Ecology and behaviour of the Osprey Pandion haliaetus on the Farasan Islands, Red Sea, Saudi Arabia.
This study (1994-1996) was concerned with the distribution, abundance, reproduction and diet of the Osprey (Pandion haliaetus) on the Farasan Islands in the southern Red Sea region
of Saudi Arabia. It was sponsored by the central conservation agency of Saudi Arabia, to provide base-line data on the Osprey, an important flagship species for conservation within
the Farasan Marine Protected Area. The Marine Protected Area consists of 128 islands with a total area 3,300 krrr'. A range of tropical marine and coastal habitats is represented across the archipelago. Initial aerial counts of Ospreys along the eastern Red Sea coast and from a survey of the literature, showed that 70 % of all occupied nests across the Arabian Peninsula are now situated within the Red Sea region. Most mainland nest sites have become abandoned with
breeding now largely confined to islands. Within the Red Sea region most (ca. 15 % ) occupied nests occurred on the Farasan Islands. The majority of nest occurred on the ground and are potentially vulnerable to predation. Within the Farasan Islands, observations on the distribution and breeding biology were made on 126 pairs of Ospreys over a two-year period. Chick survival and food were measured at 28 focal nests, 16 on three small un-inhabited islands, and 12 from the two largest inhabited islands. Nests were widely dispersed across the archipelago with an average nearest neighbour distance along the shoreline of 0.2 occupied nests/km. Above average nesting
densities were associated with wide fringing coral reef biotopes. Ospreys were recorded at 50% of the 267 nests present. A wide range offish species was recorded at Ospreys nests. Prey were most diverse (16 fish families) at nest sites adjacent to habitats with mangroves, sea-grass beds and shallow reefs. Species from all trophic levels were recorded although larger predatory species were most common from nests adjacent to fringing coral reef and deep-water regions. Pairs nesting on
islands close to coral-dominant reefs had the greatest diet overlap, reflecting the ecology and range of fish species present at these sites. Large numbers of fish were found in the shallow water biotopes close to nest sites. Using reference collections, the average weight of prey delivered to chicks was estimated at 322 g. Indices of diet breadth (Levin's index range 0.20 - 0.46) were comparatively high for the species.
Breeding occurred during the winter, from November to January (median egg laying date=30th November). The mean clutch size was 2.8 (range 2 - 4). Most birds produced only one brood, although a few second broods were recorded following loss of the first. Not all birds at nests produced eggs (82 pairs out of 13S occupied nests). Approximately 27% of eggs failed to hatch. Eggs from larger clutches were on average, 10% smaller than eggs from smaller clutches. Eggs were small for the species, but consistent with a clinal trend with
latitude. Of those eggs that hatched, 82% successfully fledged. Mortality rates were highest during the egg stage (0.8 eggs lost Inest) and lowest during the chick to fledgling stage (0.3
chicks lost Inest). The most successful nests were those having more than two adult birds (range 2 - 4) in attendance. Estimates of productivity were high compared to those of
temperate populations. A total of 140 nestlings were coloured ringed in the nest prior to fledging. Of those chicks that hatched 87% were estimated to be males. The maximum interval
between the first and last chick hatched was eight days (median interval = 4 days from 19 broods). Mortality was highest on the third chick, which hatched on average 3.9 days later than the first chick (from the smallest egg) and had the lowest survival rate (89.4% first hatchlings, 54.8% third hatchling). Growth rates of female chicks were higher than those of males and conformed to a logistic growth model. The average asymptotic weights (wts) for male and female chicks were 1369g and 1512 g respectively) representing 96% and 84% of adult weights. There was no obvious difference in the growth rates of chicks between nests.Overall, the sex ratio of fledglings was biased towards males (ratio 1.3: 1).These results suggest that Ospreys can modify the pattern of recruitment though differential
parental investment in the number and sex composition of chicks produced. Brood reduction,a feature normally associated with restrictions in food supply, was surprisingly common.Chick mortality through sibling aggression was largely confined to the smallest chick, usually a male (fratricide). Brood reduction was associated with differences in the age and size of
chicks caused by asynchronous hatching and sexual size dimorphism, rather than an adjustment to food resources.
Since most females are thought to disperse from the natal site on fledgling some selection for males may occur where local conditions are favourable. For non-migratory resident
populations it may be more important to increase the production of males which are likely to be adapted to local conditions and remain close to parent birds. These results are examined in
the context of contemporary theories of population regulation. Further studies are needed to monitor the settlement of ringed birds to establish the age at first breeding and fidelity to
natal nest site. At present, the main threat to Ospreys on the Farasan Islands comes from ground predators, especially the mongoose (Ichneumia albicauda) and a continuing scarcity of suitable nest sites. Experimental provision of nest platforms (n=10) enabled Ospreys to colonise previously unsuccessful sites on inhabited islands. Additional threats include the damage to coral reefs and loss of feeding sites, from gill net and industrial fishing within the region. With increasing disturbance, birds are likely to become confined to more remote islands, causing population fragmentation, increased emigration and perhaps colonial breeding.However, a mechanism appears to exist within the behavioural ecology of the species to
produce the largest number of "quality" offspring through adjustments to the size and growth of chicks and modification to the brood sex ratio. The Osprey is an ideal model species to
investigate how small scale selection processes impact on the behavioural ecology of birds.