by Simon Aspinall & Philip A.R. Hockey
Picture by Hanne & Jens Eriksen
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.
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Arabian Wildlife. Volume 3, Number 1
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