by Paul R. Fisher and Hany Tatwany
Picture by Hanne &Jens Eriksen
The osprey (Pandion haliaetus) is a fish-eating bird of prey (raptor). Its success as a predator and the ability to breed in extreme temperate and tropical climates by migrating in the winter months has enabled this majestic cosmopolitan bird to range over all the world's continents except Antarctica.
Osprey populations found within the Kingdom of Saudi Arabia are localized along the Red Sea and Arabian Gulf with breeding largely restricted to islands. Migrating ospreys may also be seen inland near large sewage pools or reservoirs. Aerial surveys by NCWCD and records from ornithologists (contained within the Atlas for Breeding Birds of Arabia) estimate a breeding population of around 350 pairs for the whole Red Sea region. The largest colonies are found in the Al Wejh and Farasan Islands, both archipelagos maintain 60-80 breeding pairs. The Dahlak Islands off Eritrea and Tiran Island off Sinai, along with a few other northern Red Sea islands maintain populations of around 30-50 pairs. A maximum of ten breeding pairs are known in the Gulf where climate extremes are much greater and marine flora and fauna less diverse.
The Farasan Islands in the southern Red Sea, located 40 km off the Saudi Arabian port of Gizan, are recognized by the National Commission for Wildlife Conservation and Development (NCWCD) and the World Conservation Union (IUCN) for their diverse marine and coastal habitats, important for commercial fish stocks, turtles, cetaceans, dugongs and migratory birds. Protected areas have also been established on the islands for unique terrestrial fauna such as the mountain gazelle (Gazella gazella farasani )and the extensive stands of Avicennia and Rhizophora mangroves. The recent comprehensive survey of marine resources and the impact of human utilization of the Farasan Islands Protected Area provides a platform for developing and integrating other studies to monitor key wildlife species, and the marine environment as a whole.
The Farasan Islands are thought to hold around 25 per cent of the total Red Sea osprey population, therefore the Farasan Islands Protected Area osprey studies which commenced in 1994 are designed to address three main issues: the status of the Farasan Islands osprey population, the impact of this species on local fish populations, and the gathering of baseline data on their breeding biology to facilitate formulation of management recommendations. Studies are conducted as a part of an exchange programme involving researchers from NCWCD and the Manchester Metropolitan University, England, supported by the British Council. There has only been one other study of the osprey in the Arabian peninsula region, focusing on the diet of a resident colony on Tiran Island in the northern Red Sea.
It is thought that the Red Sea and Gulf breeding populations are resident, with a small number of over-wintering migrating individuals from Fenno-Scandinavia. A ringing programme has commenced, using metal NCWCD and two colour rings (for individual identification in the field) to attempt to learn more information about their movements, dispersal and life histories. Records of any sighting of birds with rings, or individuals found dead should be forwarded to NCWCD.
Breeding success has not been monitored throughout the Red Sea region, but recent surveys of ospreys within the Kingdom suggest that ospreys are vulnerable to the disturbance and demise of breeding pairs within loose colonies: increased development of the Red Sea coast and islands are thought to be responsible for this. Ospreys are particularly vulnerable to disturbance near their nests from either fishermen using small islands for fishing settlements, or coast guard patrols. Disturbance at the nest may cause pairs to abort breeding attempts, particularly during the early stages of egg laying. Osprey nests should not be visited by the public. Small mammals such as the natural predator, the white-tailed mongoose (Ichneumia albicaudia) are also known to predate osprey eggs and small chicks. Both the white-tailed mongoose and the feral cat (Felis domesticus) are present on the larger Red Sea islands, possibly increasing their range by scavenging from garbage tips and supplementing their diet with birds eggs, chicks and migrant birds. Avian scavengers such as ravens, gulls and vultures are not thought to pose a threat to a breeding pair but may take occasional eggs or chicks on an opportunistic basis.
Until further studies are conducted, it is not known whether Red Sea ospreys are exposed to chemical pollutants and whether they might have a detrimental affect on their breeding success.
Ospreys breed in the southern Red Sea from early November through to May. Most pairs lay eggs from mid-November into December. Ospreys at more northern Red Sea latitudes generally lay eggs a month later in early January. It is likely that ospreys have adapted to breed in the winter months to avoid the extreme summer Red Sea temperatures and solar radiation, particularly as eggs require continual incubation and protection from solar radiation. The higher tides and more favourable southerly currents and winds in these winter months may also play an important role, particularly in areas of extensive shallow water, which are often used as hunting areas by ospreys.
To date, studies suggest considerable variation in the time of breeding between pairs, separated by as many as 12 weeks within a colony. This is likely to be attributed to the age and experience of a pair, nest site availability and, to some degree, courtship feeding.
The female osprey typically lays a clutch of 2-4 eggs. Long term studies of breeding pairs have shown that clutch size appears to be related to the age of the female and experience of the pair. Fewer larger eggs tend to be laid by older more experienced females. One egg clutches are rare (probably misclassified due to poor survey techniques), as is the other extreme of two females sharing the same nest with two clutches.
The incubation period is around 7-8 weeks, with migratory ospreys incubating slightly shorter periods on average. Differences are possibly due to migratory osprey fledglings requiring time to learn how to capture prey and prepare themselves for their first long distance migration.
Ospreys are successful because of their ability to adapt toward variable environmental conditions. This is exhibited in their breeding strategies which usually incorporate asynchronous egg-laying and hatching to facilitate facultative brood reduction when food is unpredictable. Eggs are normally laid 1-3 days apart, with incubation commencing with the first egg.
This strategy results in chicks hatching at different times, providing a competitive advantage for the first hatched chick. Facultative brood reduction (selective chick mortality adaptive to the local environment) usually occurs when sufficient food is not available to be fed to all the chicks in the nest. This may be due to adverse weather conditions preventing foraging for fish, and inducing starvation or aggression between chicks, or lack of parental investment, where parental quality is poor.
Nests or eyries are made from a characteristic mass of twigs and beach litter, often including dead seabirds or other animals picked from the high tide line. Since the Red Sea coast is typically of barren sand or rock and sparsely vegetated, most ospreys nest on the ground, in stark contast to the relatives in colder climes that nest at the top of tall trees. Ospreys will nest on power pylons or towers if available near hunting grounds. Red Sea osprey nests can be over 2 m in height, built over many years. Farasan osprey nests average about 1.6 m in height and 1.4 m in diameter. The cup of the nest (usually one per nest) which holds the eggs is lined with various sea sponges. Each pair has 1-3 nests but only one is used during breeding each year unless disturbed. The male uses the nest, along with a courtship display, to attract a potential mate. Mangrove stands are more extensively distributed in the southern Red Sea, and occasionaly used as a platform for an osprey nest.
Alternative nests may be used when nests are occupied by other birds, following an aborted breeding attempt due to disturbance or actual predation of eggs or young. Such nests also appear to be important as roosting sites for individual fledglings whilst learning to forage and still dependent on their parents. Pair bonds are commonly maintained for life, up to 15 years.
Incubation is conducted by the male and female, though it is still unclear why there is so much variation in individual parental investment between pairs. Fish prey are caught by the male during incubation and for the first four-five weeks of the chick rearing period. Both parents usually forage for prey once the chicks are large enough to fend for themselves, and food demands are greater.
Ospreys exhibit reversed-sexual dimorphism, i.e the female is larger in body size than the male. This dimorphism is also exhibited at the chick stage, when females tend to increase in weight more than males after four weeks old. The chick rearing period is around 50 days. Dimorphism is also shown in colour markings where males tend to show more white feathers on the breast and neck, providing greater contrast with the darker flight feathers; most likely associated with sexual signalling toward females, and used in conjunction with body postures to warn off other competitive males or potential predators. Females are more similar in appearance to feathered nestlings, which may facilitate begging for food to provision hungry chicks.
Ospreys are restricted to marine habitats as they are the only diurnal raptor that feed specifically on fish. Most other raptors feed on smaller birds. Preliminary studies on the Farasan Islands show that parrotfish (Scaridae), rabbitfish (Siganidae), needlefish (Belonidae), wrasse (Labridae), and angelfish (Pomacanthidae) are common prey of the osprey. An osprey, whilst provisioning chicks, may catch up to eight fish a day, each fish weighing as much as 800g.
The diversity of osprey fish diet is generally determined by the type of marine habitat found within the foraging area (usually close to the nest), where shallow lagoons and gently sloping reef platforms are favoured over narrow fringing reefs. Studies have monitored foraging in different marine biotopes (coral, algae, seagrass, mangrove and sandy substrates), as identified in the Farasan Islands Marine Management Plan.
A number of the small mangrove islands, free from mammal predators, with adjacent coral and algae reefs, hold high densitites of breeding osprey pairs: mangroves are known to be important fish nurseries for both neighbouring and distant reefs. The NCWCD are presently engaged in a programme to restore the Avicennia mangroves in Farasan port area which were damaged due to the construction of a causeway for sand extraction, limiting tidal flow. Banks are being created in the causeway to allow the tide to flow into the upper reaches of the lagoon, and seedling nurseries are being planted to encourage new growth.
There are still a number of important questions that we would like to answer to ensure that osprey conservation is managed in the most effective manner. Where do young osprey go to when they leave the nest in the spring? Are Red Sea ospreys different in size and shape and reproductively isolated from the northern temperate ospreys? Are Red Sea ospreys exposed to chemical contaminants via the marine environment causing a reduced reproductive effort? Such questions require long term studies to monitor colonies, providing opportunities for sharing knowledge, experience and ideas between scientists and the public, encouraging a greater understanding of the environment, and awareness of Arabian Wildlife.dency h
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Arabian Wildlife. Volume 3, Number 1
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