Tagula Butcherbird (Cracticus louisiadensis) is endemic to Tagula (= Sudest) Island, Junet (=Panatinani) Island, Panawina Island, and Sabara Island in the Louisiade Archipelago of Papua New Guinea (Coates 1990; Goulding et al. 2019a,b). It occurs in intact forest interior and adjoining forest edge, particularly in tall trees for pre-dawn singing during the breeding season. It is absent from heavily fragmented landscapes lacking contiguous forest cover, and absent near some large human populations and areas with a history of past disturbance, e.g. old coconut plantations on Tagula Island (Goulding et al. 2019).
Logging has degraded some of the lowland forest on Tagula (Beehler 1993). Forests on Junet and Panawina are under increasing pressure from subsistence agriculture due to growing human populations on Grass and Sabara Islands. Commercial gold prospecting has been occurring in forests of Sudest Island in recent years. In 2019, there were plans to commercially log forest on Sudest Island between Reuwo on the south coast and Rambuso Creek on the north coast. Associated roads have also been discussed, which would dissect the island between these two points and along the north coast to Tagula Station (W. Goulding in litt. 2020). The species avoids highly disturbed habitat and prefers forest with a relatively intact canopy (Goulding et al. 2019a). However, forest loss on Tagula has been relatively slow from the 1970s-2010s (Goulding et al. 2019b). Forest loss between 2000 and 2014, caused largely by subsistence gardening, was 1.7% on Tagula, 4.1% on Junet Island, 2.2% on Panawina Island and 0.3% on Sabara Island (Hansen et al. 2013).
Changing climate and more extreme weather events such as cyclones pose a threat to forest integrity and resources for this species (Goulding et al. 2016a). Around Araetha village on the north coast of Sudest Island, the number of breeding pairs dropped from five to three after Cyclone Ita (W. Goulding in litt. 2016). Individuals that were known by local people to have called from particular locations for numerous years have disappeared. Species in the genus Cracticus are also known to be vulnerable to disease and parasites (Peirce et al. 2005, Bennett and Gillett 2014, Goulding et al. 2016b). Avian Pox has been observed on Rossel Island, and may pose a risk to Tagula Butcherbird if it spreads (W. Goulding in litt. 2016). Although several invasive predatory species occur within the range of the Tagula Butcherbird, there is no evidence of predation.
Until recently, very little has been known about the species’s range, population size and threats, and so Tagula Butcherbird has been listed as Data Deficient. Following surveys across its range, more information on these subjects has now been published (Goulding et al. 2019a). Our current information on the species’s conservation status will now be compared to all Red List Criteria:
Criterion A – The generation length of Tagula Butcherbird has been estimated to be 5 years (Bird et al. 2020*). Hence, reductions are here quantified over a period of 15 years.
There are no direct data on population trends. Data on tree cover loss indicates a loss of 2.7% of tree cover with at least 75% canopy cover within the range from 2003-2018 (Global Forest Watch 2020). The species is known to prefer intact forest and is therefore suspected to have undergone a population reduction of 1-5% over the past three generations (15 years). In 2019, there were plans for a large logging operation on Tagula, which could affect up to 10% of the species’s range. The population is therefore suspected to undergo a reduction of 1-15% over the next three generations. There is no indication of a population reduction approaching 30% within the last ten years, or over the next decade. Tagula Butcherbird is assessed as Least Concern under this criterion.
Criterion B – Based on the area of a minimum convex polygon around the species’s range, the Extent of Occurrence (EOO) is estimated to be 1,960 km2. This meets the initial threshold for Endangered under Criterion B1.The Area of Occupancy (AOO) has not been quantified, but based on a 4 km2 grid placed over the area of mapped range, must be smaller than 1,344 km2. Given that given that forest with >75% canopy cover covers 88% of the range (Global Forest Watch 2020), the AOO is unlikely to fall beneath 500 km2. The AOO meets the threshold for Vulnerable under Criterion B2.To list the species as threatened under Criterion B, at least two further conditions must be met.
Tagula Butcherbird is not severely fragmented. “The term ‘location’ defines a geographically or ecologically distinct area in which a single threatening event can rapidly affect all individuals of the taxon present” (IUCN 2012). The species is threatened by forest loss and degradation, which have been progressing slowly, although there are plans for a large logging operation on Sudest Island (W. Goulding in litt. 2020). Nevertheless, based on the threat of forest degradation or clearance, there are likely to be at least ten locations. It is unclear how severe the impact of a future cyclone or drought may be on the species’s population. Following Cyclone Ita in 2014, the number of breeding pairs dropped from five to three in the surroundings of Araetha village on the northcoast of Sudest Island (W. Goulding in litt. 2016). If a cyclone hitting the Louisidade Archipelago could rapidly reduce the whole population, or a large proportion of the population, then the species may have just one or two locations. Tagula Butcherbird occurs in intact forest and is known to avoid areas without an intact canopy (Goulding et al. 2019a). However, cyclones are common to the region and the habitat remains relatively intact. Based on the threat of cyclones, the number of locations is probably more than ten, but it may not be much higher.
Ongoing logging is degrading the forest within the range, and data on tree cover loss indicates a loss of 2.7% of tree cover with at least 10% canopy cover within the range from 2003-2018 (Global Forest Watch 2020). The species required intact habitat with canopy cover, so it may be inferred to be undergoing a slow decline in population size, area and quality of habitat. Condition b is therefore met.
The EOO falls beneath the threshold for listing the species as Endangered and its AOO falls beneath the threshold for Vulnerable. The species meets condition b, but it is unclear whether it also meets condition a, and hence whether it qualifies as threatened under this criterion. It is therefore proposed to assess Tagula Butcherbird as Near Threatened, approaching a listing a threatened under Criterion B1ab(iii,v)+2ab(iii,v). However, if there is evidence that ten or fewer cyclones could eliminate or rapidly reduce the population size, the species may qualify as Vulnerable under this Criterion.
Criterion C – The population size has been estimated to be between 11,500-23,200 mature individuals, although it is noted that this may be a slight overestimate (Goulding et al. 2019a). This does not meet the threshold for listing the species as Vulnerable under this criterion, but it does approach it.
Data on tree cover loss indicates a loss of 2.7% of tree cover with at least 75% canopy cover within the range from 2003-2018 (Global Forest Watch 2020). Given that the species prefers intact forest and avoids regrowth areas lacking a canopy, the population size is inferred to be undergoing a slow continuing decline. Data does not suggest a rate of continuing decline approaching 10% in three generations, so the species is assessed as Least Concern under Criterion C1.
Tagula Butcherbird has been observed crossing water barriers of at least 500 m, so the species is suspected to have a high dispersal ability (Goulding et al. 2019a). However, vocal differences between individuals from Sabara and other islands suggest distinct subpopulations. The island with the largest population is Tagula, which was estimated to have a population of 10,000-21,500 mature individuals (Goulding et al. 2019a). Tagula Butcherbird does not qualify as threatened under Criterion C2a(i). The population on Tagula could represent up to 93% of the total population. This does not meet the thresholds for the % of mature individuals in one subpopulation under Criterion C1a(ii), although it is not far below. The species is therefore assessed as Near Threatened, approaching the threshold for listing as threatened under Criterion C2a(ii).
Criterion D – Based on the estimate described above, the population size does not meet or approach the threshold for Vulnerable under Criterion D. The species is therefore assessed as Least Concern under this criterion.
Criterion E – To the best of our knowledge no quantitative assessment of the probability of extinction has been conducted for this species, and so it cannot be assessed against this criterion.
Based on the above assessment, it is proposed to list Tagula Butcherbird (Cracticus louisiadensis) as Near Threatened, approaching the threshold for listing as threatened under Criteria B1ab(iii,v)+2ab(iii,v); C2a(ii). However, if there is evidence to suggest that there may be ten or fewer locations, the species could qualify as Vulnerable under Criteria B1ab(iii,v)+2ab(iii,v). Conversely, if evidence suggests that the population size is not declining, the species may be assessed as Least Concern. Comments on the proposed listing are welcomed. To allow us to achieve a clearer assessment of the species’s status, the following information is requested:
- What would be the potential impact of a cyclone on the species’s population size?
- Is the slow ongoing habitat loss/degradation likely to be impacting on the species’s population size?
Please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of the species’s Red List status. Therefore, please make sure your comments are about the proposed listing. By submitting a comment, you confirm that you agree to the Comment Policy.
*Bird generation lengths are estimated using the methodology of Bird et al. (2020), as applied to parameter values updated for use in each IUCN Red List for birds reassessment cycle. Values used for the current assessment are available on request. We encourage people to contact us with additional or improved values for the following parameters; adult survival (true survival accounting for dispersal derived from an apparently stable population); mean age at first breeding; and maximum longevity (i.e. the biological maximum, hence values from captive individuals are acceptable).
An information booklet on the Red List Categories and Criteria can be downloaded here and the Red List Criteria Summary Sheet can be downloaded here. Detailed guidance on IUCN Red List terms and definitions and the application of the Red List Categories and Criteria can be downloaded here.
Beehler, B. M. 1993. Biodiversity and conservation of the warm-blooded vertebrates of Papua New Guinea. In: Beehler, B.M. (ed.), Papua New Guinea – conservation needs assessment, pp. 77-121. Biodiversity Support Program, Washington, DC.
Bennett, M. D. and Gillett, A. 2014. Butcherbird polyomavirus isolated from a grey butcherbird (Cracticus torquatus) in Queensland, Australia. Veterinary microbiology 168(2-4): 302-311.
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