its nesting sites, and
may roost there, although some birds appear to wander more widely at sea. Juvenile birds leave the birth site and
will stay at sea for several years until returning to nest. It appears that not all juveniles return to their birth
sites however, as numbers at several sites only increase at a relatively slow pace, whereas numbers born is high.
Photo: Red-footed Booby colony at Kilauea Point
NWR, Kaua'i.
Individuals can live up to 22 years old and adult survival has been estimated at 90% in
Hawai'i, whereas chick survival varied from year to year: 52% in 1984 and 27% in 1989.
Estimated Maximum Number of Breeding Pairs in 1984. Taken
from Harrison 1990.
Note however that by 1998 at least 2000 pairs
(4000+ individuals) were present at Kilauea Point, Kaua'i and a rough estimate of at least 1500 birds was made
at Lehua and so numbers appear to have dramatically increased within a roughly ten year period (Melgar 2001).
Nihoa
2,000
Necker
750
French Frigate Shoals
600
Gardner Pinnacles
0
Laysan
300
Lisianski
450
Pearl & Hermes Reef
60
Midway
500
Kure Atoll
450
Kaua'i
600
Ni'ihau
?
Lehua
150
Ka'ula
350
O'ahu
1,200
Estimated Numbers of Pairs of Red-footed Boobies Worldwide.
Most data is over ten years old, although population estimated at <300,000 pairs in 1996.
Hawai'i
10,500
Caribbean
13,750
Atlantic Ocean
1,400
Eastern Pacific
260,000
Central and Southern Pacific
21,200
Western Pacific
100
Indonesia
10,000
Indian Ocean
26,000
World Total
345,000
The main food of the Red-footed Booby is flying fish and squid, which are caught by plunge-diving from heights
of up to 40 feet, although most is done from a height of 4 - 8 metres. Almost all feeding is done away from the
sight of land, although occasionally small groups will feed nearer land if a large school of fish is present. Food
articles are swallowed and carried back to the nest site, where food is regurgitated for the chicks. Birds returning
to the colony in the evening are particularly liable to attack by Great Frigatebirds (Fregata
minor), which chase the Boobies until they disgorge part of their meal or they
reach the safety of the nest.
The species is an aerial master but very clumsy when on land, walking with a rather laboured waddle. Birds are
seldom seen on the ground, except when young and inexperienced or when collecting nesting material. Individuals
prefer to sit in trees or low vegetation with their heads retracted onto the body. When nervous or alert birds
will extend their head and neck in alarm and will take flight readily if not nesting. Birds attending chicks or
eggs will remain on the nest, even if provoked to fly for a very long time, only leaving at the last moment or
if the annoyance persists. In experiments, birds would remain on eggs/with chicks even if poked with a stick.
On the water birds will swim readily and do so after catching prey, when preening & bathing and when resting.
In flight the species has a strong flapping flight which are interspersed with long glides. Birds have been reported
gliding for a mile without flapping, although this is rare. When returning to colonies the species will often form
lines of birds which follow each other along the coast, before breaking up at the approach to the colony, presumably
to avoid attention from Frigatebirds.
When at rest the species spends large amounts of time preening with its bill and a specially adapted claw, which
is serrated on the longest edge. When asleep the species may tuck its head under its wing but may also sleep with
its head hanging out of the nest and pointing groundward (see photo). The same "dead bird" posture may
also be used when alarmed, perhaps to fool any passing predators.
Right: Red-footed Booby, injured juvenile, Kaua'i,
Summer 1999. Note how the bird is positioned in the box with head left hanging downwards.
In Hawai'i the species has few predators, especially in the northwest chain. In the Main Islands birds may be preyed
upon by introduced cats, dogs, rats and mongoose and accidental introduction of the Brown Tree Snake would undoubtedly
have a disastrous effect, as it has on other bird species in other areas, such as Guam. Individuals, particularly
inexperienced juveniles may die from Frigatebird attacks, when wings can easily become broken or dislocated. Although
birds are not killed outright by the attack, they die fairly quickly afterwards as they cannot feed themselves
if they cannot fly.
In many other areas the species is predated upon by the same species as those in Hawai'i, but loss is higher due
to lack of protection and concerted predation attacks, including those by humans. In many areas the species is
taken for food as eggs, chicks and adults. In Hawai'i the species is fully protected by law and even "natural"
predators have little effect on the population as colonies are either on remote islands where predators do not
occur or have been removed, or are in areas where full protection is granted, such as through fence erection and
trapping of predator species on reserves.
Where to see Red-footed Booby in Hawai'i.
Colonies of Red-footed Boobies can be seen in the Main Islands on O'ahu and Kaua'i. On O'ahu colonies exist on
Moku Manu Island and at Kane'ohe marine Base, both on the windward side of the Island. Birds can also be commonly
seen from the coast on the windward and north shores, especially during the evening when birds return to the colony.
On Kaua'i birds can be observed at Kilauea Point National Wildlife
Refuge on the North Shore. Birds may be encountered at any time of the day and any month of the year, but a visit
between November and June will provide the greatest numbers. In the evenings when birds return from feeding trips,
several thousand birds may be seen over the cliffs and hillside at the refuge, and although the refuge is closed
in the evening, birds can still be seen from the entrance overlook. Elsewhere on Kaua'i birds may be seen offshore
from any of the coasts, but particularly along the south-east, east and north shores.
Pelagic trips from the Main Islands will usually produce this species, although during the winter months numbers
encountered may be very low. Trips which start from Kaua'i and go to Lehua Rock (just off Ni'ihau) have an almost
100% hit rate. A sizeable colony (up to 1000 pairs) also breeds on Lehua Rock, although the views are not as close
as those colonies on O'ahu or Kaua'i.
Elsewhere in Hawai'i the species is only present off the coasts of the Main Islands in small numbers and in the
inaccessible northwestern islands. Midway also has birds which can be easily viewed, but access to Midway is not
always possible, for example when closed during certain periods, such as occurred when Midway was closed to visits
in early 2002.
Masked Booby
The largest of the Boobies in
Hawai'i. Birds nest alone or in small loose colonies on the ground, where they make a nest or scrape with a few
items of vegetation. One or two eggs are laid, but the first chick born often rolls the second egg out of the nest
or kills the second chick soon after hatching.
The species is found throughout the tropical Pacific as well as the Caribbean, tropical western and central, Indian
Ocean and Australasia. Atlantic In Hawai'I over 7,000 birds are present, although up to a quarter of these are
non-breeders. Large colonies are located on Laysan, Lisianski and French frigate Shoals.
Estimated Maximum Number of Breeding Pairs in 1984. Taken
from Harrison 1990.
Note that by 1998 at least 20 pairs were nesting
off O'ahu.
Nihoa
300
Necker
150
French Frigate Shoals
600
Gardner Pinnacles
25
Laysan
425
Lisianski
350
Pearl & Hermes Reef
160
Midway
10
Kure Atoll
75
Kaua'i
0
Ni'ihau
?
Lehua
10
Ka'ula
400
O'ahu
4
Adult birds are large, white with black primaries and secondaries, which in flight extend the whole way across
the rear wing in a thick band (see difference in photos of this species and Red-footed Booby in flight). The head
is white with a black face patch at the base of the bill, which is deep-based yellow-grey in colour. The tail is
black (white or pale brown in Red-footed Booby).
Juveniles are darker brown than juvenile Red-footed Boobies and are perhaps more confusable with Brown Booby. As
the juvenile ages it becomes dark brown with an obvious white collar, which is lacking in Brown Booby. The flight
is similar to all other Boobies, consisting of deep flaps followed by long glides, however the species appears
to usually have a slightly different flight style, enabling identification of individuals even at great distance
and without seeing full plumage details.
The species is quite vocal, its calls which consists of loud screeching squawks and guttural noises, mostly consisting
of low "krauks" and "Aaks", are especially uttered during the breeding season. The Hawaiian
name for this species is also 'A, strangely the Hawaiians did not appear to differentiate between the three species
found in Hawai'i.
The main food of the Masked Booby is flying fish, mackerel scads and squid, which are caught by plunge-diving from
heights of up to 30 feet, although most is done from a height of 6-10 metres. Their diet is dominated by large
flying fish and account for over three-fifths of their prey. They also eat many dolphinfish and green halfbeaks.
Almost all feeding is done away from the sight of land. Food articles are swallowed and carried back to the nest
site, where food is regurgitated for the chicks.
The species is also graceful in the air like other Boobies but also clumsy when on land, walking with a rather
laboured waddle. Birds are seldom seen on the ground, mostly when young and inexperienced or when nesting. On the
water birds will swim readily and do so after catching prey, when preening & bathing and when resting. In flight
the species has a strong flapping flight which are interspersed with long glides.
Predation of individuals is much the same as for Red-footed Boobies, with introduced species being the major threat,
although the more remote areas inhabited by the species means that populations tend to be less affected than Red-foot
colonies. Frigatebirds seldom chase Masked Boobies.
Where to see Masked Booby in Hawai'i.
This is the most difficult of the three species to see in Hawai'i. Birds nest only on O'ahu in the Main Islands,
and possibly on Lehua Rock off Ni'ihau. A few birds may be observed on the islands along the windward coast of
O'ahu, such as Moku Manu, where up to 30 pairs nest. On Lehua Rock nesting has not been confirmed in recent years,
but regular sightings of birds nearby seem to suggest that nesting does occur. Sightings elsewhere in the Main
Islands are irregular but most common off windward and north shores of O'ahu at locations such as Ka'ena Point.
Birds are also regularly, although infrequently seen at Kilauea Point NWR on Kaua'i, where birds are seen particularly
during the evening, presumably tempted in by the number of Red-footed Boobies around. Up to three different individuals
have been recorded off the Point in a single evening. Elsewhere on Kaua'i birds have been seen from Ha'ena Point,
Nawiliwili Lighthouse, Makahuena Point and Polihale.
Elsewhere in Hawai'i the species is present in the inaccessible northwestern islands. Midway also has birds which
can be easily viewed, but access to Midway is not always possible, for example when closed during certain periods,
such as occurred when Midway was closed to visits in early 2002.
Brown Booby
The Brown Booby has a pantropical
distribution, with populations in the Pacific Ocean, the tropical Atlantic Ocean, the Indian Ocean and the Caribbean.
Birds along the continental coast of the North American west coast have been split into a separate species, Nazca
Booby. About 2,000 adults nest in the Hawaiian Chain, although over 3,000 birds are present. Most birds nest on
Kaula Rock and Nihoa, with smaller numbers on other islands in the chain. Individuals nest on the ground on a small
nest made of twigs and vegetation and usually a single egg is laid. Often if two eggs are laid the first born chick
will eject the second egg or chick.
This species is also quite vocal, and its vocabulary, like other Boobies consists of loud screeching squawks and
guttural noises, mostly consisting of low "krauks" and "Aaks". The Hawaiian name for this species
is also 'A.
Adults are dark chocolate-brown on the whole of the upperparts and entire neck and have white bellies and mid-underwing.
The bill is different in each sex during the breeding season, with males exhibiting a blueish bill, whereas females
show a yellow-green colouration on the bill, feet and face. Juveniles are a little paler than adults and do not
show the clear-cut neck - breast demarcation of the adults. Adult plumage is acquired by the third year. The oldest
recorded Brown Booby in Hawai'i was sixteen, although birds of an older age have been recorded elsewhere.
Estimated Maximum Number of Breeding Pairs in 1984.
Taken from Harrison 1990.
Note that in 1998 none were confirmed nesting
on Kaua'i although possible nesting took place on Moku'ae'ae, off Kilauea Point and along the Na Pali Coast. Numbers
on Lehua had increased by year 2000 to at least 150 pairs (estimate)(Melgar 2001).
Nihoa
200
Necker
25
French Frigate Shoals
60
Gardner Pinnacles
10
Laysan
34
Lisianski
25
Pearl & Hermes Reef
60
Midway
0
Kure Atoll
60
Kaua'i
?
Ni'ihau
?
Lehua
40
Ka'ula
500
O'ahu
75
The main food of the Brown Booby is flying fish and squid, which are caught by plunge-diving from heights of up
to 30 feet, although most is done from a height of 4 - 8 metres. Brown Boobies will also repeatedly dive from as
little as a couple of feet, if there is a fish source directly below the water. Feeding may take place commonly
within the sight of land. Food articles are swallowed and carried back to the nest site, where food is regurgitated
for the chicks. Birds present near Red-footed Booby colonies will often join Great Frigatebirds in chasing the
Red-foots until they disgorge part of their meal or they reach the safety of the nest and will swoop in to grab
any missed food, or sit on the waters surface feeding on any stray fish particles. Brown Boobies take smaller prey
than Masked Boobies and up to twelve fishes may be present in a birds stomach, compared to two or three large fish
in Masked. Flying fish make up about one-twentieth of the diet of this species compared to one-quarter in Red-footed
Boobies.
Where to see Brown Booby in Hawai'i.
Brown Boobies can often be seen offshore in any of the Main Islands, and is more likely to be encountered from
shore from Maui and Hawai'i than the other two species. A few pairs breed on the windward coast of O'ahu and birds
have bred on Mokuaeae Island, offshore of Kilauea Point on Kaua'i. Quite a number also nest on Lehua Rock (off
Ni'ihau), where up to 300 birds have been observed in a single visit. Birds may also have bred on the Na Pali coast
on Kaua'i, although this could not be confirmed.
Elsewhere in the Main Islands single birds or small groups can be encountered almost anywhere, but harbour entrances
where there are navigation buoys are particularly productive, as Brown Boobies enjoy sitting on these structures
when not fishing.
Red-footed Booby colony at Kilauea Point, Kaua'i. Summer 1998.
References
HARRISON, C. 1990. Seabirds of Hawaii. Natural History and Conservation. Cornell University Press, Ithaca.
HARRISON, P. 1983. Seabirds - An identification Guide. Helm, London.
HARRISON, P. 1987. Seabirds of the World. A photographic Guide. Christopher Helm, London.
HAWAII AUDUBON SOCIETY. 1997. Hawaii's Birds. The Hawaii Audubon Society, Honolulu.
MELGAR, C.W. 2001. Field observations of birds in Hawai'i. Unpublished field notes.
PRATT, H.D., P.L. BRUNER and D.G. BERRETT. 1987. The Birds of Hawaii and the Tropical Pacific. Princeton University Press. New Jersey.
SCHREIBER, E.A, R.W. SCHREIBER and G.A. SCHENK. 1996. Red-footed Booby (Sula sula). In The Birds of North America, No. 241 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists' Union, Washington, D.C.
Adult Red-footed Booby on nest at Kilauea Point, Kaua'i.
Christian Melgar. Worthing, West Sussex, UK. 2002.
