Forest thrush (Turdus lherminieri): request for information

BirdLife species factsheet for Forest thrush: http://datazone.birdlife.org/species/factsheet/forest-thrush-turdus-lherminieri

Forest thrush (Turdus lherminieri) is endemic to the Lesser Antilles, where it is uncommon on Montserrat (to U.K.), Dominica and Guadeloupe (to France), and rare (perhaps extirpated) on St Lucia.

The species underwent a significant population reduction over recent decades, particularly during 1995-1997, when the range on Montserrat was reduced by two-thirds by the effects of volcanic eruptions (G. Hilton in litt. 2000). On St Lucia, there has been just one recent record (at Des Chassin in 2007), but it was considered numerous in the late 19th century, indicating a serious long-term decline, if not extirpation (Keith 1997, B. Ibene in litt. 2014, L. John in litt. 2016). The magnitude of the species’s overall population reduction was inferred as 30-49% over the past three generations (13.2 years) and the species is therefore currently listed as Vulnerable under Criterion A2cde (see BirdLife International 2017).

However, more recent data has suggested that the species’s population may no longer be declining. Modelling of the results of surveys from 2000-2013 indicated that the population on Montserrat increased from 2000 to 2004 (perhaps following a decline caused by volcanic ash fall), then remained stable with moderate fluctuations from 2004 to 2016 (Parashuram 2013; S. Oppel in litt. 2016).

Monserrat only holds a small proportion of the species’s total range, and little is known about population trends on other islands. A recent study based on surveys and habitat mapping on Guadeloupe has suggested that previous population estimates have been substantial underestimates, with the Guadeloupe population estimated at 46,900 – 49,500 pairs (93,800 – 99,000 mature individuals; Eraud et al. 2012). However, the estimate was based on distance sampling which can be highly inaccurate in dense tropical forests given that most Forest Thrush detections are aural. On Dominica, the species occurs at low densities and has been observed in suitable habitat in the northern, western, central, south-eastern and southern regions (J. Arlington in litt. 2007).

If the evidence suggests that the species’s population size has not declined by at least 30% over the last three generations (13 years), the species may no longer qualify for listing as Vulnerable under Criterion A2.

In September 2017, the region inhabited by the Forest thrush was struck by Hurricane Maria, causing severe damage on Dominica and some damage on Guadeloupe. Dominica was reported to have been ‘stripped of vegetation’ (Phipps 2017). We do not currently have information on any impact of the hurricane on the populations of Forest thrush.

We are therefore requesting information on the current population size and trends of the Forest Thrush across its range since the late 1990s.

Our current information on the species’s conservation status will now be compared to all Red List Criteria.

Criterion A

The species underwent a significant population reduction on Montserrat in 1995 – 1997, when its range was reduced by two thirds due to a volcanic eruption (G. Hilton in litt. 2000). The population is also likely to have declined on St Lucia, where it may now be extirpated (Keith 1997, B. Ibene in litt. 2014, L. John in litt. 2016). However, in December 1999, the Montserrat population was estimated at 3,100 birds (Arendt et al. 1999), representing an increase of c.50% since December 1997, with further increases up until 2004, followed by a period of stability at least until 2016 (Parashuram 2013).

Little is known about population trends on other islands, and the population on Guadeloupe is now thought to comprise the majority of the total population (Eraud et al. 2012). The area of forest in the species’s range has been estimated to have declined by 1.5% between 2000 and 2012 (Tracewski et al. 2016).

Recent trends on Montserrat mean that the species is unlikely to qualify as Vulnerable under Criterion A2 based on a population reduction on this island alone. If, however, there is evidence that the species may have declined on other islands at such a magnitude that the species’s total population has reduced by at least 30% over the last three generations (13 years), then the species may still qualify for Vulnerable under Criterion A2.

Criterion B

The species’s extent of occurrence (EOO; based on a single minimum convex polygon fitted around all populations) is currently estimated at 12,000km2, and therefore falls beneath the threshold for listing the species as Vulnerable under Criterion B. For the species to qualify as Vulnerable, or a higher category of threat, under this criterion, two of three conditions must also be met. Turdus lherminieri is not considered to be severely fragmented, is currently assessed as having more than 10 locations and is not undergoing extreme fluctuations (according to IUCN definitions) and is not currently considered to be undergoing a continuing decline. Unless there is evidence that the species now has fewer than ten locations (according to the IUCN definition) and is undergoing a continuing decline in population, range or habitat quality, the species warrants listing as Least Concern under Criterion B.

Criterion C

Young (2008) estimated a population of 3,100 individuals on Montserrat, and Eraud et al. (2012) estimated 93,800 – 99,000 mature individuals on Guadeloupe alone. However, both estimates were based on distance sampling which can be highly inaccurate in dense tropical forests given that most Forest Thrush detections are aural. On Montserrat there were 1,174 individuals (95% credible interval 624-2,178) at the 88 point count stations monitored in the Centre Hills in 2016. These point count stations cover only 20-40% of the available forest habitat (Oppel et al. 2014), hence the population on Montserrat could be between 1,500 – 10,000 individuals. The sizes of the populations on Domenica and St Lucia are unknown, although the latter is assumed to be very small, if not zero. The total population is therefore placed in the band 100,000-499,999 mature individuals. The species therefore warrants listing as Least Concern under Criterion C.

Criterion D

The species has >1,000 mature individuals and does not qualify for listing as Vulnerable under Criterion D1. The area of forest within the species’s range was estimated at 1,191km2 in 2012, based on analysis of remote sensing (Tracewski et al. 2016) and the area of mapped range is currently 1,440km2 (1,107km2 without St Lucia). The species is therefore unlikely to qualify for listing as Vulnerable under Criterion D2, although it may be approaching the threshold. There are plausible future threats to the species (such as hurricanes), but none that are likely to drive the species to Critically Endangered or Extinct within a very short period of time (i.e. one or two generations [c.4.5-9 years]). The species therefore warrants listing as Least Concern under Criterion D.

Criterion E

To the best of our knowledge no quantitative analysis of the extinction risk of this species has been conducted. Therefore, it cannot be assessed against this criterion.

Thus to get a clearer assessment of the species’s status, information is requested on the population size and trends of the Forest Thrush across its range since the late 1990s.

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.

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.

 

References

Arendt, W. J., Gibbons, D. W. and Gray, G. (1999) Status of the volcanically threatened Montserrat Oriole Icterus oberi and other forest birds in Montserrat, West Indies. Bird Conservation International 9: 351-372.

BirdLife International (2017) Species factsheet: Turdus lherminieri. Downloaded from http://www.birdlife.org on 21/12/2017.

Eraud C., Magnin H., Van Laere G., Arnoux E. and Levesque A. (2012). Biologie des populations et statut de conservation des oiseaux endémiques des Antilles et Petites Antilles en Guadeloupe. Rapport d’étude ONCFS-Parc National Guadeloupe.

Keith, A. R. (1997) The birds of St Lucia, West Indies: an annotated check-list. British Ornithologists Union, Tring, UK.

Parashuram, D. (2013) Population trend and habitat associations of the forest thrush (Turdus lherminieri) on Montserrat, West Indies. MSc dissertation, University of East Anglia.

Phills, C. (2017, September 21) Hurricane Maria: Dominica ‘in daze’ after storm leaves island cut off from world. The Guardian. Retrieved from http://www.theguardian.com.

Tracewski, Ł.; Butchart, S. H. M.; Di Marco, M.; Ficetola, G. F.; Rondinini, C.; Symes, A.; Wheatley, H.; Beresford, A. E.; Buchanan, G. M. 2016. Toward quantification of the impact of 21st-century deforestation on the extinction risk of terrestrial vertebrates. Conservation Biology 30: 1070-1079.

Young, R.P. (ed.). 2008. A Biodiversity Assessment of the Centre Hills, Montserrat. Durrell Conservation Monograph No. 1. Durrell Wildlife Conservation Trust, Jersey, Channel Islands.

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2 Responses to Forest thrush (Turdus lherminieri): request for information

  1. I have been participating in and analysing the data from the annual forest bird monitoring on Montserrat since 2011, and I can confirm that on Montserrat the Forest Thrush is the third-most abundant species (after Pearly-eyed Thrasher and Bananaquit), which is commonly observed in all natural forest areas. Ongoing monitoring indicates a marginally positive trend (not significant), but definitely not a decline, and the estimates for 2017 indicate that >800 individuals occurred around the point count locations alone. There may well be twice as many in the Centre Hills forest overall, and probably another 200+ in the South Soufriere Hills forest, in the Volcanic Exclusion Zone (where no data exist). As long as the forest on Montserrat can be retained, and no new invasive species are introduced, the Forest Thrush appears to be reasonably secure on Montserrat.

    Data up to 2016 are described in Bambini et al. 2017:
    http://www.birdscaribbean.org/jco/index.php/jco/article/view/403

    Monitoring in April 2018 will show whether the hurricanes in 2017 have affected the population, but forest damage on Montserrat has been minimal.

    In 2016 I visited St Lucia and we did not hear or see (m)any Forest Thrushes there, so the status of the Forest Thrush on St Lucia is indeed extremely worrisome (if it still exists at all). We covered a reasonably large amount of forest, not exhaustively, but no Forest Thrush was recorded at any point count.

  2. Hannah Wheatley (BirdLife) says:

    The following comment was received as a personal communication from
    Lyndon John:

    Steffen is correct in his assessment of the St. Lucia forest thrush. It is very rare with very few sightings reported over the decades.

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