Archived 2020 topic: Great Blue Heron (Ardea herodias) is being split: assessment of newly recognised taxa

BirdLife species factsheet for Great Blue Heron

Following a taxonomic reassessment, Great Blue Heron (Ardea herodias) has been split into Great Blue Heron (A. herodias) and Great White Heron (A. occidentalis) due to morphological, genetic and behavioural differentiation (see McGuire et al. 2019). The newly-split Great Blue Heron is widespread across North and Central America, as well as the Caribbean. Great White Heron partly overlaps with Great Blue Heron in what has been described as a ‘secondary contact zone’ (McGuire et al. 2019): Great White Heron occurs from southern Florida (Florida Bay and southern Biscayne Bay) and the Florida Keys through Cuba (McGuire et al. 2019, J. Kushlan in litt. 2020). There may be additional breeding populations on islands off the coast of Venezuela, the US and British Virgin Islands, in coastal Yucatán (Mexico) and possibly on other Caribbean islands (McGuire et al. 2019, J. Kushlan in litt. 2020, Vennesland and Butler 2020). It is however unclear whether these are indeed Great White Herons or whether they are breeding individuals or merely vagrant (J. Kushlan in litt. 2020). The largest part of the population is breeding in Florida (J. Kushlan in litt. 2020).

The population size of Great Blue Heron is estimated at 500,000-4,999,999 mature individuals (Partners in Flight 2019). The population of Great White Heron appears to be much smaller. In southern Florida, surveys detected 175 nests, equating to 350 mature individuals, in 2018 (per Cook and Baranski 2019, J. Kushlan in litt. 2020). Up to 200 nests, equating to 400 mature individuals, are reported from the Florida Keys, and a further 30-50 nests, equating to 60-100 mature individuals, from the southern Biscayne Bay (Meyer et al. 2018, J. Kushlan in litt. 2020). The population in Cuba has not been quantified, but is reported to be small, with the majority of the population breeding in Florida (J. Kushlan in litt. 2020). The global population of Great White Heron is therefore tenetatively placed in the band 1,000-2,499 mature individuals, though the true population size may be closer to the lower end of the estimate.

Both species are wading birds and inhabit coastal marine and freshwater wetlands (Vennesland and Butler 2020). Threats to them include loss and degradation of habitat through wetland drainage, infrastructural developments and agricultural expansion, as well as decreasing food supply due to the depletion of fish stocks (Powell 1983, McGuire et al. 2019, J. Kushlan in litt. 2020, Vennesland and Butler 2020). The newly-split Great White Heron is further threatened by climate change impacts like increased hurricane frequency and storm surges, and by introduced predators like the Burmese Python (Python bivittatus) in Florida (McGuire et al. 2019). While Great Blue Heron shows a high level of resilience to these threats with increasing population trends, Great White Heron appears to be in steep decline. 

The pre-split species was listed as Least Concern (BirdLife International 2020). However, following the taxonomic split, new estimates of the population trends suggest that both species warrant a thorough reassessment, which is provided below.

Criterion A:

Great Blue Heron (A. herodias)– Although trends differ between populations, the species overall appears to be increasing slowly (Partners in Flight 2019, Wetlands International 2020). Trends have been quantified across North America, where the population has been growing at >10% in total since the 1960s (Sauer et al. 2017, Meehan et al. 2018). Despite some uncertainty surrounding the quantification of the increase rate (see Vennesland and Butler 2020), the species is assessed as Least Concern under Criterion A.

Great White Heron (A. occidentalis) The species appears to be undergoing a steep decline, but the rate of decline is difficult to quantify as there are no range-wide estimates. Locally, the species may have stable trends (e.g. in the lower Florida Keys; Meyer et al. 2018); however, this is not thought to compensate for the overall decline in the global population. Comparing historical count data from Florida (>800 nests, equating to >1,600 mature individuals in the early 1980s; per Meyer et al. 2018 and J. Kushlan in litt. 2020) to current high counts of c. 300 nests in 2020 (per J. Kushlan in litt. 2020) indicates that the species is declining at a rate of c. 45% the last three generations (24 years; Bird et al. 2020)*. We have no information on the rate of population change in Cuba. Tentatively, we can assume that the species is undergoing similar declines elsewhere in its range and that declines are continuing at the same rate into the future, though these assumptions require confirmation. The overall rate of decline is thus preliminarily estimated at 30-49%. Great White Heron therefore qualifies for listing as Vulnerable under Criterion A2abce+3bce+4abce.

Criterion B:

Great Blue Heron (A. herodias) – The Extent of Occurrence (EOO) of Great Blue Heron has been calculated as 34,200,000 km2. This is too large to warrant listing as threatened under Criterion B, and the species may be considered Least Concern under this criterion. The Area of Occupancy (AOO) has not been quantified following IUCN Guidelines (IUCN Standards and Petitions Committee 2019), and therefore the species cannot be assessed against Criterion B2.

Great White Heron (A. occidentalis) – The EOO of Great White Heron has been calculated as 263,000 km2. This is too large to warrant listing as threatened under Criterion B, and the species may be considered Least Concern under this criterion.  The Area of Occupancy (AOO) has not been quantified following IUCN Guidelines (IUCN Standards and Petitions Committee 2019), and therefore the species cannot be assessed against Criterion B2.

Criterion C:

Great Blue Heron (A. herodias) – The global population is thought to number 500,000-4,999,999 mature individuals (Partners in Flight 2019). This is too large to meet the threshold for listing as threatened under Criterion C, and the species is thus considered Least Concern under this criterion.

Great White Heron (A. occidentalis) – The population size has preliminarily been placed in the band 1,000-2,499 mature individuals. This number meets the initial threshold for listing as Endangered under Criterion C. However, to do so, the species must meet further conditions. Great White Heron is estimated to decline at 30-49% over three generations (24 years). This equates to a decline of 21-36% over two generations (16 years) and 11-20% over one generation (8 years). The species thus meets the threshold for subcriterion 1 at the level of Endangered. Given the disjunct distribution range, it is likely that the species forms at least two subpopulations in Florida and in Cuba, with potentially additional subpopulations on other Caribbean islands, in Venezuela and in Mexico. We have no exact information on the sizes of the subpopulation. Assuming that true overall population size is closer to the lower end of the estimate of 1,0000-2,499 mature individuals and further assuming that the largest subpopulation breeds in Florida, no subpopulation would number more than 1,000 mature individuals. Therefore, subcriterion 2a(i) is met at the level of Vulnerable. To conclude, Great White Heron warrants listing as Endangered under Criterion C1. Additionally, the species may qualify for Vulnerable under Criterion C2a(i), but in order to comprehensively assess the species against this criterion, we seek recent information on the size and structure of subpopulations, particularly from outside Florida.

Criterion D:

Great Blue Heron (A. herodias)– The population size and range of Great Blue Heron are too large to warrant listing as threatened under Criterion D and therefore the species is considered Least Concern under this criterion.

Great White Heron (A. occidentalis)– The population size is estimated at 1,000-2,499 mature individuals. Thus, the species warrants listing as Near Threatened, approaching the threshold for listing as threatened under Criterion D1.

Criterion E – To the best of our knowledge there have been no quantitative analyses of extinction risk carried out for either of these species. Therefore, they cannot be assessed against this criterion.

Therefore, it is proposed that Great Blue Heron (Ardea herodias)be listed as Least Concern and Great White Heron (Ardea occidentalis) be listed as Endangered under Criterion C1. We welcome any comments on this proposed listing and specifically request up-to-date information on the population size and trend of Great White Heron, particularly from Cuba.

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

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: Ardea herodias. http://www.birdlife.org (Accessed 28 April 2020).

Cook, M. I.; Baranski, M. (eds.). South Florida Wading Bird Report. South Florida Water Management District, West Palm Beach, FL, USA.

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.

Meyer, K.; Powell, A.; DiRodio, J. 2018. A comparative study of the distribution and population trend of nesting Great White Herons (Ardea herodias occidentalis) in the Florida Keys National Wildlife Refuge Complex, 2017-2018. Avian Research and Conservation Institute, Gainesville, FL, USA.

McGuire, H. L.; Taylor, S. S.; Sheldon, F. H. 2019. Evaluating the taxonomic status of the Great White Heron (Ardea herodias occidentalis) using morphological, behavioural and genetic evidence. The Auk Ornithological Advances 136: 1-18.

Meehan, T. D.; LeBaron, G. S.; Dale, K.; Michel, N. L.; Verutes, G. M.; Langham, G.M. 2018. Abundance trends of birds wintering in the USA and Canada, from Audubon Christmas Bird Counts, 1966-2017, version 2.1. National Audubon Society, New York, NY, USA.

Partners in Flight. 2019. Avian Conservation Assessment Database, version 2019. http://pif.birdconservancy.org/ACAD.

Powell, G. V. N. 1983. Food availability and reproduction by Great White Herons, Ardea herodias: A food addition study. Colonial Waterbirds 6: 139-147.

Sauer, J. R.; Niven, D. K. ; Hines, J. E.; Ziolkowski, Jr, D. J.; Pardieck, K. L.; Fallon, J. E.; Link, W. A. 2017. The North American Breeding Bird Survey, Results and Analysis 1966 – 2015. Version 2.07.2017 USGS Patuxent Wildlife Research Center, Laurel, MD, USA.

Vennesland, R. G. ; Butler, R. W. 2020. Great Blue Heron (Ardea herodias), version 1.0. In: Poole, A. F. (ed.). Birds of the World. Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/19.2173/bow.grbher3.01 (Accessed 28 April 2020).

Wetlands International. 2020. Waterbird Population Estimates. wpe.wetlands.org (Accessed 28 April 2020).

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4 Responses to Archived 2020 topic: Great Blue Heron (Ardea herodias) is being split: assessment of newly recognised taxa

  1. I agree completely with the proposal that the Great White Heron be considered a full species and that it be listed as endangered. Jim Kushlan, Founder and Co-chair Heron Specialist Group and co-author of “The Herons” (Oxford) and “Heron Conservation” (Academic Press).

    Further: The published, peer-reviewed biological evidence for its recognition as a species is overwhelming. I have had the opportunity to study its occurrence in some detail. It occupies a very limited range in extreme southern Florida with individuals also occurring in low numbers in Cuba. Other records are ephemeral or unsubstantiated as to the species involved. The population in Florida Bay has experienced a long and dramatic decline, owing to demonstrated changes in food availability. In my opinion the population is actually less than 1000 birds accepted by this analysis. Sea level rise, increased human use of its habitat, and continuing deterioration of the environmental condition of Florida Bay threaten its persistence.

  2. 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

  3. Red List Team (BirdLife International) says:

    Preliminary proposal

    While the newly-split Great Blue Heron appears to be at low risk of extinction, the situation of the Great White Heron is likely far worse. Uncertainties regarding population size and the distribution range (with the possible misidentification of individuals in the Caribbean and in Mexico) complicate the assessment.

    Based on available information, our preliminary proposal for the 2020 Red List would be to list

    Great Blue Heron as Least Concern and

    Great White Heron as Endangered under Criterion C1. If the population size of Great White Heron were below 1,000 mature individuals, the species would additionally meet the threshold for VU under Criterion D1.

    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, following further checking of information relevant to the assessments by both BirdLife and IUCN.

  4. Red List Team (BirdLife International) says:

    Recommended categorisation to be put forward to IUCN

    The final categorisations for these species have not changed.
    Great Blue Heron is recommended to be listed as Least Concern.
    Great White Heron is recommended to be listed as Endangered under Criterion C1.

    Many thanks for everyone who contributed to the 2020 GTB Forum process. The final 2020 Red List categories will be published on the BirdLife and IUCN websites in December 2020/January 2021, following further checking of information relevant to the assessments by both BirdLife and IUCN.

Comments are closed.