Crab plover (Dromas ardeola)
|Size||Length: 33 – 36 cm (2)|
Classified as Least Concern (LC) on the IUCN Red List (1).
The crab plover is remarkable for being the only representative of the Dromadidae family, which means that in evolutionary terms this unusual shorebird has no close living relatives (3). This species has a distinctive appearance, with white plumage on the body and head, contrasting with jet-black primaries and back feathers, and a long, black, gull-like bill. Like most waders, the legs are long with partially webbed toes, and the tail is short. Juvenile crab plovers lack the bright white adult plumage, appearing uniform grey-brown instead (2).
Outside the breeding season, the crab plover occurs over a large range, extending throughout much of the Indian Ocean, from Natal, South Africa, east to the Andaman Islands. During the breeding season, however, the known breeding colonies are found within a much more restricted range at a small number of sites around the southern Red Sea, the Gulf of Aden, the Gulf of Oman and the Arabian Gulf (2) (3).
The crab plover inhabits sandy and muddy shores on mainland coasts and on islands, as well as intertidal sandflats and mudflats, estuaries, lagoons and exposed coral reefs (1) (2). During breeding, this species has a specific requirement for sandy islands or extensive dunes in which nesting burrows can be excavated (1).
Noisy and gregarious, the crab plover is commonly encountered in small groups foraging on the shore for its preferred prey of crabs (1) (2). These foraging groups usually comprise around twenty to thirty birds, but may number as many as 400 outside of the breeding season (2). The crab plover is active during the day and night (4), taking advantage of crabs, marine worms and mudskippers exposed by the receding tide (2) (4). This species’ large, powerful beak allows it to tackle crabs without suffering injury, swallowing smaller individuals whole, and smashing larger specimens against the ground, to be consumed piecemeal (2).
The breeding season occurs between April and August, at which time dense colonies form around areas that have an abundance of crabs on which the young can be fed (1) (3). The crab plover is unique amongst the waders for its habit of constructing its nest in an underground burrow (2). After moving inland from the shore to sand dunes, the birds excavate an extensive network of interconnected burrows, 1 to 2.5 metres long. The entrances of the burrows are initially angled downwards, before curving upwards and terminating in a nest chamber situated a short distance from the surface (1) (2). The burrow is believed to serve two important functions, one of which is to insulate the egg against the extremely high temperatures that occur during the breeding season (3) (5). The second function is that the chamber acts as a solar incubator, keeping the egg at an optimum temperature, which means that only a small amount of direct incubation by the parent birds is necessary (5). A single white egg is laid, which is very large and provides the developing chick with sufficient energy that after the 32 to 33 day incubation period it hatches very well-developed and is quickly able to walk (2) (3). Despite this fact, the chick remains in the nest chamber until fledged, where it is fed live crabs by both parent birds (2).
After the breeding season, some crab plovers remain in the vicinity of the breeding colonies, while the majority fly southwards or eastwards to wintering sites around the Indian Ocean. Interestingly, while surveys of birds at the wintering sites appears to indicate that this species could have a global population of between 60,000 and 80,000 birds, the total population at the known breeding grounds represents only a fraction of this figure. Hence, there must be some large breeding colonies of crab plover that have yet to be discovered (3) (6). Most recently, in a survey conducted between 2002 and 2004, the largest colony yet discovered was found in the Dahlak and Howakil archipelagos, off the coast of central Eritrea. However, while this comprised an estimated 4,800 to 6,500 individuals—half of the known breeding population—it still does not account for the large numbers of birds observed at the wintering grounds. The authors of the study therefore speculate that the missing colonies probably lie in southern Eritrea, Sudan and Somalia (6)
With a large overall population, the crab plover is not considered to be globally threatened (1). Nevertheless, due to the concentration of the global population within a relatively restricted range during breeding, a localised threat such as an oil spill or the introduction of nest predators, could have catastrophic results (1) (3).
There is some concern that the planting of artificial mangrove stands on the south coast of the Arabian Gulf could have a negative impact on this species by decreasing nesting habitat, although in contrast to this caveat, it may also increase food availability (3). The expansion of the tourist industry, along with coastal development and the collection of eggs from some colonies are also potential threats, although fortunately many of the known breeding colonies occur on relatively small, isolated and undeveloped islands (3) (6).
Within the United Arab Emirates the crab plover’s breeding colonies, located on two islands in the Abu Dhabi Emirate, receive formal protection, as does the colony on the Farasan Islands, Saudi Arabia (3) (7). In order to safeguard this species against potential threats, the other known breeding colonies, in particularly the large Eritrean colonies, would also benefit from the implementation of protective legislation (6).
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IUCN Red List (March, 2009)
- Burton, M. and Burton, R. (2002) International Wildlife Encyclopedia. Marshall Cavendish, New York.
- Aspinall, S.J. and Hockey, P.A.R. (1996) The Indian Ocean’s crab-loving plover. Arabian Wildlife, 3: 32 - 35.
- Fasola, M., Canova, L. and Biddau, L. (1996) Foraging habits of crab plovers Dromas ardeola overwintering on the Kenya coast. Colonial Waterbirds, 19: 207 - 213.
- de Marchi, G., Chiozzi, G. and Fasola, M. (2008) Solar incubation cuts down parental care in a burrow nesting tropical shorebird, the crab plover Dromas ardeola. Journal of Avian Biology, 39: 484 - 486.
- de Marchi, G., Chiozzi, G., Semere, D., Galeotti, P., Boncompagni, E. and Fasola, M. (2006) Nesting, overwintering, and conservation of the crab plover Dromas ardeola in central Eritrea. Ibis, 148: 753 - 764.
BirdLife International (July, 2009)