The natural rarity of the crab plover appears to be a function
of limited availability of suitable nest-sites in close proximity
to an abundant supply of crabs, which dominate their diet.
The crab plover (Dromas ardeola) is a large, long-legged, pied,
exclusively marine wader. The sole representative of the family
Dromadeidae, it is highly ‘sought after’ by birdwatchers because
of its unusual appearance and rarity, having a very limited world
distribution. At a distance it superficially resembles an avocet
(Recurvirostra avosetta) but is easily distinguished by its heavy,
dagger-like black bill. Pinpointing which wader, or even non-wader,
is its closest relative, has puzzled ornithologists for a long
time. In fact recent studies of the genetics of the worlds’ birds
indicate that the closest relatives of the crab plover are pratincoles
and coursers! These lineages, however, are thought to have diverged
during the Oligocene epoch. This means that the crab plover has
been on an evolutionary monorail for some 35 million years, if
the genetic ‘clock’ is to be believed. Even if we do not know
exactly where it came from, one thing is in no doubt – it has
no close living relatives but has comfortably withstood the test
of time.
Compared with the majority of the worlds’ coastal breeding waders,
crab plovers have a very restricted breeding range, stretching
from Somalia in the west via the coast of Arabia to Iran in the
east. Most waders with such a characteristic are confined to one,
or a few, islands and several have become very rare while others
are now extinct.
Colonial nesting, 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, than
of waders. Only a dozen or so crab plover breeding colonies are
known and several of those that have been pinpointed have not
been visited in recent years. All are on islands and conservation
is effectively an ‘all or nothing’ act: an entire population can
be saved, or just as easily lost, in one fell swoop. The same
applies in the winter quarters, albeit to a lesser degree, because
they remain sociable year-round, feeding in a limited number of
particularly favoured areas.
In the early part of this century, Archer and Goodman reported
colonies from Somalia (British Somaliland’), on Saad al Din island;
Meinertzhagen described crab plovers as being common in the Arabian
Gulf and placed colonies on the island of Umm al-Haradim, Kubbar
and Auhah, adjacent to the coasts of Kuwait and northeastern Saudi
Arabia. Other Gulf colonies from where there are no recent reports
include Warba, Boonah and Dara and from the end of the last century,
Montafis Island in Iran.
Since 1970, only nine active colonies have been reported. There
have been no recent counts from Somalia but crab plovers certainly
still occur and presumably breed there. The present most westerly
colony lies in the Farasan Islands (belonging to the Kingdom of
Saudi Arabia) in the southern Red Sea. Four other colonies are
known around the Arabian peninsula, two in the United Arab Emirates
(UAE) in Abu Dhabi; one on an islet off Umm al Karam. This colony,
the largest known, held 1500 pairs in the early 1970s (Scott pers.
comm; Evans 1994) and may well still survive at the present time.
In the early 1970s, the colony at Auhah was still active, but
it was reported in 1990 that there was ‘no longer any evidence
of breeding’ on the coast of Kuwait.
On the basis of numbers of birds counted during the non-breeding
season, there must be some undiscovered, perhaps substantial,
colonies still to be found. Other colonies, unvisited since the
early 1970s, may of course still be occupied. Nonetheless, it
seems possible perhaps probable, that the entire world population
breeds at less than 15, or even less than ten, sites.
At the end of the breeding season, most crab plovers head away
from their colonies, generally southwards or eastwards. Their
winter range spans a considerable latitudinal band, unlike the
restricted tropical and sub-tropical breeding zone.The principal
wintering concentrations are found along the East African coast,
on Madagascar, on other Indian Ocean islands such as the Seychelles
group and at various localities on the coasts of India and Sri
Lanka. A few penetrate even further east, to the Bay of Bengal,
with a handful reaching Thailand and, rarely, Malaysia.
Apart from some predictable observations of a north-south movement
along the eastern African coast and off eastern Arabia, we know
almost nothing about the migrations of crab plovers to and from
their breeding grounds. The non-breeding distribution of crab
plovers suggests that they have two distinct migration routes,
with the majority moving south, about 20 per cent remaining within
the breeding range, and the remaining 10 per cent or so migrating
in a south-easterly direction. There are no ringing recoveries
– very few have ever been ringed – so we have no direct evidence
of who goes where and when. A satellite tracking study would solve
this situation almost overnight and it is sure to come, until
such time however, all that is possible is to make some ‘informed’
speculation.
What we do know is where some of the major concentrations of crab
plover occur, and from these we can put together a seemingly plausible
‘scenario’. In the western parts of the non-breeding range, the
greatest numbers appear to be in Tanzania. The estimated population
here is 20,000 – 26,000 birds (with 3000 on Mafia Island alone).
Further north, in Kenya, there are about 2000 birds, of which
about one third are found at Mida Creek near Malindi. An unknown
number visit, and perhaps overwinter, in Somalia. To the south,
in Mozambique, there are far fewer, the national total probably
not exceeding 500 birds.
Crab plovers are fairly common on the northern and southern coasts
of Madagascar, with up to 1500 at Baie des Assasins. In the northern
Indian Ocean, Aldabra is known to hold more than 1000 individuals
in winter but none of the other islands are known to support large
populations.
Within the breeding range, but during the non-breeding season,
a maximum of 600 has been counted at Khor al Beidah when just
over 500 were present on Merawah island, both sites being in the
United Arab Emirates. The Abu al Abyadh colony, 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
at Khor al Beidah have come from one or more Iranian colonies.
Their occurrence at this locality has been erratic and unpredictable
in recent years. Elsewhere in the Arabian Gulf, Kuwait and Saudi
Arabia have more than 200 birds each, and between 1250 and 1750
spend the nonbreeding season on the (Mekran and Baluchistan) coast
of Iran. In Oman, there are records of 2000 at Masirah island
(also a breeding station) and 3000 at Barr al Hikman, although
these may refer to the same wintering group. East of these western
and northwest Indian Ocean resorts, the only major 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 midpoint of ranges, gives a total
of around 42,000 birds of which 70 per cent are on the coasts
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 based on a single count (although
this could theoretically produce an overestimate) and secondly,
because smaller concentrations are ignored. In 1994, Rose & Scott’s
(IWRB) published world population was estimted as 43,000. This
is probably (almost certainly) an underestimate, and 50,000 may
be nearer the mark. However, whether 43,000 or 50,000 is closer
to the real world population it is irrelevant in highlighting
one major anomaly: the numbers of birds breeding at the known
colonies accounts for well under a quarter of the world population.
There have to be undiscovered colonies, some of which ought to
be large. So where are they?
Returning to the question of likely migration routes may give
us some clues. It seems likely that birds moving east or remaining
within the breeding range in the Arabian Gulf either breed in
the Arabian Gulf or in the Gulf of Oman. If this is the case,
the three colonies to the west of here, in the Red Sea and Gulf
of Aden, would have to be the source for the African /Madagascan
populations. These alone are much too small for this to be possible.
This can only point to one conclusion: somewhere along the coast
of Somalia (and probably including Saad el Din island) are major
breeding grounds of crab plovers- to the point that they may contain
half or more of the world population. It seems likely that these
‘armchair-theory’ colonies are the principal source of birds migrating
to East Africa, Madagascar and associated islands. The challenge
now is to find them! The Bajun Islands 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 unusual or unique
among waders, furthermore they contribute to its rarity. For a
start, colonial breeding among waders is very unusual. Apart from
crab plovers, the only waders that are habitually colonial are
the pratincoles (Glareolidae )and the banded stilt (Himantopus
leucocephalus) of Australia. In the case of the latter, coloniality
is forced on them by environmental factors because they breed
in ephemeral 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 excavate their own nest burrows, more than two
metres long and half 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, finding substrate soft enough to excavate
and cohesive enough not to continually collapse isn’t necessarily
that easy, particularly when you consider that it must be close
to a plentiful supply of crabs (see below). Unusually for a wader,
only a single egg is laid and uniquely, although predictably,
the egg is white (the eggs of most concealed nesters are white).
All other wader eggs are elaborately camouflaged with spots, scrolls
and hieroglyphics on a coloured background. Whether the large
eye size of crab plover allowed underground nesting or was developed
as a result, is a ‘chicken and egg’ question, however.
Nesting on islands is one way of avoiding predators; in general,
the further from the mainland the less the likelihood of ground
predators being present. Burrow nesting avoids the excessive heat
experienced at the surface and this is likely to be the real reason
for crab plovers digging their burrows, since the predator-free
island they occupy would not necessitate disappearing underground.
Crab plovers breed later in the year than other waders in the
Middle East. Coast species such as Kentish plover (Charadrius
alexandrinus) start to breed in March (the end of northern winter),
whereas crab plover do not lay eggs until well into May. The first
crab plover chicks to fledge each year do so at the very end of
July or beginning of August when the temperatures and humidity
are at 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 catching crabs
without getting injured yourself, but their almost complete reliance
on crabs probably explains why their destribution is largely resticted
to tropical and sub-tropical areas. Although many other wader
eat crabs, none do so to the same extent throughout the year.
Crabs are most abundant in intertidal areas in hot climes, particularly
where mangroves occur, and spend much of their time on the surface
outside of their own burrows. It would seem prudent for the plovers
to synchronize their breeding so that the chicks hatch at a time
coincident with the peak availability of crabs.
A simple energetic equation would dictate why only a single egg
is laid and also why it is eye-wateringly large in proportion
to the size of the bird (the chick develops fast and fledges earlier
than would be the case if the egg was smaller). Speed of development
is probably all important since it minimizes the period when adults
have to feed their chick in the burrow at the colony. Also, this
would explain why the chicks leave the colony with their parents,
who continue to provision them, as soon as they can fly. It must
be very costly flying backwards and forwards carrying a single
prey item to the colony each time. Stationing yourself and your
chick together in a rich feeding area being a far more cost-effective
strategy.
Although some crab plovers remain in their breeding range during
the non-breeding season, they do not remain at their breeding
sites. Crabs continue to dominate the diet throughout the non-breeding
season. Just as there are few sites which combine suitable nesting
and feeding conditions, it also appears that there are few sites
which are capable of supporting large numbers of crab plovers
outside of the breeding season.
Animals that are naturally rare are predisposed to local, regional
or global extinction. Similarly, animals that congregate at certain
stages in their life history tend to be more vulnerable than those
whose populations are widely dispersed, although they are also
actually easier to protect. In terms of their present and future
conservation status and prospects, crab plovers satisfy both the
above criteria for being a ‘high 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 coast
of the Arabian Gulf may, strangely, present 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 actually damage or even destroy colony sites instead. In the
non-breeding grounds the felling of mangroves for timber or charcoal
could pose a threat by reducing the abundance of crabs. Coastal
developments are also proceeding apace in many areas, often with
little or no regard for wildlife interests. Happily, however,
at least the two UAE colonies receive formal protection.
Oil pollution is an ever present problem. Pollution of mangrove
areas could prove disastrous for mangroves, crabs, crab plovers
and host of other wildlife including much of commercial interest
and high economic value. Collection of both eggs and young for
human consumption, as certainly happened in the early years of
this century until relatively recently (1970s), may still pose
a serious threat.
Whilst any evidence for an historical decrease in the world’s
crab plover population is little more than anecdotal, the precautionary
principle should nonetheless prevail. Its lifestyle clearly makes
it a prime candidate for conservation concern.
The global whereabouts of crab plovers are better known in the
non-breeding season than in the breeding season. Coupled with
this, young birds are still easily identified outside the breeding
seaon on the basis of plumage characters. Their proportion in
the population can therefore be assessed quickly and easily –
much more easily than on the breeding grounds where they leave
the colony almost immediately on fledging. An international monitoring
programme should target these key wintering sites, certainly until
such time as the ‘missing’ colonies are found and safeguarded.
The evolutionary eccentricities of the unique crab plover are
only just beginning to come to the surface but there remains a
long way to go their survival for the next 35 million years can
be assured.