This EBA covers the islands of Mitiaro, Atiu, Mauke, Rarotonga and Mangaia. Aitutaki, one of the northernmost islands of the Southern Cook Islands group, is treated separately (Secondary Area s135) as there are no reliable records that restricted-range species have historically been shared between it and the islands of the EBA. The Cook Islands are self-governing but, since 1965, have been in free association with New Zealand.

Rarotonga is the most populated, the largest (67 km²) and the highest island, reaching 652 m at Te Manga; it supports secondary rain forest at lower elevations with almost pristine montane forest above. The other islands consist of much-weathered volcanic centres covered with fernlands and disturbed forests, surrounded by a raised coral limestone platform (the 'makatea') covered with both undisturbed and disturbed forest and scrub. Freshwater marshes and swamps are present on all islands.

Most of the restricted-range species are typically confined to native forest (disturbed and undisturbed) in inland Rarotonga or to the makatea forest of the raised atolls.

The pattern of distribution between the islands is very patchy, but it seems likely that at least some of the species were more widespread in the group prior to 1800, but were extirpated as a result of the activities of Polynesian settlers (see, e.g., Kirch et al. 1992). Kuhl's Lorikeet Vini kuhlii, a restricted-range species from Rimatara in French Polynesia (EBA 211), is also likely to have been widespread in this EBA, but was extirpated through exploitation for its red feathers (Steadman 1991, McCormack and Künzle 1993).

Today four species are single-island endemics, being confined to Atiu (one species), Rarotonga (two) and Mangaia (one).

The ranges and densities of all restricted-range species are likely to have been affected by the clearance of forest by man and its degradation through browsing by introduced herbivores-and by the resulting fragmentation of native primary habitat. The four single-island endemics, which have tiny world ranges and small populations, are therefore considered threatened.

Pomarea dimidiata (Rarotonga only) is classified as Critical and was close to extinction in the early 1980s. A remnant population, of a bird recorded as widespread and common on the island in the mid-1800s, survived in 1.5 km² of native forest in the south-east, in the lower foothills and steep V-shaped valleys at 100-250 m. However, suitable habitat was available elsewhere and introduced rats (especially black rat Rattus rattus introduced by Europeans) were identified as the main obstacle to successful nesting; subsequent rat control has resulted in an increasing population, starting with a low of 29 birds in 1989, 56 in 1992 and increasing to over 100 in 1995. In August 1997 the population stood at a minimum of 144 birds and, as the numbers have been above 50 birds (all mature birds capable of breeding) for nearly five years, the species is a likely candidate for the down-grading of its threat status—a fitting reflection of the success of this conservation initiative. The site where the species survives—Takitumu Conservation Area—will be managed by the three land-owning families for sustainable development (ecotourism), and the feasibility of transferring the species to an island free of black rats elsewhere in the southern Cook Islands will be assessed (McCormack and Künzle 1990, Robertson et al. 1994, Robertson 1995c, G. McCormack in litt. 1996, H. Robertson in litt. 1996).

It is likely that rats affect other species too and may have contributed to the extinction of Aplonis mavornata. This species is based on a single specimen previously of unknown origin (hence its English name), but it has been linked to a description of a -Sturnus- collected from Mauke in 1825 (the type was then subsequently apparently lost). Observations at the time of the collection of 'quantities of rats', perceived to be similar to brown rat Rattus norvegicus, are thus of considerable interest (Olson 1986).

Common Myna Acridotheres tristis, originally introduced to the Cook Islands at the beginning of the twentieth century to control insects, may be detrimental to Todirhamphus ruficollaris (Mangaia only), estimated to have a population of 250–450 birds in 1992, as interference with nesting has been observed and there is competition between these two bird species for food; cats and rodents (black rat is present) are also potential predators (Rowe and Empson 1996).

Threats leading to rarity in the other two threatened endemic species are not so apparent. Aplonis cinerascens (Rarotonga only) was regarded as abundant early in the twentieth century but is now estimated to number only a few hundred birds, though no reasons for the decline are known. For Collocalia sawtelli (Atiu only, where black rat appears to be absent), the major (natural) causes of mortality have been identified as starvation of chicks after falling out of the nest and predation by crabs, although disturbance by tourists could be a problem in the future; a survey in 1995–1996 recorded 175 active nests in the only two caves where it breeds (Tarburton 1990, G. McCormack in litt. 1993, 1994, 1996).

Bristle-thighed Curlew Numenius tahitiensis, a threatened (Vulnerable) restricted-range species which breeds in Western Alaska (Secondary Area s002), winters among Pacific Islands including the islands of this EBA.

Recommended citation
BirdLife International (2024) Endemic Bird Area factsheet: Southern Cook Islands. Downloaded from https://datazone.birdlife.org/eba/factsheet/216 on 23/11/2024.