Ecology of Sulawesi. Tony Whitten
Читать онлайн книгу.
(Watling 1983). The bill is pale green and red at the base. Maleo are primarily inhabitants of forest, but only lay eggs where the ground is sufficiently hot for incubation—that is, near hot springs (Wiriosoepartho 1979), near volcanic vents, or on sandy beaches. The megapodes and the Egyptian plover Pluvianus aegyptius are the only living birds which do not use the heat of their own bodies for incubation.
Pairs arrive at a nesting area the night before eggs are laid. The following morning, amid much duck-like quacking and turkey-like gobbling, the birds examine holes and make trial digs. When a suitable spot is found, both male and female start digging, throwing earth or sand behind them using their strong legs and claws. The toes are slightly webbed at the base which must help when scratching away loose sand (Wallace 1869). As the hole becomes deeper, so the birds take it in turns to dig and drive away other maleos that venture too close.
This digging can take over three hours, particularly where the sand is loose, after which the female lays her enormous egg, 11 cm long and 240-270 g in weight15 (Guillemard 1889), in the bottom of the pit. Subsequent eggs are laid at approximately 10-day intervals. The refilling takes nearly as long as the digging and is lengthened by the digging of false pits near the real one to divert predators such as monitor lizards and pigs. Against humans who value maleo eggs as a delicacy, however, these precautions are of little use. During the nesting period the maleos seek food such as figs, and fruit of Macaranga (Euph.) and Dracontomelum (Anac.) in the beach forest and roost primarily in Casuarina (Casu.) trees (Wiriosoepartho 1980).
Maleos are communal nesters and on the largest known site at Bakiriang, on the south coast of the north-east peninsula, more than 600 birds nest early in the year with the holes only two or three metres apart. Two hundred of the birds nest on just 1 ha of sand (Wading 1983).
The surface of a sandy beach can become extremely hot, over 50°C and 80°C on white and black sand respectively (MacKinnon 1978), yet just a few centimetres below the surface, the temperature is relatively stable at about 36°C. It seems as if most eggs, on beaches or elsewhere, are laid in positions where the temperature is between 32°C and 38°C (MacKinnon 1978; Wiriosoepartho 1980). The depth of the hole might be thought to be critical, and it has been suggested that the bare head of the maleo is efficient at sensing temperature but, in reality, the exact depth and temperature (within certain limits) are not so critical. Instead it seems that the eggs are laid as deep as possible for protection against predators.
Hatching takes about three months and if the chicks survive the one-or two-day scramble to the surface, for ants are a major predator of chicks in the ground (R. Dekker pers. comm.), they are able to fly away immediately, already having adult plumage. The manner in which they 'explode' from the sand and rush away is probably an adaptation to avoid the attention of predators (Watling 1983). The great size of the egg is related to the need to produce a chick strong enough to struggle up to the surface (Guillemard 1889).
The size of the egg makes it an attractive source of food for humans and maleo nesting beaches have probably been exploited since man first arrived on Sulawesi. Unfortunately, however, over-exploitation has been a common phenomenon: for example, the beach at the Batuputih16 just north of the present Tangkoko-Batuangus Reserve, where Alfred Wallace (p. 59) watched maleo nesting in 1859, was at one time visited by egg collectors in an apparently more or less sustainable manner, but within six years of a settlement being established at Batuputih in 1913, maleos no longer visited the beach (MacKinnon 1978). In 1947 about 10,000 eggs were laid in 2 ha of the Panua Reserve on the coast near Marisa, Gorontalo (fig. 2.34) (Uno 1949), but the present total is less than 10% of this (Anon. 1982a). The total number of breeding hens is between 25% and 67% of the total 40 years ago (Wiriosoepartho 1980).
Figure 2.34. Panua Nature Reserve between Gorontalo and Marisa, showing the maleo nesting site, the large expanse of Rhizophora forest, coastal forest and the two small lakes.
After Wiriosoepartho 1980
Figure 2.35. Differential use of habitats by maleo at Panua to demonstrate the importance of lowland forest to beach-nesting maleo.
Based on Wiriosoepartho 1980
The largest site, at Bakiriang, is only a few kilometres away from a transmigration site. The lowland forest the birds depend on behind the beach is being felled and unless this is protected the demise of this population seems almost certain (fig. 2.35). The Bakiriang site is so special that, until 50 years ago, the raja of Banggai, on Peleng Island 100 km away, determined who should collect the eggs and he received a revenue from the eggs collected. The first 100 eggs were sent to the raja and only after he had approved these could they be consumed by local people. Although the Banggai rajas were notorious pirates and unacceptable in many ways, they were among Indonesia's first resource managers. Now, however, eggs are taken despite legal prohibition and they can be found, wrapped in individual palm-leaf baskets, in the markets of Ujung Pandang and even Jakarta.
Experiments by the head of the Gorontalo Forest Service in the mid-70s showed that maleo eggs could be collected and reburied in a cage so that predation was avoided, and then hatched with significant rates of success (MacKinnon 1978). This was tried again in the Tangkoko-Batuangus Reserve and a hatching rate of 78% was achieved. This technique, together with the control of pig and lizard predators and the clearing of undergrowth to increase the area with a suitably high soil temperature, could make a significant contribution to increasing maleo populations affected by overexploitation where forest areas are sufficient (MacKinnon 1981).
Work is currently being conducted in Bogani Nani Wartabone National Park on the management of an inland population of maleo birds and results are awaited with interest.
Seabirds
The coasts and islands of Sulawesi are visited by at least a dozen species of seabirds which, unlike waders, spend months or even years at sea without returning to land. They tend to nest in large colonies, often on small islands, which are extremely sensitive to disturbance (fig. 2.36; table 2.7). Some islands, like the precipitous Batu Kapal off the northeast coast of Lembeh Island, appear to be roosting rather than nesting sites (Hickson 1889). It is likely that seabirds once nested on or near beaches on the mainland and that human disturbance is the cause of the nesting pattern seen today. Indeed, the seabird populations of Indonesia are experiencing a serious decline in numbers (de Korte 1984).
The habit of nesting in large colonies is disadvantageous because disturbance can have so serious an effect, but it has evolved for at least three important reasons: for ease of pair formation (most seabirds are solitary or live in small groups and range over vast distances when they are not breeding), for defence against predators (there is less risk to one individual of becoming the prey), and for the information shared concerning locations of the abundant food necessary for feeding young birds (Nelson 1980). Fishermen well know the value of these birds since they help to locate schools of tuna and other fish. Indeed, along the north coast of North Sulawesi frigatebirds enjoy a traditional protection because of the service they give (Polunin 1983). Since seabirds are top predators,17 they tend to concentrate some pollutants, the effects of which only become obvious when the levels exceed a certain limit and reproduction is disrupted. Monitoring programs of pollution levels in seabird tissues can thus be extremely valuable in assessing levels of marine pollution.
Seabirds that visit Sulawesi are boobies (Sulidae), frigatebirds (Fregatidae), and terns and noddies (Laridae) (fig. 2.37). Terns can take off easily and consequently can nest directly on the ground. The red-footed booby and frigatebirds, on the other hand, are masters of soaring flight on their long wings and so they need to nest a few metres above the ground in order to take off successfully.
The nutrients which seabirds contribute to the islands on which they nest can be quite considerable. For example, colonies of the white-capped noddy Arums minutus, known in Sulawesi only from the small Sangisangian Island north of Kalaotoa Island in the Flores Sea, deposit in their faeces an average of about 2g dry matter/m2/day. This is equivalent to 1,030, 220, 140 and 50 kg/ha/yr of nitrogen, phosphorus, potassium and