in the Inner Gulf of Thailand
By Philip D. Round
This article was originally published in Stilt 50
(2006) the journal for the Australasian
Wader Studies Group and was kindly submitted by Philip
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As a predominantly lowland country with a long shoreline, Thailand
is of major importance for waterbirds, both passage and wintering
species, and residents and local dispersants. A total of 64 species
of shorebirds are found both in coastal mudflat and mangrove habitats,
and also inland on the paddylands and marshes of alluvial basins;
many of these species occur in internationally important concentrations.
Scott (1989) listed 42 sites in Thailand that were wetlands of probable
international importance, while Tunhikorn & Round (1995) considered
that 14 sites, nine coastal and five inland, were of international
importance for waders.
In view of the time that has elapsed since the latter review, and
the great amount of new information collected, an up-date on Thai
wader sites would be timely. I focus here on one of these, the Inner
Gulf, probably Thailand’s single most important wetland site
because of the numbers and species diversity of shorebirds it supports.
Southern Hemisphere readers should note that the winter referred to
here is the Northern Hemisphere winter, the nonbreeding season for
Extending east and west of the city of Bangkok, the Inner Gulf encompasses
a roughly 100 km length of shoreline at the head of a 350,000 km2
enclosed shallow bay (45–80 m depth), lying on the Sunda Shelf.
In addition to the delta of the Chao Phraya River (on which Bangkok
is situated), the Inner Gulf also encompasses the mouths of the Bang
Pakong River to the east, and three further rivers to the west, the
Thachin (a deltaic branch of the Chao Phraya), the Mae Klong (better
known as the River Kwai), and the much smaller and shorter Phetchaburi
River (Figure 1). Together these areas form the second or third largest
river delta in south-east Asia.
1: Inner Gulf of Thailand
800–1000 km2 of mudflats, salt-pans, prawn-capture ponds and
unused coastal flats lie adjacent to, and grade into, one of the largest
rice-growing areas in the world, Thailand’s Central Plains.
There is a long history of human use of coastal habitats in the Inner
Gulf. Salt-pan usage in the western parts of the area dates back perhaps
800 years (Reid 1988) and salt pans continue to occupy c. 106 km2.
Low intensity prawn-capture ponds including some abandoned, unutilised
areas occupy around 400 km2 while approximately 235 km2 of mudflats
lie offshore (Erftemeijer & Jukmongkol 1999). Mangroves (129 km2)
are now limited to a narrow (100–200 m wide) belt along the
coast, with largest areas (approximately 80% of the total) in the
western gulf, around Phetchaburi. Clearance of mangroves probably
took place as long as 100 years ago, and the area remaining is largely
secondary regrowth, dominated by Avicennia spp., Rhizophora
mucronata and R. apiculata.
The tidal pattern is characterised as mixed semi-diurnal; two high
and two low tides occur each lunar day, but one tide is much smaller
and often negligible. During much of December and January, for example,
the tidal flats of the Inner Gulf are inundated throughout the daylight
hours and exposed only during the hours of darkness. Thus mudflat
usage cannot be assessed at that season.
Human use of the Inner Gulf is intensive. In addition to onshore activities,
the mudflats are exploited for molluscs, and coastal waters support
inshore fisheries for fishes, molluscs and crustacea, and plankton.
OF WADER STUDIES
The importance of the site for waterbirds, and especially as a wintering
and staging area for migratory shorebirds, has long been recognized.
W.J.F. Williamson collected Great Knots, Asian Dowitchers and a range
of other shorebirds from the gulf early in the twentieth century (Williamson
1918), as did C.J. Aagaard a decade or so later (Jørgensen
1949). Only a handful of shorebirds numbered among the more than 185,000
birds banded in Thailand during the (1963–1971) Migratory Animals
Pathological Survey (McClure 1974).
Small-scale banding and surveys of shorebirds have taken place from
1980 onwards (e.g. Melville 1982, Parish & Wells 1985), and more
recently, since September 2000, by the author and colleagues. Since
September 2005, all shorebirds banded in the Inner Gulf have been
marked with leg-flags under the East Asian–Australasian Shorebird
Observer coverage of the gulf has improved markedly since 1999. In
particular, better access to, and knowledge of, shorebird habitats
in the western sectors of the gulf has greatly improved our understanding
of the status of many species, and led to the discovery of a regular
wintering population of Nordmann’s Greenshanks and two regular
wintering locations for Spoon-billed Sandpipers. Extensive midwinter
coverage of the Inner Gulf during the Asian Midwinter Waterbird Census
(AWC) was achieved in three years since 2000 (2000, 2005 and 2006).
Many birdwatchers and photographers who are now active around Bangkok
have contributed greatly to recent coverage.
Fifty-six shorebird species have occurred in the Inner Gulf (Table
1); 49 species are winter visitors or passage migrants. Seven species
breed locally: Pheasant-tailed Jacana, Bronze-winged Jacana, Black-winged
Stilt, Eurasian Thick-knee, Oriental Pratincole, Red-wattled Lapwing
and Malaysian Plover.
1: Shorebirds known from the Inner Gulf of Thailand. Largest
midwinter counts are the largest single site concentration or largest
coordinated count made within the study area. Maximum counts are provided
where these are larger than midwinter numbers. Waterfowl populations
exceeding 1% of the estimated flyway population levels (following
Wetlands International, 2002) are considered globally significant
according to criteria set down by the Ramsar Convention. Numbers and
concentration in Inner Gulf of probable international importance or
definite international importance according to currently accepted
criteria. Asterisk (*) indicates figure for 1% level no longer applicable
due to recent findings; that for Oriental Pratincole is now much larger
than listed (Sitters et al., 2004) while that for Spoon-billed
Sandpiper is now thought to be much lower (Zöckler et al.,
2006). In the count columns: na = Not available. Source: Round and
Gardner in press; unpubl. AWC results from January 2006.
presented are midwinter count maxima. Midwinter concentrations of
20 species are thought to be of international importance. The most
numerous wintering shorebirds are Lesser Sand Plover (6,298), Red-necked
Stint (3,447), Black-tailed Godwit (3,078), Marsh Sandpiper (2,324),
Black-winged Stilt (2,726), and Common Redshank (1,523). Based on
allowance for turnover, Erftemeijer and Jukmongkol (1999) estimated
the numbers of wintering and staging shorebirds that use the site
as 100,000–135,000 per year, and the numbers using the site
in midwinter as 30,000– 40,000.
Indications are that the western sectors of the gulf receive higher
usage than the sectors to the more heavily industrialised east of
the Chao Phraya River, where it is thought that mudflat usage may
be constrained by a shortage of suitable onshore roosting areas (see
below under Status of Habitats).
Globally threatened species
Small numbers of Spoon-billed Sandpipers are now known to winter at
two localities in the Inner Gulf, having first been recorded in 1995,
and have been found in every winter period since late 1999. The largest
single site count is 16 birds. Most sightings have been made on temporarily
out-ofuse, shallow-flooded salt pans where the birds evidently feed.
The first birds arrive in mid-October and remain until mid-April (exceptionally
A regular presence of Nordmann’s Greenshank at two localities
in the Inner Gulf has only been recognised since November 2003. The
largest single count was a single flock of 60 birds on 24 December
2005 while up to 30 birds have been seen at a second site some 60
km distant, so there could potentially be as many as 90 birds wintering
in total. There is very little local information on ecology since
most birds have been roosting on ponds at high tide during the daylight
hours and are presumed to fly out to feed on mudflats as the tide
drops, usually during the hours of darkness.
Undoubtedly the most important shorebird breeding population is that
of Black-winged Stilt. Nesting of this species on coastal flats near
Samut Sakhon was mentioned by Madoc (1950), and breeders are common
and widespread, laying their eggs on, for example, pond margins in
brackish water areas in the coastal zone but also on floating vegetation
in freshwater sites well inland. There have been no counts of the
breeding population, though it probably numbers over 1000 pairs. Those
recorded in winter (~3000) are assumed to be mainly or entirely local
birds, although the occurrence of northern migrants is also to be
Roughly 10–15 pairs of Malaysian Plovers nest at the south-west
margins of the gulf, on the site’s only sand-beach habitat,
on an accreting 3 km long sandspit. Although this is not the largest
single population, it is of national significance, given the threats
from disturbance posed by tourism at other, more extensive sand-beach
habitats elsewhere in the peninsula.
The Inner Gulf as a staging area for migrants
Counts are too few, and coverage too uneven, to be able to reliably
chart seasonal changes in numbers in the many shorebirds in the gulf.
Paradoxically, midwinter counts for most species now generally outnumber
those during spring and autumn, perhaps because those seasons are
less wellsampled, and also due to turnover.
The clearest evidence of the importance of the Inner Gulf as a staging
area comes from observations of Asian Dowitcher. The c. 400 Asian
Dowitchers observed in the Inner Gulf in April 1984 (Round 1985) was
then the largest number known anywhere until a major wintering concentration
in Sumatra was discovered in the autumn of that same year. Although
Asian Dowitchers are now known to winter in the gulf in significant
numbers (Table 1), midwinter numbers are greatly exceeded by spring
maxima, usually recorded in the first half of April. In autumn, Asian
Dowitchers begin to arrive in mid- to late July and passage continues
into October, though numbers are generally lower than those in spring,
perhaps because the passage is more protracted.
Small numbers of Grey-tailed Tattlers are also recorded on spring
and autumn passage; none winter. Most other species are recorded both
on passage and in midwinter.
Flag-sightings and recoveries/controls have also begun to inform on
movements. Two flag-sightings and one control indicate that some Common
Redshanks that move through the Inner Gulf migrate further, to winter
in West Malaysia or Singapore. Sightings of a Singaporean leg-flagged
Lesser Sand Plover, and of Curlew Sandpipers from north-west and southern
Australia, and Red-necked Stints from the east Asian seaboard have
also been reported (Table 2).
All Lesser Sand Plovers handled, and those observed in breeding dress
in spring, have shown the characteristics of the atrifrons
group of races that breed in central Asia. So far as known there are
no records in Thailand of the north-east Asian breeding mongolus,
which presumably passes further east.
2: Trans-national recoveries or resightings of shorebirds
from the Inner Gulf of Thailand
in status of species
Trends in numbers through time cannot be tracked reliably for most
species. Although recorded maxima for most have increased in recent
years, this is almost certainly due to better coverage. An increase
in the wintering population of Black-tailed Godwits Limosa limosa
appears, however, to be genuine as the numbers at a single frequently
covered site (Bang Pu) increased from 300 in December 1996 to 800
in December 1997, and 1,200 one year later (Round & Gardner, in
press). The midwinter count throughout the gulf in January 2006 was
3,078. Increased numbers of Great Knots (previously thought to be
only a spring and autumn passage migrant) have also been recorded
in midwinter. Roughly 60 were counted in year 2000, but 800 in January
2005, and over 1,450 in January 2006.
The trend detected in Asian Dowitcher runs counter to that for most
other shorebirds and is possibly of concern. Single day concentrations
of 200–400 birds were regularly found in spring during the 1980s.
The largest reliably documented count is 600 birds on 22 April 1989
(Round & Gardner, in press) although there are anecdotal
reports of “about one thousand” during peak spring passage.
Coverage during April has subsequently been very limited, and relatively
few have been recorded in recent years (maximum 93 in April 1999;
Erftemeijer and Jukmongkol 1999) and during March–May 2006 (120;
S. Nimnuan, in litt.) indicating that further study is needed.
It is not clear whether this represents a genuine decline in the population
using the gulf, or a local shift in areas used so that some birds
Other than shorebirds, there are at least another 11 species of waterbird
for which the populations in the Inner Gulf are of known, or probable,
international importance (Round & Gardner, in press).
These are: Brown-headed Gull Larus brunnicephalus, Caspian
Tern Sterna caspia, Common Tern S. hirundo, Whiskered
Tern Chlidonias hybrida, Little Cormorant Phalacrocorax
niger, Indian Cormorant P. fuscicollis, Little Egret
Egretta garzetta, Great Egret Casmerodius albus,
Javan Pond Heron Ardeola speciosa, Spot-billed Pelican Pelecanus
philippensis and Painted Stork Mycteria leucocephala.
The number of Brown-headed Gulls, roughly 10,000 of which winter,
may be one of the largest wintering concentrations known.
The importance of the Inner Gulf is owed largely to the relatively
great expanse of low intensity ponds, so-called “supratidal
habitats” that occur in proximity to the extensive mudflats.
While the importance of traditional aquaculture ponds and salt-pans
as shorebird roosting areas has long been recognised, such areas also
provide feeding habitat, with shorebirds showing similar rates of
energy intake on salt pans as they do on semi-natural wetlands (Yasué
and Dearden, in prep.). For some species, such as Long-toed
and Red-necked Stints, and Broad-billed Sandpipers, salt-pans possibly
support the majority of birds throughout all stages of the tidal cycle.
Industrial and urban expansion
The integrity of the Inner Gulf is threatened by a number of factors.
Virtually none of this onshore habitat is protected; no zoning is
in place to prevent piecemeal loss from landspeculation, creeping
urbanisation, and industrialisation associated with the spread of
Bangkok and the provincial capitals of Samut Prakan, Samut Sakhon,
Samut Songkhram and Phetchaburi. The area has suffered from a proliferation
of inappropriate constructions, and ribbon development along some
roads. In addition, several new highways are either being constructed,
or are planned within the coastal zone. The eastern sectors of the
gulf, to the east of the Chao Phraya River, have borne the brunt of
the industrialisation up to the present. However, industrialisation
of the western sectors is now beginning, with the construction of
an oilrefinery on 1 km2 of land close to one of the two Spoon-billed
Sandpiper wintering areas on the Inner Gulf (Manopawitr and Round
Loss of mudflats
Although mudflat reclamations have occasionally been proposed, no
reclamation on any significant scale has taken place. A more significant
threat is posed by coastal erosion. Over 80% of the shoreline is suffering
erosion rates of 5–25 m/year (study on coastal change by the
Department of Mineral Resources; data supplied by N. Chaimanee, in
litt.). The problem is compounded by the extraction of groundwater
for industrial and household use, which causes compaction of sediments
and land subsidence, and also by reduced outflow of sediments from
the major rivers, most of which are dammed.
Responses to erosion include ad hoc mangrove plantings on mudflats
(which may exacerbate the loss of shorebird feeding areas), and the
construction of concrete sea-walls or boulder embankments on some
stretches of shoreline, which may alter tidal flow pattens and worsen
erosion on unprotected sections of coast.
The Inner Gulf was spared the worst of the (post-1980) boom in intensive
prawn-farming which has blighted many areas of south-east Asia and
which would have destroyed or damaged onshore feeding and roosting
areas for shorebirds. Following an initial boom in the mid- to late
1980s, poor water circulation and inappropriate management caused
the industry to collapse within about four years (around 1990) due
to the proliferation of fungal diseases and the accumulation of pollutants.
Most areas then reverted to low intensity prawn-capture ponds including
some abandoned, unutilised areas, which continued to support significant
numbers of shorebirds.
The cycle may now, however, be being repeated. The current practice
in some areas is to remove the accumulated pond sediments for landfills,
converting once shallow, traditional ponds into deep, steep-sided
ponds for aquaculture of prawns and crabs combined, or in some cases
for cultivation of molluscs. If the trend towards conversion to deep
water-filled ponds continues, this will greatly reduce onshore feeding
and roosting areas for waders, and could conceivably increase the
susceptibility of the coastal zone to erosion, or perhaps even catastrophic
Another disturbing trend is the use of polythene pondliners on salt
pans in some areas, rendering them unavailable as feeding areas. So
far, however, use of these is limited.
A wide array of organic and inorganic pollutants enters the gulf and,
because of poor circulation, they tend to accumulate. However, there
are no data on the effects of these on waterbirds. Occasional deaths
of small numbers of birds have been recorded, possibly associated
with dinoflagellate blooms.
Direct persecution of shorebirds (netting for supply to local markets
as food) still occurs, but probably on too small a scale to have a
major impact. Awareness is generally high and most species are fully
protected in law under the Wild Animal Reservations and Protection
No traditional protected areas exist in the Inner Gulf. Although
archaic legislation, such as the 1913 Thai Waterways Act, and some
land ownership laws, restricts development in coastal areas without
specific permission, there is no conventional conservation or protected
area legislation that can easily be applied to areas that support
human populations or otherwise receive human use. Additionally,
government agencies are usually exempt from the requirement to obtain
Environmental Impact Assessments before undertaking most construction
and public works activities.
24 km2 of coastal habitats, and an undetermined area of offshore
mudflats and shallow coastal waters at Don Hoi Lot in the western
sector of the gulf, were declared as a Wetland of International
Importance under the Convention on Wetlands. However, the area receives
no special protection in law and, additionally, offshore flats receive
heavy human use from shellfish collectors so that shorebird usage
is relatively low compared with some other sectors that receive
no special recognition.
government has prepared its own national inventory of wetlands (OEPP
1999; 2002), the Inner Gulf, listed as of international importance
by IUCN (Scott 1989), was perversely downgraded to only national
importance in the national inventory. This suggests that designation
was not based on objective scientific criteria, and that the lowered
importance level of the Inner Gulf may have been due to political
pressure applied to the inventory compilers.
the Inner Gulf of Thailand is one of the most important coastal
wetlands in Asia, yet also one of the most threatened. It is accorded
no special recognition by government agencies, even though Thailand
is a party to the Convention on Wetlands.
FOR FUTURE WORK
1) Monitoring of numbers and usage. There is a clear need for more
frequent and systematic counting of shorebirds throughout the
Inner Gulf so as to better track numbers and
usage. In particular, more emphasis needs to be placed on:
i) identifying key mudflat feeding areas;
ii) investigating how differential usage of
feeding areas relates to the density and distribution of shorebird
iii) investigating how conditions of onshore
habitats influences mudflat usage.
2) Determining origins and movements of birds through increased emphasis
on banding and colour-flagging.
3) Increasing awareness of the importance of the Inner Gulf among
Thai government agencies. Sufficient information already exists to
warrant nomination of at least the western sectors of the Inner Gulf
as a shorebird reserve network site, and such a designation would
usefully provide focus for those government agencies involved in coastal
resource management. Additional information on shorebirds that is
collected could be integrated into a comprehensive zoning plan for
coastal habitats in the gulf.
I thank the Bird Conservation Society of Thailand and the many shorebird
watchers who have contributed to our knowledge of waders in the gulf,
and Wetlands International and The Wetland Trust (UK) for providing
support. Sittichai Jinamoy (Hornbill Project Thailand) prepared the
Erftemeijer P.L.A. & R. Jukmongkol. (1999). Migratory
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Round, Assistant Professor and Regional Representative, The Wetland
Trust, Department of Biology, Faculty of Science, Mahidol University,
Rama 6 Road, Bangkok 10400, Thailand Email: email@example.com