The Sooty Oystercatcher (Haematopus fuliginosus), inhabits rocky shorelines and coastal islands along the coast of Australia. It forages in the littoral zone, the region between high and low tide marks and preys upon a variety of molluscs and crustaceans. It appears to breed mainly on marine islands, a limited breeding resource which appears to place an upper limit on its population size. Non-breeding birds inhabit rocky shore areas all year round.
Sooty Oystercatchers require access to a larger area of littoral zone during the breeding season to supply all the food requirements of both parents and chicks. This is achieved by the pairs maintaining both breeding and post breeding territories. The size of these territories depends upon prey densities. On many islands littoral zones are restricted to vertical rock faces which are not readily accessible to oystercatchers. Such islands provide limited habitat and may support only one or two pairs. More rarely the littoral zone may occur as a rocky platform offering a relatively large foraging area for oystercatchers, resulting in much higher population densities such as those on Flinder’s Island at the Five Islands Nature Reserve. However, overall the New South Wales Sooty Oystercatcher population is small and widely distributed, with only 100 individuals recorded in counts conducted by Wader Study Groups (Lane 1987).
Breeding Sooty Oystercatchers are not easy to study, as acquisition of useful data is a prolonged process requiring a great deal of effort and persistence. The work can often be hazardous and requires a good boat and captain with excellent seamanship. This is essential when attempting to gain access to islands in poor weather or sea conditions.
As a result of these factors, the Sooty Oystercatcher remains one of the least known of all the world’s Oystercatchers, with little written on the species' breeding biology. This information is critical to the long-term survival of this species, as the Sooty Oystercatcher is now listed as a threatened species as a result of its small and vulnerable population size. In an attempt to rectify this situation, in 1994 members of the Southern Oceans Seabird Study Association (SOSSA) began studying the breeding biology of the Sooty Oystercatcher, and in particular, this species breeding distribution on Five Islands Nature Reserve and the post breeding dispersal of fledglings.
The Five Islands group is situated off the coast of Port Kembla, and includes Big Island No. 1, Big Island No. 2, Martin, Flinder’s and Bass Islands. In New South Wales, most offshore islands, including the Five Islands, are either in national parks or are nature reserves under the control of the NSW National Parks & Wildlife Service Also there is a considerable length of as yet relatively undisturbed rocky coastline along NSW. However, small populations must always be considered vulnerable, particularly if breeding biology and population regulation parameters are unknown, as is the case with the Sooty Oystercatcher.
Population and status
Allen Keast (pers. comm.) indicated that during his visits to the Five Islands with Consett Davis between 1940-42, the Sooty Oystercatcher was regarded as a rare breeding resident, with one pair breeding on Flinders Island and another pair possibly breeding on Big Is No.2 in 1941. He also indicated that there were thought to be fewer than 3 pairs breeding on the islands in 1942 and the Sooty Oystercatcher was generally regarded as quite rare in the Sydney region.Thirty years later, Battam (1976) reported that up to 3 pairs of Sooty Oystercatchers were still breeding on Flinder’s Island. The presence of breeding pairs on other islands in the group was not recorded. Our studies suggest that the small population on the Five Islands Nature Reserve appears to have significantly increased and this small group of islands may now hold the largest breeding population of Sooty Oystercatchers along the seaboard of NSW. Below we present some of our preliminary results.
Breeding population and distribution
From 1994 to 1998/9 there was a total of 16 pairs of Sooty Oystercatchers breeding on Five Islands. These pairs were distributed throughout the group as follows: 1 Pair on Big Island No. 1
3 Pairs on Big Island No. 2
1 Pair on Martin Island
9 Pairs on Flinders Island
2 Pairs on Bass Is.
The breeding season of the Sooty Oystercatcher on Five Islands extended from mid-September till February or more rarely, March. Eggs are usually laid in late October, although eggs were laid as early as 13th October in 1996, with chicks usually fledging from the islands by mid-February.
At the Five Islands, Sooty Oystercatchers maintain breeding territories for the entire breeding season. The size of each territory varies considerably, depending on the quality of each site. Territories with a higher prey density are considerably smaller. During incubation and for much of the chick rearing period males forage in nonbreeding or post breeding territories and were rarely observed in their breeding territory during this period. Foraging in the breeding territory was confined to the female and chick.
On Five Islands the Sooty Oystercatcher nests in a variety of sites, from 3 metres above the high water line to elevated situations up to 16 meters above the high water mark. Nest sites were rarely found more than 20 metres from the waters edge. The distribution of nest sites is largely dependent on the availability of suitable nest sites within suitable foraging areas. For example, on Flinder’s Is, 3 nests containing eggs belonging to 3 separate pairs were found within 3 metres of each other, while only 1 pair bred on the much larger but poorer foraging habitat at Big Is. No. 1.
Nests are constructed amongst patches of pebbles, shells, rocks and vegetation, most are often quite exposed. The sitting bird usually has a good view of its surrounds and in most cases is able to skulk away undetected at the approach of possible danger. The nest of the Sooty Oystercatcher is as variable as the nest site itself. In many instances it is no more than a scrape in the pebbles amongst low vegetation or on occasions underneath small shrubs. This scrape is made by the bird digging with it's bill as it turns it's body in a "top" like manner, excavating the loosened material with it's feet. In some cases the scrape is well lined with pebbles, shell fragments, vegetation or even small bones. Often more than one nest scrape is found in a breeding territory, the purpose of these additional scrapes is not known.
On the Five Islands two eggs form the usual clutch though one egg clutches are often encountered. Typical eggs weigh 58 grams and are 62.1 x 41.5mm. Predation rates of eggs is low, although Kelp Gulls (Larus dominicanus) and Australian Ravens (Corvus coronoides) have been observed taking Sooty Oystercatcher eggs. Both of these species also breed and regularly forage on Flinder’s Island.
The earliest recorded hatching date of Sooty Oystercatcher chicks on the Five Islands was 16th November 1993. Most eggs hatch during December and generally one of the chicks is lost to starvation / siblicide / act of God soon after hatching, usually within 6 or 7 days.
Newly hatched chicks are covered with dark sooty grey down peppered with black, four narrow black lines run the length of the body. The crown is also striped and peppered with black. Soft parts, namely the bill, legs and eye-ring are a mid-grey colour. The chicks are active within hours of hatching, and when not brooded by the parents will scramble around in close proximity of the nest site. Small chicks are brooded by the parents at or near the nest sight. Generally the chick is fed at a site which offers plenty of cover and allows them to hide quickly to avoid predators. This feeding station is often remote from the nest site.While the adults have a much more varied diet, chicks are primarily fed Limpets and Chitons. The feeding station soon becomes littered with discarded shells. To the trained eye of a predator/researcher, this site is obvious and fresh shells suggests the near presence of a chick. Oystercatcher chicks grow rapidly and at 2 weeks of age are covered in body feathers with down persisting along the back and on the head.At this age the chicks will accompany the adults along the rocky shore-line of their breeding territory. Here they are taught how to forage and fed by the female.
At the sounding of an alarm call given by the female/one of the parents, chicks will hide under rocks, often with water breaking over them, and will remain hidden until given the all clear by one of the parents/female. Chicks can swim quite well at this age and will also dive to escape danger, or a researcher!. They can swim well under water, and do so if pursued, using their wings in a manner similar to that of penguin’s flippers when underwater. Chicks disperse with adults from the islands at 7-8 weeks of age. Adult pairs and chicks move to their non-breeding season foraging territories on the mainland. The chicks remain with the parents for up to a further 8 months, before eventually being driven away.Generally ousted birds will join up with a nomadic flock of non-breeding (non-territory holders) and juvenile birds, which is referred to as the "Club" (Goss Custard 1996). They will remain in the Club until such time as they are able to secure a foraging and breeding territory of their own. Both of these are at a high premium ensuring that only the fittest and most experienced birds breed.
The contribution of the Five Islands Sooty Oystercatcher breeding population towards the total NSW population is as yet unknown. Further research at other breeding sites in NSW is required and planned for 1999/2000.The Five Islands group was originally thought to present the best opportunity for studies into the breeding biology of Sooty Oystercatchers in New South Wales. However the small size of Flinder’s Island and high concentration of breeding Oystercatchers results in high disturbance levels to breeding birds with every visit to the islands. It was therefore decided that further disturbance of intensive studies may effect the well being and breeding success of the birds. Mindful of this researchers have restricted their visits to the island in the past two years to one or two visits during the peak egg laying period in mid-October to late November. All chicks are banded with metal and colour bands on one or two visits in January and February.Any sightings of banded individuals can be reported to SOSSA headquarters and would be greatly appreciated.Additional studies examining breeding biology and chick growth rates of Sooty Oystercatchers have been conducted at this location by SOSSA members and are currently in preparation for scientific publication. Hopefully further studies can be conducted at additional sites which are less disruptive to the species, thus allowing us to gain a more enlightened understanding of the breeding biology and needs of this handsome and vulnerable species.
Battam, H. (1976). Seabird Is. 39,40, 41. The Australian Bird Bander March 1973.
Goss-Custard, John. D. (1996) The oystercatcher: from individuals to populations/edited by John D. Goss-Custard. (Oxford ornithology series; 7).
Lane, Brett A.(1987) Shorebirds in Australia. RAOU.
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