Crab Plovers (Dromadidae)

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Crab plovers

(Dromadidae)

Class Aves

Order Charadriiformes

Suborder Charadrii

Family Dromadidae

Thumbnail description
Medium-sized birds with long blue-gray legs, contrasting black-and-white plumage, and a large, all-black, heavy, dagger-like bill with an angled lower mandible

Size
Height 13.0–16.1 in (33–41 cm); 0.5–0.7 lb (230–325 g); wingspan 29.1–30.7 in (74–78 cm)

Number of genera, species
1 genus; 1 species

Habitat
Coastal dunes, intertidal mudflats, and coral reefs

Conservation status
Not threatened

Distribution
Maritime coast of east Africa, Persian Gulf, and Arabian Peninsula

Evolution and systematics

Crab plovers (Dromas ardeola) are classified as Charadriiformes. The species was named by Paykull in India, in 1805. However, these birds are distinct enough that most taxonomists consider them to be a monotypic family with uncertain affinities to other groups. For instance, the tarsal scutellation and unpatterned down of chicks indicate a grouping with the gulls (family Laridae), whereas their burrow-nesting habits suggest a close relationship with the auks (family Alcidae). In general, the most widely accepted classification is to group crab plovers within the shorebirds or waders of the suborder Charadrii.

The Charadrii comprise a large and diverse group. Based on plumage and initial appearance, crab plovers closely resemble avocets (family Recurvirostridae), yet there are greater similarities in skeletal characteristics and external morphology with thick-knees (family Burhinidae) and coursers and pratincoles (family Glareolidae). DNA-DNA hybridization work of Sibley and Ahlquist (1990) support grouping crab plovers within the Glareolidae and divergence of crab plovers from other shorebirds during the Oligocene, approximately 35 million years ago. However, the methodology of this work has received sufficient criticism to cast doubt on results. A more detailed examination of the origin of crab plovers and its taxonomic affinities must await results from detailed comparisons of mitochondrial and nuclear DNA.

Physical characteristics

Crab plovers are striking long-legged birds with black-and-white plumage and a large heavy bill. The distinct and powerful black bill is dagger-shaped and well-suited for stabbing and consuming crabs. Adult crab plovers have predominantly white plumage contrasting with a black mantle, primary and greater coverts, and primaries. The tail is pale-gray. In juvenile birds, the crown, hindneck, mantle, and lesser and median wing coverts are also gray. The legs are blue-gray with partially webbed feet and a well-developed first toe, potentially associated with digging nesting burrows. Sexes are similar in appearance but males have slightly longer and heavier bills.

Distribution

The distribution of crab plovers is restricted to maritime coastal areas of the Indian Ocean. Breeding occurs from Somalia and Madagascar east to areas of western India. The limited breeding distribution is partly a result of the need for areas with sandy substrate suitable for burrow construction in conjunction with sufficient foraging sites. During the nonbreeding season, birds disperse and occupy sites that extend from South Africa east to Thailand, although most of the population winters in India, Kenya, Tanzania, and parts of the Arabian Peninsula.

Habitat

Crab plovers are a coastal marine species that occurs in desert and semi-desert regions. Generally, activities are restricted to areas within 0.6 mi (1 km) of the coast. Breeding colonies construct burrows in large expanses of coastal

sand dunes or sandflats. Birds forage on exposed mudflats and shallow-water areas of the intertidal zone.

Behavior

Crab plovers are gregarious birds that congregate throughout the year at breeding colonies, foraging areas, and roost sites. Flock sizes at foraging sites are as large as hundreds of birds. At traditional roost sites birds may travel as far as 15.5 mi (25 km) to join flocks of up to a thousand individuals. In these groups, birds are noisy and emit a constant chatter of barking "ha-how" or "crow-ow-ow" calls. There are reports of flocks being audible from distances of 1 mi (1.5 km). Birds are most active at dawn and dusk in addition to nocturnal periods. This activity pattern is attributed to avoidance of the intense mid-day temperatures of regions they inhabit. Both migratory and sedentary populations.

Feeding ecology and diet

Crab plovers are specialized predators that forage on exposed mudflats and shallow intertidal areas. As their name implies, crabs are the major component of both adult and chick diets throughout the year. Crab plovers forage in noisy groups and walk or run after prey, stabbing it with their heavy bills. Smaller crabs are swallowed whole whereas larger prey are dismembered and then eaten. In addition, crab plovers also feed on various other crustaceans, fish, marine worms, and assorted invertebrates. Adults do not regurgitate food to young chicks but instead pass bits of mashed prey. Older chicks are given whole prey items.

Reproductive biology

Various aspects of crab plover reproduction are fascinating and in some cases unique. For instance, these birds nest during the hottest and driest times of the year (April to June) when temperatures in the shade can exceed 104°F (40°C). Presumably, the timing of nesting coincides with the period when crabs are plentiful enough to sustain young growing chicks. As a response to these extreme temperatures, crab plovers nest underground and are the only shorebird or wader (Charadrii) to construct burrows. These birds use their bills and feet to construct burrows in sandy substrates. The downward-sloping burrow tunnels are from 47.2–74.0 in (120–188 cm) long and also provide protection from potential nest predators.

Crab plovers are colonial nesters with burrows situated close together, resulting in a honeycomb effect at breeding sites. The mating system is not well understood but birds are presumed to be monogamous. As many as 10 birds have been observed at a single burrow, leading some investigators to suggest that crab plovers are communal. Complicating the situation is the suspicion that there are helpers at nests.

In contrast to the clutches of two to four eggs laid by other Charadrii, crab plovers lay a single white egg of approximately 0.1 lb (45 g). Relative to body mass, the egg of the crab plover is one of the largest laid by any bird species. Because crab plovers nest in burrows, the duration of both incubation and fledging periods are unknown. The role of the sexes in parental care is also not well understood, but of 10 adults pulled from burrows, all were female. Crab plover chicks are precocial and semi-nidifugous. Young birds remain at nest burrows for extended periods. During this time, adults provide

food for the young and may travel as far as 7.5 mi (12 km) roundtrip on single foraging trips.

Conservation status

Crab Plovers are not listed as threatened or endangered. Based on extrapolations from large-scale winter surveys, the global population is estimated at 43,000–50,000 individuals with the greatest numbers found in Tanzania (20,000–26,000 birds). Counts at nine known colonies sum to 4,000–5,000 pairs. Includes Iran: 1,500 pairs; United Arab Emirates: 300 pairs; Oman: 85 pairs; Saudi Arabia: 110 pairs. However, with only nine identified breeding colonies worldwide, the locations of most breeding sites remain unknown.

One cause for concern is that large concentrations of crab plovers occur near to oil production sites. Due to their small population size, low reproductive rate, and narrow habitat requirements, the species would take a long time to recover from a catastrophic event. Other potential threats are the destruction and degradation of mangrove and other coastal habitats from pollution and development. This fascinating species warrants further studies, as well as increased survey and monitoring efforts to ensure its continued persistence.

Significance to humans

Crab plovers occur in relatively remote and harsh areas such that they have little contact with humans. During the early 1900s, birds and eggs were exploited by people in Iraq as a food source, but the current status of this practice is unknown. Reports of egg collecting from other areas occurred as recently as the 1970s. Both known crab plover breeding colonies in the United Arab Emirates receive formal protection from the ruling family.

Resources

Books

Cramp, S., ed. The Birds of the Western Palearctic. Vol. 3, Waders to Gulls. London: Oxford University Press, 1983.

Rands, M. R. W. "Family Dromadidae (Crab Plovers)." In Handbook of the Birds of the World. Vol. 3, Hoatzin to Auks, edited by Josep del Hoyo, Andrew Elliot, and Jordi Sargatal. Barcelona: Lynx Edicions, 1996.

Sibley, C. G., and J. E. Ahlquist. Phylogeny and Classification of Birds: A Study in Molecular Evolution. New Haven and London: Yale University Press, 1990.

Urban, E. K., C. H. Fry, and S. Keith, eds. The Birds of Africa. Vol. 3. London: Academic Press, 1986.