McGuire et al. (2019) review in detail the status of the all-white taxon occidentalis, which although formerly given species status (‘Great White Heron’ [GWH]) has long been considered a subspecies of Great Blue Heron [GBH] Ardea herodias, in great part because the two taxa interbreed with regularity. However, given that the evidence points to assortative mating where possible, they conclude that occidentalis is indeed better treated as a species.
It is separated by diagnostic plumage (white vs. blue), behavior (mate choice and timing of mating) and habitat (saltwater-island vs. fresh/brackish water mainland).
There are, in fact, a few additional morphological considerations, with GWH having a shorter occipital plume but longer bill, tarsus and middle toe than GBH (supplementary material online with measurements not accessed). Applying the Tobias criteria to the situation, the taxonomic status of GWH involves a consideration of its
· all-white vs grey/blue plumage (an exceptional difference: score 4);
· shorter occipital plume (seemingly fairly distinctive from the authors’ Figure 3: allow 1);
· longer tarsus (apparently the most distinctive of the longer characters measured, but perhaps insufficient to be allowed a score: ns);
· timing of mating or habitat choice (only one of these permitted as a score: 1);
· narrow hybrid zone (not more than 50 km as measured on Google Earth: 2).
On this basis BirdLife accepts Ardea occidentalis (Handbook of the Birds of the World and BirdLife International 2020) as a separate species.
Handbook of the Birds of the World and BirdLife International. 2020. Handbook of the Birds of the World and BirdLife International digital checklist of the birds of the world. Version 5. Available at: http://datazone.birdlife.org/userfiles/file/Species/Taxonomy/HBW-BirdLife_Checklist_v5_Dec20.zip.
|Migratory status||full migrant||Forest dependency||Low|
|Land mass type||Average mass||-|
|Extent of Occurrence breeding/resident (km2)||14,500,000||medium|
|Extent of Occurrence non-breeding (km2)||32,000,000||medium|
|Number of locations||-|
|Value||Data quality||Derivation||Year of estimate|
|No. of mature individuals||500000-4999999||poor||estimated||2019|
|Decline (3 years/1 generation past)||-||-||-|
|Decline (5 years/1 generation past)||-||-||-|
|Decline (10 years/1 generation past)||-||-||-|
|Decline (10 years/3 generation future)||-||-||-|
|Decline (10 years/3 generation past and future)||-||-||-|
|Number of subpopulations||-||-||-|
|Percentage in largest subpopulation||-||-||-|
|Generation length (yrs)||8||-||-||-|
Population justification: The population size of Great Blue Heron is estimated at 500,000-4,999,999 mature individuals (Partners in Flight 2019).
Trend justification: 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 (Meehan et al. 2018, Pardieck et al. 2018).
|Anguilla (to UK)||N||Extant|
|Antigua and Barbuda||N||Extant|
|Aruba (to Netherlands)||N||Extant|
|Bermuda (to UK)||N||Extant||Yes|
|Bonaire, Sint Eustatius and Saba (to Netherlands)||N||Extant||Yes|
|Cayman Islands (to UK)||N||Extant|
|Curaçao (to Netherlands)||N||Extant||Yes|
|Greenland (to Denmark)||V||Extant|
|Guadeloupe (to France)||N||Extant||Yes|
|Martinique (to France)||N||Extant|
|Montserrat (to UK)||N||Extant|
|Puerto Rico (to USA)||N||Extant|
|Sint Maarten (to Netherlands)||N||Extant||Yes|
|St Kitts and Nevis||N||Extant|
|St Martin (to France)||N||Extant||Yes|
|St Pierre and Miquelon (to France)||N||Extant||Yes|
|St Vincent and the Grenadines||N||Extant|
|Trinidad and Tobago||N||Extant|
|Turks and Caicos Islands (to UK)||N||Extant|
|United States Minor Outlying Islands (to USA)||N||Extant||Yes|
|Virgin Islands (to UK)||N||Extant||Yes|
|Virgin Islands (to USA)||N||Extant||Yes|
|Mexico||Pantanos de Centla|
|Mexico||Bahía de Ceuta - Cospita|
|Mexico||Cuenca baja del Río Papagayo|
|Mexico||Presa de Valsequillo|
|Habitat (level 1)||Habitat (level 2)||Importance||Occurrence|
|Forest||Subtropical/Tropical Mangrove Vegetation Above High Tide Level||suitable||breeding|
|Forest||Subtropical/Tropical Mangrove Vegetation Above High Tide Level||suitable||non-breeding|
|Marine Intertidal||Mud Flats and Salt Flats||suitable||breeding|
|Marine Intertidal||Mud Flats and Salt Flats||suitable||non-breeding|
|Wetlands (inland)||Bogs, Marshes, Swamps, Fens, Peatlands||major||breeding|
|Wetlands (inland)||Permanent Freshwater Lakes (over 8ha)||suitable||breeding|
|Wetlands (inland)||Permanent Rivers/Streams/Creeks (includes waterfalls)||suitable||breeding|
|Altitude||0 - 2600 m||Occasional altitudinal limits|
|Purpose||Primary form used||Life stage used||Source||Scale||Level||Timing|
|Pets/display animals, horticulture||-||-||International||Non-trivial||Recent|
BirdLife International (2021) Species factsheet: Ardea herodias. Downloaded from http://www.birdlife.org on 21/09/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 21/09/2021.