Malabar Grey Hornbill (Ocyceros griseus): revise global status?

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.

References

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.

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5 Responses to Malabar Grey Hornbill (Ocyceros griseus): revise global status?

  1. The estimate of total population along the Western Ghats is likely a significant overestimate for the following reasons. It appears to have been estimated based on the assumptions that the bird occurs in the 23,000 km2 occupied range (i.e., 10% of total range) at population densities as estimated in the Anamalai Hills. However, the Anamalai Hills is one of the areas with the highest known densities of the bird (as indicated in Mudappa and Raman 2009). In the Anshi-Goa landscape, further north, the bird occurs at a density of 9.4 / sq km (95% CI= 6.1 – 14.4 / sq. km), but this estimate is also mostly from within protected reserves (Mudappa and Raman 2009). While density estimates are not available from other areas, encounter rates are lower suggesting that densities will also be lower in much of the remaining parts of the species’ range. Hence, the estimated total population reported here (c. 269,000-400,000 individuals) and mature individuals population (180,000-260,000 mature individuals) are both likely to be significant overestimates. This affects the interpretation under Criterion C.

    Similarly, given that occurrence outside protected reserves may be lower, the EOO and AOO may also be overestimated and may need to be properly estimated and revised. This affects interpretation under Criterion B as well.

    Criterion D may also need a re-look given the above.

  2. Praveen J says:

    I agree with the concern that the total population may have been over-estimated. My only callout would be verify whether it really matters for this specific purpose of redlisting.

    What I understand from the summary

    Area of mapped range = 230,000 km2

    This is neither EOO nor AOO.
    However, an MCP over the mapped range would give an EOO. Hence, mapped range is always lower than EOO.

    I used the below program to calculate EOO (MCP) and AOO (at a square size of 1/16th of a degree) based on eBird records.
    https://paintedstork.shinyapps.io/mcpcal/

    EOO: 203,064 sq.km (11% less than BLI’s AOH)
    AOO: 43,316 sq.km

    These values are much higher (~10 times) than the EOO and AOO thresholds that matter for IUCN (EOO: 20,000, AOO: 2,000) to be considered for any threatened category under Criteria B. Hence, this Criteria would not apply for this species.

    If we were to take a precautionary 10% on the EOO, we may get a 11% lesser value for the population. It is arguable that 10% is the right precautionary measure while using EOO (vs mapped range). However, will it ever approach the threshold of Vulnerable for Criteria C (10,000 mature individuals)?
    Even if we take 1% as the right precautionary measure (considering variations in density estimates), 18,000 mature individuals is still higher than the Vulnerable thresholds. Hence, in my opinion – Criteria C may not apply for this species even after considering very stringent approximations.

    Criteria D has an even more stronger limit for threatened status (1,000 mature individuals) and hence out of contention.

    Hence, in my opinion – Criteria A is the only one that matters for this species.
    More on it in the next post.

  3. Praveen J says:

    We are writing to bring in whatever information can be added from the recently published State of India’s Birds (SoIB) 2020 report (www.stateofindiasbirds.in), in which birdwatchers’ lists uploaded to eBird were used to estimate long-term (c.25y) and current (5y) trends in an index of abundance (the frequency of reporting), and to estimate range size (Area of Occupancy: AOO) for a large number of species. For an endemic species, all information in the report and associated data is relevant. We agree that the short term trends in this case has a wide variance and cannot be used directly. However, the long-term trends can still be used effectively to assess the declines and some of the statements in the summary note are clarified.

    Statement 1: “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.”

    We agree with this.
    1. Pre-2000 is an open ended period, which makes IUCN trend calculations difficult though we can argue that the median is 25 years. It creates an uncertainty in the declines that might need more justification. We decided to ignore this particular open-ended period.
    2. The remaining years (2000-2018) may need extrapolation into the past for three generations for some species. However, it may not be justifiable to extrapolate into the past a decline that occurred in the last 18 years. Hence, all analysis on the long-term data was done without extrapolations.
    3. Some of the years are pooled in SoIB dataset – specifically 2000-2006, 2007-2010 and 2011-2012. In all these pooled years, we took the widest band to calculate the trend. In other words, instead of selecting the median year of 2000-2006, we used 2000 itself as the start year. If there is a decline when using the beginning year, then the decline would only by worse if any year in that band of pooled years is selected (unless there are extreme fluctuations). Hence, this a safe lower bound with highest confidence.
    4. Using these years, we created 10 decline intervals, and we used these decline intervals for red listing – with three generation for a species as the upper bound. If there is a decline in lesser number of years, then the decline would atleast be as much in the longer 3 generation interval. (Except cases with extreme fluctuations).

    The main intent of this analysis is to remove of any uncertainty in the species decline values in SoIB which were done initially for a different purpose (three concern categories) but now is being refurbished for the red list assessment.

    Malabar Grey Hornbill: Generation length 5.0-5.5y (as in the summary note, BLI factsheets have a different value) and hence, only the trends falling in similar 3-GEN values are provided. We provide the most optimistic estimate (upper bound of the 95% confidence interval) and the best estimate (mean)

    2000-2016: -30.92% (Optimistic). Best: -59.25%

    Hence, with abundant caution – SoIB data indicate that the species has declined by at least 30% in a 3 GEN period, and most likely by over 50%.

    Statement 2: “if older records are biased towards including scarcer species, then the data will demonstrate a pseudo decline in reporting.”

    This may also be a valid concern for some species. We have no mechanisms to estimate for this bias. However, Malabar Grey Hornbill is a wide-spread species which was not pursued-after by bird-watchers, historically (or even now). It is one of the first endemics that will be ticked by even visiting bird-watchers. Hence, we do not see this perceived bias influencing the SoIB long-term trend estimates.

    In summary, SoIB dataset do provide a 3 GEN decline that would breach the thresholds of Vulnerable and all available auxiliary evidence on likely estimates and potential bias indicate that this decline is at least 30%, and most likely over 50%. Hence, this species should be uplisted to Vulnerable under Criterion A2b

    Suhel Quader, Praveen J and Ashwin Viswanathan

    For the SoIB partnership:
    Ashoka Trust for Research in Ecology and Environment
    Bombay Natural History Society
    Foundation for Ecological Security
    National Biodiversity Authority
    National Centre for Biological Sciences
    Nature Conservation Foundation
    Salim Ali Centre for Ornithology and Natural History
    Wetlands International South Asia
    Wildlife Institute of India
    Worldwide Fund for Nature—India

  4. Red List Team (BirdLife International) says:

    Many thanks to everyone who has contributed to this discussion. We greatly appreciate the time and effort invested by so many people in commenting. The window for consultation is now closed. We will analyse and interpret the new information and post a preliminary decision on this species’s Red List status on this page in early July.

    Thank you once again,
    BirdLife Red List Team

  5. Red List Team (BirdLife International) says:

    Preliminary proposal

    Based on available information, our preliminary proposal for the 2020 Red List would be to adopt the proposed classifications outlined in the initial forum discussion.

    There is now a period for further comments until the final deadline in mid-July, after which the recommended categorisations will be put forward to IUCN.

    Please note that we will then only post final recommended categorisations on forum discussions where these differ from the initial proposal.

    The final 2020 Red List categories will be published on the BirdLife and IUCN websites in December 2020/January 2021 (information on the IUCN Red List update process can be found here), following further checking of information relevant to the assessments by both BirdLife and IUCN.

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