Island Thrush (Christmas Island) (Turdus poliocephalus erythropleurus)

Advice to the Minister for the Environment and Heritage from the Threatened Species Scientific Committee (TSSC) on Amendments to the list of Threatened Species under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act)
12 April 2005

1. Scientific name, common name (where appropriate), major taxon group

Turdus poliocephalus erythropleurus (Island Thrush (Christmas Island))

2. Description

The Island Thrush (Christmas Island) is a medium-sized bird that is brown-grey above and plain pale grey to whitish below with orange sides to the lower belly. It has a yellow bill and a distinctive song.

The Island Thrush lives in most habitats on Christmas Island, apart from very dense regrowth, post-mining clearings or Pandanus thickets. It has a varied diet of fruit and litter invertebrates and normally feeds on or near the ground.

3. National context

This subspecies of Island Thrush is restricted to Christmas Island. It is not listed as threatened under any State or Territory legislation, nor is it currently listed under the EPBC Act.

EPBC Act criteria.

TSSC judges the subspecies to be eligible for listing as endangered under the EPBC Act. The justification against the criteria is as follows

Criterion 1 - It has undergone, is suspected to have undergone or is likely to undergo in the immediate future a very severe, severe or substantial reduction in numbers.

The most serious threat to the Island Thrush (Christmas Island) is the spread of Yellow Crazy Ants (hereafter called Crazy Ants). The Crazy Ants are an aggressive invasive species that were accidentally introduced to Christmas Island between 1915 and 1934 and are now widespread throughout the rainforest and settled areas on the island. They can build up into colonies with extremely high numbers called supercolonies. Before recent abatement programs, Crazy Ant supercolonies had formed in 24.4% of island's rainforest.

Crazy Ants have both direct and indirect effects on this species. Direct attacks on injured Island Thrushes (Christmas Island) have been observed in supercolony areas, though it was unclear whether attack by Crazy Ants was the initial cause of injury, as healthy birds appeared to be able to dislodge the ants before serious injury occurred.

Although the presence of Crazy Ant supercolonies had no detectable impact on the abundance of adult Island Thrushes, there appeared to be a significant effect on juvenile Island Thrushes (Christmas Island) with an approximately 84% decrease in sightings in ant-invaded forest than uninvaded forest (Davis 2002). It is unknown why this reduction in abundance occurs. It may represent either displacement of juveniles or breeding to intact forest nearby, or reflect poor reproductive success in ant-invaded forest.

Of 33 observed nesting attempts by Island Thrushes (Christmas Island), none were successful in ant-invaded areas, while higher success ratios were observed in sites not affected by Crazy Ants (Davis 2002). While this trend was not statistically significant, it indicates a negative impact of Crazy Ants on reproductive success. The low numbers of juveniles and lack of breeding success in ant-infested areas strongly suggests that the breeding success of the Island Thrush (Christmas Island) is substantially reduced in ant-infested areas.

Some behaviours of the Island Thrush (Christmas Island) were altered in areas where ant supercolonies were present (Davis 2002). As Crazy Ants occur in the highest densities on the ground and low in the vegetation, the Island Thrush's normal habits of perching on low branches for extended periods and foraging on the ground would appear to make it susceptible to harassment or attack by Crazy Ants. As Thrushes also feed on litter invertebrates, it is possible that they may compete for food with Crazy Ants.

Crazy Ants also impact the Island Thrush (Christmas Island) indirectly through the removal of the keystone forest species, the Red Land Crab (Gecarcoidea natalis). In areas of forest where Crazy Ants have eliminated Red Crabs, it was found that the seeds that were previously consumed by the crabs had germinated, and the forest structure had altered, with dense layers of seedlings covering the forest floor. This makes foraging on the ground more difficult for the Island Thrush (Christmas Island) and may result in a possible long-term alteration of forest structure and composition.

Predation by Black Rats (Rattus rattus) is thought to have been responsible for extinction of thrushes on other islands. While rats do not commonly occur in the rainforest possibly due to the presence of the Red Land Crabs, reduction of the crabs may facilitate rainforest invasion by feral mammals, including cats and rats, and the introduced Wolf Snake (Lycodon capucinus) and may lead to an increased threat.

The impact caused by the mutualistic relationship between Crazy Ants and scale insects may also have an indirect effect on the Island Thrush (Christmas Island). Infestations of scale insects associated with Crazy Ants, have detrimental effects on the health of the canopy and may also lead to long-term decline in the habitat.

Although, an island-wide aerial baiting campaign in September 2002 has abated the threat of Crazy Ants resulting in a 98-100% reduction in ant activity within two weeks, the baiting program was based on control rather than eradication and new invasions and the reformation of a number of new, small supercolonies has been reported.

A decrease of 84% in sightings of juveniles and a decrease in nesting success of Island Thrush (Christmas Island) were observed in ant-invaded forest as compared with uninvaded forest (Davis 2002). There is also a possibility that Crazy Ants are competing directly for food and indirectly effecting foraging habits of Island Thrushes. As supercolonies covered 24.4% of the Island Thrush's (Christmas Island) habitat, the island's rainforest, prior to baiting, it could be inferred that the species has been negatively impacted. However, there are insufficient quantitative data detailing the rate of decline and it is unclear what the future rate of decline may be in response to the abatement of the threat. Therefore it is not eligible for listing under this criterion.

Criterion 2 - Its geographic distribution is precarious for the survival of the species and is very restricted, restricted or limited.

This subspecies of Island Thrush (Christmas Island) is endemic to Christmas Island. The species has a restricted geographic distribution occurring over the entire island, an area of approximately 135 km2. The restricted geographic distribution is considered precarious for the survival of the species as the Island Thrush (Christmas Island) occurs only on Christmas Island where its habitat is seriously threatened by the spread of Crazy Ant supercolonies. Crazy Ants have both direct and indirect effects on this species (see Criterion 1).

A decrease of 84% in sightings of juveniles and a decrease in nesting success of Island Thrush (Christmas Island) were observed in ant-invaded forest as compared with uninvaded forest (Davis 2002). There is also a possibility that Crazy Ants are competing directly for food and indirectly effecting foraging habits of Island Thrushes. As supercolonies covered 24.4% of the island's rainforest, prior to baiting, it could be inferred that the species has had a decline in quality of approximately 25% of its preferred habitat for foraging and nesting and has undergone some reduction in numbers and reproductive success as a result of the spread of the Crazy Ants.

There has been some abatement of the threat as a result of the baiting programs. However, although baiting appeared successful in the short term, its success in the longer term is not clear and the threat may increase yet again.

The future impact of the threat of Crazy Ants on Island Thrushes (Christmas Island) is also not clear. The longer term effects of the change in species composition and structure of the rainforest may result in more habitat becoming unsuitable or unavailable for the species.

The Island Thrush (Christmas Island) has a restricted geographic distribution which is considered to be precarious for the survival of the species as long as Crazy Ants are present on Christmas Island in supercolonies. Therefore the species is eligible for listing as endangered under this criterion.

Criterion 3 - The estimated total number of mature individuals is limited to a particular degree and: (a) evidence suggests that the number will continue to decline at a particular rate; or (b) the number is likely to continue to decline and its geographic distribution is precarious for its survival.

The total population of Island Thrush (Christmas Island) is estimated to be 4000 breeding birds (Garnett and Crowley 2000), however this number was extrapolated from the size of territories of other species of Thrush. A more recent analysis suggests up to 15,000 breeding birds based on an estimate of nest density for the Island Thrush on Christmas Island (Davis 2002). Therefore it is not eligible for listing under this criterion.

Criterion 4 - The estimated total number of mature individuals is extremely low, very low or low.

The Island Thrush (Christmas Island) has an estimated population of up to 15,000 breeding birds. The subspecies is not eligible for listing under this criterion.

Criterion 5 - Probability of extinction in the wild

There are no quantitative data available against this criterion. Therefore the subspecies is not eligible for listing under this criterion.

5. Conclusion

The Island Thrush (Christmas Island) is endemic to Christmas Island. The species has a restricted geographic distribution with an extent of occurrence of approximately 135 km². The estimated population is 15,000. There has been an observed decline in the habitat and reproductive success of this species and an inferred decline in the population due to the spread of Crazy Ant supercolonies. The direct threat of Crazy Ants has not yet been eliminated and the long term effects of the change in habitat are yet unknown. The species is therefore eligible for listing as endangered under criterion 2.

6. Recommendation

TSSC recommends that the list referred to in section 178 of the EPBC Act be amended by including in the list in the endangered category:

  • Turdus poliocephalus erythropleurus (Island Thrush (Christmas Island))

Conservation Advice

The Island Thrush (Christmas Island) is a medium-sized bird endemic to Christmas Island. It occupies most habitats on Christmas Island, apart from very dense regrowth, post-mining clearings or Pandanus thickets. The estimated population is up to 15000 birds. The key threat to the subspecies is the invasion of its habitat by Yellow Crazy Ant supercolonies. Christmas Island is part of the national NHT funding process and is not subject to a separate NRM regional plan.

The Christmas Island National Park Management Plan, the Action Plan for Invasive Ants on Christmas Island, and the planned Threat Abatement Plan (TAP) for Tramp Ants (including Crazy Ants), identify threat abatement actions needed to address the Crazy Ant invasion on Christmas Island.

The priority recovery and threat abatement actions required for this species are:

  • To continue effective and comprehensive Crazy Ant threat abatement actions

This list does not encompass all actions that may be of benefit to this species, but highlights those that are considered to be of the highest priority at the time of listing.

There is no recovery plan in place for the Island Thrush (Christmas Island).

Priority for the development of recovery plan: Low.

Publications used to assess the nomination

Davis, N. (2002) The Invasive Yellow Crazy Ant (Anoplolepis gracilipes) on Christmas Island, Indian Ocean: Impacts on the Frugivorous Bird Fauna. Honours thesis, Monash University.

Hannecart, F. & Letocart, Y. (1983) New Caledonian Birds

Stokes, T. (1988) A review of the birds of Christmas Island, Indian Ocean. Australian National Parks & Wildlife Service Occasional Paper 16.