BirdLife species factsheet for Malabar Grey Hornbill
Malabar Grey Hornbill is endemic to the Western Ghats of India. It inhabits wet evergreen and moist forests (Balasubramanian et al. 2004, Mudappa and Raman 2009), but may also be found in gardens or plantations of timber, shade coffee and cardamom adjacent to forests (Raman 2006, Sidhu et al. 2010). The species is threatened by the loss and degradation of its habitat (del Hoyo et al. 2001, T. R. S. Raman and D. Mudappa in litt. to the Hornbill Specialist Group 2020).
Along line transects in the Anamalai landscape in 2017-2018, population densities of 17.5 individuals/km2 (CI = 12.8-23.9 individuals/km2) were found inside protected areas, and densities of 11.7 individuals/km2 (CI = 7.1-19.3 individuals/km2) were found outside protected areas (P. Pawar, D. Mudappa and T. R. S. Raman in litt. to the Hornbill Specialist Group 2020). Based on an area of mapped range of c. 230,000 km2, and precautionarily assuming that about 10% of the range is occupied, the population is estimated at c. 269,000-400,000 individuals. Assuming further that 2/3 of the population consists of mature individuals, the population size would fall in the band 180,000-260,000 mature individuals, though this requires confirmation.
Previously, Malabar Grey Hornbill has not been considered at risk of extinction as it has a large range within which high densities were often recorded (Mudappa and Raman 2009). The species has consequently been listed as Least Concern (BirdLife International 2020). The IUCN SSC Hornbill Specialist Group has now raised concern over the species’s rate of population reduction, after two lines of evidence suggest a moderate to rapid decline has occurred over the past two decades. Given this new information, the species is here re-assessed against all Red List Criteria:
Criterion A – The species has a generation length estimated at 5.5 years (Bird et al. 2020)*, hence 16.5 years is the appropriate period over which to consider population reduction.
In 2017-2018, repeats of line transects in the Anamalai landscape previously surveyed during 2004-2005 have resulted in population densities 38.6% lower within protected areas, and 55% (CI 42.2 – 61.8%) lower in rainforest fragments (Mudappa and Raman 2009, P. Pawar, D. Mudappa and T. R. S. Raman in litt. to the Hornbill Specialist Group 2020).
The recent publication of the State of India’s Birds report (SoIB 2020) has given a second line of evidence to suspect that there are considerable declines occurring. For both the reported ‘long-term’ trend and ‘current’ trend a decline is suggested from the incidence of reporting the species in complete checklists within the citizen science platform eBird (eBird 2020). ‘Long term’ refers to a comparison of pre-2000 data with recent reporting rates, hence all of this data was entered considerably after the observations were made. An important assumption here is that pre-2000 records are entered in an equivalent fashion to current records: if older records are biased towards including scarcer species, then the data will demonstrate a pseudo decline in reporting. Equally the start and end date to use to scale such a decline to a three-generation period are difficult to define, as records many years prior to 2000 may contribute to the initial prevalence value. But the ‘current’ trend should suffer less from this issue, as this is derived from data entered between 2014-2019.
With these caveats, the long-term trend reported is a 66.8% decrease (CI 43.6 – 89.9%), but scaling this to the appropriate three-generation period does not appear possible. The current trend is given as a 3.3% annual decline, but with very large confidence intervals (±10.7%). Hence the predicted trend is uncertain, equivalent to an estimated three generation trend of between a 92% decline and a 225% increase. So while the State of India’s Birds data does indicate a population decline, the uncertainty surrounding the trend estimate makes the assessment against Criterion A difficult. Tentatively, the rate of decline is here placed in the band 30-49% over three generations, which is in line with the comparison of the transect data collected in 2004-2005 and again in 2017-2018 in the Anamalai landscape. Malabar Grey Hornbill would therefore qualify for listing as Vulnerable under Criterion A2bc+3bc+4bc.
Criterion B – The Extent of Occurrence (EOO) for this species is 271,000 km2. This is too large to warrant a listing as threatened under Criterion B1, and thus Malabar Grey Hornbill is assessed as Least Concern under this criterion. The Area of Occupancy (AOO) has not been quantified according to IUCN Guidelines (IUCN Standards and Petitions Committee 2019), and thus the species cannot be assessed against Criterion B2.
Criterion C – The population is tentatively estimated at 180,000-260,000 mature individuals. This is too large to qualify as threatened under Criterion C, and as such the species is assessed as Least Concern under this criterion.
Criterion D – The population size and range are too large to warrant a listing as threatened under Criterion D, and thus the species is considered Least Concern under this criterion.
Criterion E – To the best of our knowledge no quantitative analysis of extinction risk has been conducted for this species. Therefore, it cannot be assessed against this criterion.
Therefore, it is proposed that Malabar Grey Hornbill (Ocyceros griseus)be listed as Vulnerable under Criterion A2bc+3bc+4bc. We welcome any comments on this proposed listing and specifically request up-to-date information regarding the population trend. Based on records on eBird, the species seems to have deteriorated in status since c. 2004. We assume that the species has been stable until about 2004, when it started to undergo population declines. We further assume that declines approached the threshold for listing as threatened during 2008-2012, qualifying the species for listing as NT, and gradually accelerated, so that the species met the threshold for listing as VU during 2016-2020.
Please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of its Red List status. Therefore, please make sure your comments are relevant to the discussion outlined in the topic. 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.
Balasubramanian, P.; Vijayan, V. S.; Prasad, S. N.; Ravi, R.; Krishnakumar, N. 2004. Status and distribution of the hornbills in the Western Ghats. Project Report. Salim Ali Centre for Ornithology and Natural History, Coimbatore, India.
Bird, J. P.; Martin, R.; Akçakaya, H. R.; Gilroy, J.; Burfield, I. J.; Garnett, S.; Symes, A.; Taylor, J.; Şekercioğlu, Ç. H.; Butchart, S. H. 2020. Generation lengths of the world’s birds and their implications for extinction risk. Conservation Biology online first view.
BirdLife International. 2020. Species factsheet: Ocyceros griseus. http://www.birdlife.org (Accessed 20 May 2020).
del Hoyo, J.; Elliott, A.; Sargatal, J. 2001. Handbook of the Birds of the World, vol. 6: Mousebirds to Hornbills. Lynx Edicions, Barcelona, Spain.
eBird. 2020. eBird: An online database of bird distribution and abundance [web application]. eBird, Ithaca, New York. http://www.ebird.org (Accessed 20 May 2020).
IUCN Standards and Petitions Committee. 2019. Guidelines for using the IUCN Red List Categories and Criteria. Version 14. http://www.iucnredlist.org/documents/RedListGuidelines.pdf.
Mudappa, D.; Raman, T. R. S. 2009. A conservation status survey of hornbills (Bucerotidae) in the Western Ghats, India. Indian Birds 5: 90–102.
Raman, T. R. S. 2006. Effects of habitat structure and adjacent habitats on birds in tropical rainforest fragments and shaded plantations in the Western Ghats, India. Biodiversity and Conservation 15: 1577–1607.
Sidhu, S.; Raman, T. R. S.; Goodale, E. 2010. Effects of plantations and home-gardens on tropical forest bird communities and mixed-species bird flocks in the southern Western Ghats. Journal of the Bombay Natural History Society 107: 91–108.
SoIB. 2020. State of India’s Birds, 2020: Range, trends and conservation status. The SoIB Partnership.