The Indian Ocean's Crab-loving Plover Arabian Wildlife

The Indian

Ocean's Crab-loving

PLOVER The natural rarity of the crab plover appears to be a function oflimited availability ofsuitable nest-sites in close proximity to an abundant supply of crabs, which dominate their diet.

Simon Aspinall T H E Cl<AB PLOVER (Orolnas ardca/a) is a large, long-legged,

& Philip A. R. pied, exclusively marine wader. The sole representative

Ho ckey of th e family Dro ma de idae, it is highly 'sou ght after ' by

birdwatchers because of its unusual appearance and rarity,

having a very limited world distribution . At a distance it

superficially resembles an avocet (Rec!lrv irosfra avasefln ) but

is easily distinguished by its heavy, dagger-like black bill.

Pi npointing which w ader, or even non-'wader, is its closest

rela ti ve, has puzzled ornithologi sts for a long time. In fa ct

recent s tudies of the genetics o f the worlds' birds indicate

that the closes t relatives of the crab plover are pratincoles and

coursers I These lineages , however, are thought to have

di verged d uring the Oligocene epoch. This m eans that the

Portrait af a crab plover has been on an evolutionary monora il for some

Crab plover. 35 million yea rs, if the genetic 'clock' is to be believed. Even

32

if we do not know exac tly where it came from, one thing is

in no d o ubt - it ha s no close li ving re la tives bu t has

comfortably withs tood the test of time.

Com pa re d with the majority of th e wo rlds' coas ta l

breeding waders, crab plovers have a very restricted breeding

range, stre tching from Somali a in the west vi a the CO flS t of

Arabia to (ran in the eflSt. Most waders with such a ch(lfflc­

teristic are confined to one, or fl few , isla nd s and severid

have become very rare wh ile others are now extinct.

Colonial nes ting, an aspect in the nesting behaviour of

crab plover which has a strong bearing on their conservation,

is much more characteristic of seabirds, be they on cliffs or

on islands, th im of waders Only fl dozen or so crab plover

breeding co lon ies are known and several of those that have

been pinpointed have not been visi ted in recent years . All

are on island s and conservfltion is effectively an 'a ll or

nothing' ac t: an entire population can be s.wed, or jus t as

eas ily los t, in one fe ll swoop. The same applies in the win ter

quar ters, a lbei t to a lesser degree, because they remain

sociable year-round , feeding in a limited number of partic­

ularly favoured areas.

In the cilrly part of this century, Archer and Goodman

reported colonies from Somalia (B ritish Som aliland'), on

Saad 211 Din island; Meinertzhagen described crab plove rs as

being common in the Arabia n Gulf a nd placed colonies on

the island of Umm il l-Haradim, Kubbar and Au hil h, ildjacent

to the coasts of Kuwait and northeas tern Saudi Arabi il. Other

Gulf colonies from where there are no recent reports includ e

Warba, Boonah and Dara and from the end of the last century,

Montafis Is la nd in Iran.

Since 1970, only nine active colonies ha ve been reported .

There have been no recent counts from Somalia bu t crab

plovers certainly still occur and presumably breed there.

The present mos t wes terly colony lies in the Farasan Islands

(belonging to the Kingdom of Saudi Arabia) in the sou thern

Red Sea . Four other colonies il re known arou nd the Arabian

peninsula, two in the United Aril b Emiriltes (UAE) in Abu

Dhabi; one on an isle t off Umm al Karam. This colony, the

larges t known, held 1500 pairs in the early 19705 (Scott pers.

comm; Evans 1994) and ma y well s till survive at the present

time. In the ea rly 1970s, the colony at Au hah was s till active,

but it was reported in 1990 tha t there was 'no longer any

evidence of breedin g' on the coast of Ku wai t.

On the basis of numbers of birds counted during the non­

breeding season, there must be some undi scovered, perhflps

s ubs tantiill , colonies s till to be fo und . Other co loni es ,

unvis ited s ince th e early 19705, ma y of course s till be

occupied . N onetheless, it seems possible perhaps probable,

that the enti re world populil tion breeds a t less than 15, or

even less than ten, sites.

At the end of the breed ing season, most crab plovers heRd

away from their colonies, generally southwards or eastwards.

Their winter range spans a considerable latitudinal band ,

unlike the res tricted tropical and sub-tropica l breedin g

zone.The principal wintering concentra tions are found along

Volume 3 , Number I

- --- -Arabian Wildlife The Indian Ocean's Crab-loving Plover

the East African coast, on Madagascilr, on other Indian Ocean

islands such as the Seychelles group and at various loca lities

on the coasts of India and Sri Lanka. A few penetrate even

furth er east, to the Bay of Bengal, with a handful reaching

Thailand and, rarely, Malaysia .

A piut from some predictable observa tions of a north-south

movement along the eastern African coast and off eastern

Arabia, we know almost nothing ilbout the migfil tions of crab

plovers to and from their breeding g rounds. Th e non­

breeding distribution of crab plovers sugges ts thilt they have

two distinct migrati on routes, with the majority moving

south, about 20 per cent remaining wi thin the breed ing range,

and the remaining 10 per cent or so migratin g in a south­

easterl y direction. There are no ringing recoveries - very few

have ever been ringed - so we have no direct evidence of who

goes w here and when. A satellite tracking study would solve

this situa tion almost overn ight and it is sure to come, until

such time however, a ll that IS possibl e is to make some

'informed'speculation ,

What we do know is where some of the major concentra­

tions of crilb plover occur, and from these we can put together

il seemingly plausible 'scenario'. In the western parts of the

non-breeding rilnge, the greatest numbers appear to be in

Tanzania The es timated population here is 20,000 - 26,000

birds (with 3000 on Mafiil Island alone). Further north, in

Kenya, there are ilbout 2000 bird s, of which about one third

are found at Mida Creek near Millindi. An unkno'vv n number

visit, and perhaps overwinter, in Somillia. To the south, in

Mozambique, there ilre far fewer, the nationa l total probably

not exceeding 500 birds.

Volume 3, Number I

Crilb plovers are fa irly common on the northern a nd

southern coasts o f Madagascar, with up to 1500 at Baie des

Assasins. ln the northern Indian Ocean, Aldabra is known

to hold more than 1000 individuals in w inter but none o fthe

other is lands a re known to support large populations.

VVithin the breeding rilnge, but during the non-breeding

season, a maximum of 600 has been counted at Khor al Beidah

w hen jus t over 500 were present on Merawah island, bo th

sites bei ng in the United Arab Emirates. The Abu al Abyad h

colon y, of c300 pairs, is vacated at this time and it is thought

that these birds travel the short distance to Merawah, whereas

it is possible that birds a t Khor al Beidah have come from

one or more Iranian colonies. Their occurrence at this locality

has been erratic and unpredictable in recent yea rs. Elsewhere

in the Arabian Gulf, Ku wa it and Saudi Arabia ha ve more

than 200 birds each, and between 1250 and 1750 spend the

nonbreeding season on the (Mekran and Baluchistan) coas t

of Iran. Tn Oman, there are records of 2000 a t Masirah island

(also a breeding station) ilnd 3000 at Barr al Hikman, although

these may refer to the same wintering group. Eas t of these

western and northwest Indian Ocean resorts, the only miljor

concentration found to date, and a very important one at that,

is some 5000 birds in the southern Gulf (Rann) of Kutch.

These come from an unknown source.

The above figures, taking the mid point of ranges, gi ves a

total of around 42,000 bird s of which 70 per cent are on the

COilSts of Africa, including Madagascar and nearby islands.

The 42,000 population at the prime sites is likely to be an

underestimate, as firstly, many of the figures are bilsed on a

s ingle count (although this could theoretically produce an

33

Arabian Wildlife

C},<1b pfOFfI' overestimate) and secondly, beciluse small er concentriltions

((l lollY OJI Aim are ignored . In 1994, Rose & Scott's (IWRB) published world

II I AbYlldh, population was es timted ilS 43,000. This is probably (a lmost

A/III Dhahi. certainly) an underes timate, and 50,000 may be neilrer the

milrk. However, whether 43,000 or 50,000 is closer to the reill

world population it is irrelevant in highlighting one major

anomaly: the numbers of birds breeding a t the known

colonies accounts for well under a qua rter of the wo rld

population. There have to be undiscovered colo nies, some

of which ough t to be lilrge. So w here are they?

Returning to the questio n of likely migration routes may

give us some clues. It seems likely that birds moving east or

remaining vvithin the breeding range in the Arabian Gulf

either breed in the Arilbian Gulf or in the Gulf of Oman. If

this is the case, the three colon ies to the west of here, in the

Red Sea and Gulf of Aden, wou ld have to be the source for

the African / Madagasca n populations. These alone are much

too small for this to be possible. This can only point to one

conclusion: somewhere a long the coast of Somillia (and

probably including Saad el Din island) Me major breed ing

ground s of crab plovers- to the point that they may contain

half or more of the world population. It seems likely that

these 'a rmchair-theory' colonies are the principal source of

birds mig rating to East Africa, Madagascar a nd associated

islands. The challenge now is to find them' The Bajun Isla nds

off southern Somalia seem a likely place to look.

Apart from its isolationist evolutionary history, there are

several aspects of the biology of crab plovers which are

unu s ual or unique ilmo ng wad ers , furth e rm o re they

con tri bute to its rMity . For a StiHt, colon ial breeding among

waders is very unusuill. Apart from crab plove rs, the on ly

wade rs that are habitua ll y co lonial are the pra tincol es

(C/l7rco /idl7t' la nd the banded s tilt (Hirrumioplis /cucocep/l17/us)

of Australia. In the case of the latter, coloniality is forced on

the m by env ironmental fact ors because th ey breed in

ephemera l wetlands that rarely hold sufficient water and

food .

The crab plover differs from these other species, however,

in that it breeds underground - the only wader in the world

to do so. Crab plovers excavilte their own nest burrows,

more than two metres long and hil lf a metre deep, in raised

areas of sand close to the sea. An entire sandbank can become

honeycombed with tunnels. Such banks may be limiting,

findin g subs trate sof t e noug h to excava te and cohes ive

enough 110t to continually collapse isn' t necessari ly that easy,

particularl y when you consider that it must be close to a

p lentiful supply of crabs (see below). Unusua Il y for a wader,

only a single egg is laid and uniquely, although predictilbly,

the egg is white (the eggs of mos t concealed nes ters are

white) . All other wader eggs are elaborately camouflaged

with s p o ts, sc rolls and hi e rog lyphics o n a co loured

ba ckground . Whe ther the large eye s ize of crab plover

a llowed underg round nesting or was developed as a result,

is il 'chicken and egg' question, however.

N es ting on islands is one way of avoiding predators; in

genera l, the further from the mainl and the less the likelihood

of ground predators being present. Burrow nesting avoids

the excessive heM experienced at the su rface and this is likely

to be the rea l reason for crab plovers d igging their burrows,

si nce the predator-free is land they occupy wou ld no t

necessi ta te disa ppeil ring und ergroLi nd.

Crab p lovers breed later in the year than other wilders in

the Mid d le East. Coast species such as Kentish plover

(C/wradri u5 aleXlll1drinlls) start to breed in March (the end of

northern winter), whereas crab plover do not lay eggs until

wel l into May. The first crab plover chicks to fledge each year

do so a t the very end o f Jul y o r beg inning of Augus t w hen

the tempera tures a nd humidity Me ilt their highest.

Why do crab plovers not breed earlier in the year and

avoid the problem like other species do? The answer to this

question must lie in the most unique aspect of their biology

- their diet. The wonderfully well designed bill is just the tool

for ciltching crabs w ithout getting injured yourself, but their

a lmost complete re lia nce on crabs p robably explains why

their des tribution is la rge ly res ticted to tropica l and sub­

tropical areas. Although many other wader ea t crilbs, none

do so to the Sil me extent throughout the year. Crabs are most

abundant in intertidal areas in hot climes, particularly where

mangroves occur, and spend mu ch of their time 011 the

surface outside of their own burrows. It wou ld seem prudent

for the plovers to synchronize their breeding so that the

chi cks ha tch a t a time coincident with the peak ava ililbility

of crabs.

A simple energetic equation would dictate why only a

single egg is laid ilnd also why it is eye-wateringly large in

Volum e 3 , Number I 34

proportion to the si ze of the bird (the chick develops fast and

fledges earlier than would be the case if the egg was smaller).

Speed of d evelopm ent is probably all important since it

minimi zes the period when adults have to feed their chick

in the burrow at the colony . Also, this would explain why

the chicks leave the co lony with their parents, who continue

to provision them, as soon as they can fl y. It must be very

costly flyin g backwa rd s and forwards carrying a single prey

item to th e colony each time. Stationing yo urself ilnd your

chick together in i1 rich. feeding area being a far more cost­

effective stra tegy.

Although some crab plovers remain in their breeding range

during the non-breeding season, they do not remain at their

breedin g s ites. Cra bs continue to dominate th e diet

throughout the non-breeding season. Jus t as there are few

sites which combine suitable nesting and feeding conditions,

it also appea rs that there are few sites which are capable of

supporting large numbers of crab plovers o utside o f the

breed ing season.

Animals that are na turillly rare are predisposed to local,

reg ional o r g lobal ex tinction. Similarl y, animal s that

congregate il t certain stages in their life history tend to be

more vulnerable than those whose popula tions are w idely

dispersed , although they are also actuall y eas ier to protect.

In terms of their present and future conservCltion status and

pros pects, cr<'lb plovers satisfy both the above criteria for

being a 'hi gh risk'species: the population is small and is

concentrated in both the breeding and non-breeding seasons.

The artificial establishment of mangrove stands on the

south coas t o f the Arabian Gulf ma y, s trangely, present

Volume 3, Number I

The Indian Ocean 's Crab-loving Plover

either a threat or an advantage. This practice while leading

to an increase in the availability of food for crab plovers

(whilst reducing it for many other species it has to be said)

when injudiciously placed may C1 ctually d a milge or even

destroy colony s ites ins teild . In the no n-breeding grounds

the fe lling of mCingroves for timber or charcoal could pose

a threilt by reducing the abundClnce of crilbs. COils tal deve l­

opments are al so proceeding apace in many a reas, ofte n

with little or no regClrd for wildl ife interes ts. Happily,

however, at leil s t the tw o UAE colo nies receive form a l

protection.

Oil p ollution is all ever present problem. Po llution of

mangrove areas could prove disas trous for mangro ves,

crabs, crab plovers and host of other w ildlife including much

of commercial interest and high economic value. Collection

of both eggs and young for human consumption, as certainly

happened in the ea rly years of this century until re liltively

recently (9705), may still pose a serious threat.

Whil s t any evidence for an historical decreilse in th e

world's cra b plover popula ti on is Ii ttl e more thim anecdotal,

the preca utiona ry pri nciple should no netheless prev il iI. Its

lifestyle clearly m akes it a prime candidate for conservation

concern.

The global whereabouts of crilb plovers are better known

in the non-breeding season thilll in the breeding season.

Coupled with this, young bird s are s till easily id entified

outside the breeding seaon on th e bas is of plumage

characters . Their proportion in the population can therefore

be assessed quickl y and easil y - much more easil y than on

the breeding grounds where they lea ve the colony almos t

immedia te ly on fl edging. An international mo nitoring

programme should ta rget these key wintering sites, certainly

until such time as the 'missing' colonies are fo und and

safeguard ed.

The evolutionary eccentricities of the unique crilb plover

are only Ju st beginning to come to the surface but there Crab p /OV('l'

remains a long way to go their survival for the next 35 million b UI'I'011'

years Ciln be assured. el1 lrrlJ1 ( (, s.

35