This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is very large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.
AERC TAC. 2003. AERC TAC Checklist of bird taxa occurring in Western Palearctic region, 15th Draft. Available at: #http://www.aerc.eu/DOCS/Bird_taxa_of _the_WP15.xls#.
Christidis, L.; Boles, W. E. 2008. Systematics and taxonomy of Australian birds. CSIRO Publishing, Collingwood, Australia.
Cramp, S.; Perrins, C. M. 1977-1994. Handbook of the birds of Europe, the Middle East and Africa. The birds of the western Palearctic. Oxford University Press, Oxford.
del Hoyo, J.; Collar, N. J.; Christie, D. A.; Elliott, A.; Fishpool, L. D. C. 2014. HBW and BirdLife International Illustrated Checklist of the Birds of the World. Barcelona, Spain and Cambridge UK: Lynx Edicions and BirdLife International.
SACC. 2006. A classification of the bird species of South America. Available at: #http://www.museum.lsu.edu/~Remsen/SACCBaseline.html#.
The global population is estimated to number c.510,000-3,600,000 individuals (Wetlands International 2006), while national population estimates include: c.100,000-1 million breeding pairs, >c.10,000 individuals on migration and >c.10,000 wintering individuals in China; c.100,000-1 million breeding pairs in Taiwan; c.10,000-100,000 breeding pairs and c.50-1,000 wintering individuals in Korea and c.10,000-100,000 breeding pairs and c.1,000-10,000 individuals on migration in Japan (Brazil 2009).Trend justification
The overall population trend is decreasing, although some populations may be stable, others have unknown trends (Wetlands International 2006). This species has undergone a large and statistically significant increase over the last 40 years in North America (294% increase over 40 years, equating to a 40.8% increase per decade; data from Breeding Bird Survey and/or Christmas Bird Count: Butcher and Niven 2007) Note, however, that these surveys cover less than 50% of the species's range in North America.EcologyBehaviour
Northern populations of this species are migratory, with those breeding in the western Palearctic travelling on a broad front across the Sahara (del Hoyo et al.
1992) and those breeding in North American travelling on a narrow front along the Atlantic and Pacific coasts (Kushlan and Hancock 2005). Post-breeding southward movements occur from September to October and return northward movements occur from March to May (Kushlan and Hancock 2005). Tropical populations are not migratory but may undergo seasonal post-breeding dispersive movements (del Hoyo et al.
1992). In temperate regions breeding occurs in the local spring, with tropical and subtropical nesting generally coinciding with the rains (Kushlan and Hancock 2005). The species usually nests in small numbers (Snow and Perrins 1998) in single- or mixed-species colonies (del Hoyo et al.
1992), although sometimes groups may reach several thousand pairs (del Hoyo et al.
1992). When nesting within mixed-species colonies the species tends to form monospecific clusters (Kushlan and Hancock 2005). The species's aggregatory behaviour outside of the breeding season varies much throughout its range, some populations (e.g. in America) remaining highly gregarious throughout the year (Snow and Perrins 1998) and gathering in flocks of hundreds or even thousands to roost (del Hoyo et al.
1992), others (e.g. Palearctic breeders) being largely solitary except when roosting or on migration (Snow and Perrins 1998) (roosting flocks of 2-6 to 200 are known in Africa (Brown et al.
1992) and small flocks occur on migration) (del Hoyo et al.
1992). The species is largely crepuscular and nocturnal, but may feed diurnally especially during the breeding season (del Hoyo et al.
The species inhabits fresh, brackish or saline waters with aquatic vegetation and bamboo or trees (e.g. pine, oak or mangroves) for roosting and nesting in (del Hoyo et al.
1992), showing a preference for islands or predator-free areas for nesting sites (Kushlan and Hancock 2005). It occupies the forested margins of shallow rivers, streams, lagoons, pools, ponds, lakes, marshes and mangroves and may feed on pastures, reservoirs, canals, aquaculture ponds (del Hoyo et al.
1992) and rice-fields (up to 96 % of a colony's food resources may be taken from nearby rice-fields) (Kushlan and Hancock 2005). On migration the species may also frequent dry grasslands or marine coasts (del Hoyo et al.
1992), kelp beds (Kushlan and Hancock 2005) and estuaries (Hockey et al.
2005). It breeds up to 4,800 m (Chile) (Kushlan and Hancock 2005) but is more common at elevations of up to c.2,000 m (Snow and Perrins 1998). Diet
It is an opportunistic feeder taking fish, frogs, tadpoles, turtles, snakes, lizards, adult and larval insects (del Hoyo et al.
1992) (e.g. beetles, bugs, grasshoppers, crickets, flies and dragonflies) (Kushlan and Hancock 2005), spiders, crustaceans, molluscs, leeches, small rodents, bats and the eggs and chicks of other bird species (del Hoyo et al.
1992). Breeding site
The nest is platform constructed of sticks and vegetation (del Hoyo et al.
1992, Kushlan and Hancock 2005) placed 2-50 m above water or on dry ground near water (Snow and Perrins 1998) in trees, bushes, reedbeds, on cliff ledges (del Hoyo et al.
1992) (overhanging rivers) (Hockey et al.
2005) and on the ground (del Hoyo et al.
1992) in protected sites (Kushlan and Hancock 2005). The species nests close together in single- and mixed-species colonies (del Hoyo et al.
1992) with as many as 20-30 pairs in the same tree (Kushlan and Hancock 2005). Colony sites may be reused in consecutive years or flocks may move to new sites (usually such movements are a result of nesting trees being destroyed due to the colony's nesting activities) (Kushlan and Hancock 2005). Colony sites are dispersed throughout the landscape in relation to distance from feeding areas (Kushlan and Hancock 2005). Management information
A study carried out in north-west Italy suggests that existing nesting sites should be protected and that breeding habitats should be actively managed in order to maintain suitable habitat characteristics (Fasola and Alieri 1992). The creation of a network of new nesting sites spaced at 4-10 km in relation to available foraging habitats in zones currently without suitable nesting sites is also recommended (Fasola and Alieri 1992).Threats
The species is threatened by wetland drainage and destruction (del Hoyo et al.
1992, Kushlan and Hancock 2005) and by drought in wintering areas (Hafner and Kushlan 2002). It is highly susceptible to pesticides (del Hoyo et al.
1992, Kwon et al.
2004, Kushlan and Hancock 2005) such as organophosphates, carbamates (Kwon et al.
2004) and DDE (a breakdown product of DDT) which negatively affect hatching success (Kushlan and Hancock 2005). There are also cases of genetic damage to chicks as a result of petroleum contamination (Custer 2000). The species is susceptible to avian influenza (Melville and Shortridge 2006) and Newcastle disease so may be threatened by future outbreaks (Kuiken et al.
2006). It is also persecuted (anti-predation killing) at aquaculture facilities due to its depredation on fish stocks (Kushlan and Hancock 2005), and has suffered declines due to the exploitation of chicks from nesting colonies in the past (del Hoyo et al.
Chicks of the species are still taken for food in some areas (e.g. Madagascar) (Kushlan and Hancock 2005, Hafner 200) and adults are hunted and traded at traditional medicine markets in Nigeria (Nikolaus 2001).
Brazil, M. 2009. Birds of East Asia: eastern China, Taiwan, Korea, Japan, eastern Russia. Christopher Helm, London.
Brown, C.R., Brown, M.B. and Ives, A.R. 1992. Nest placement relative to food and its influence on the evolution of avian coloniality. American Naturalist 139(1): 205-217.
Custer, T. W. 2000. Environmental contaminants. In: Kushlan, J. A., Hafner, H. (ed.), Heron conservation, pp. 251-267. Academic Press, San Diego.
del Hoyo, J.; Elliot, A.; Sargatal, J. 1992. Handbook of the Birds of the World, vol. 1: Ostrich to Ducks. Lynx Edicions, Barcelona, Spain.
Delany, S.; Scott, D. 2006. Waterbird population estimates. Wetlands International, Wageningen, The Netherlands.
Fasola, M.; Alieri, R. 1992. Conservation of heronry Ardeidae sites in North Italian agricultural landscapes. Biological Conservation 62: 219-228.
Hafner, H. 2000. Heron nest site conservation. In: Kushlan, J. A.; Hafner, H. (ed.), Heron conservation, pp. 201-217. Academic Press, San Diego.
Hafner, H.; Kushlan, J. A. 2002. Action plan for conservation of the Herons of the world. Heron Specialist Group, Gland, Cambridge and Arles.
Hockey, P. A. R.; Dean, W. R. J.; Ryan, P. G. 2005. Roberts birds of southern Africa. Trustees of the John Voelcker Bird Book Fund, Cape Town, South Africa.
Kuiken, T.; Fouchier, R. A. M.; Rimmelzwaan, G. F.; Osterhaus, A. D. M. E. 2006. Emerging viral diseases in waterbirds. In: Boere, G.; Galbraith, C., Stroud, D. (ed.), Waterbirds around the world, pp. 418-421. The Stationary Office, Edinburgh, UK.
Kushlan, J. A.; Hancock, J. A. 2005. The herons. Oxford University Press, Oxford, U.K.
Kwon, Y. K.; Wee, S. H.; Kim, J. H. 2004. Pesticide Poisoning Events in Wild Birds in Korea from 1998 to 2002. Journal of Wildlife Diseases 40(4): 737-740.
Melville, D. S.; Shortridge, K. F. 2006. Migratory waterbirds and avian influenza in the East Asian-Australasian Flyway with particular reference to the 2003-2004 H5N1 outbreak. In: Boere, G.; Galbraith, C., Stroud, D. (ed.), Waterbirds around the world, pp. 432-438. The Stationary Office, Edinburgh, UK.
Nikolaus, G. 2001. Bird exploitation for traditional medicine in Nigeria. Malimbus 23: 45-55.
Snow, D. W.; Perrins, C. M. 1998. The Birds of the Western Palearctic vol. 1: Non-Passerines. Oxford University Press, Oxford.
Further web sources of information
Detailed species account from Birds in Europe: population estimates trends and conservation status (BirdLife International 2004)
Detailed species account from Birds in Europe: population estimates, trends and conservation status (BirdLife International 2004)
Explore HBW Alive for further information on this species
Search for photos and videos, and hear sounds of this species from the Internet Bird Collection
Species factsheet from HeronConservation - The IUCN-SSC Heron Specialist Group
Text account compilers
Ekstrom, J., Butchart, S., Malpas, L.
IUCN Red List evaluators
Butchart, S., Symes, A.
BirdLife International (2016) Species factsheet: Nycticorax nycticorax. Downloaded from
http://www.birdlife.org on 28/05/2016.
Recommended citation for factsheets for more than one species: BirdLife International (2016) IUCN Red List for birds. Downloaded from
http://www.birdlife.org on 28/05/2016.
This information is based upon, and updates, the information published in BirdLife International (2000)
Threatened birds of the world. Barcelona and Cambridge, UK: Lynx Edicions and BirdLife International, BirdLife International (2004)
Threatened birds of the world 2004 CD-ROM and BirdLife International (2008) Threatened birds of the world 2008 CD-ROM. These sources provide the information for species accounts for the birds on the IUCN Red List.
To provide new information to update this factsheet or to correct any errors, please email BirdLife
To contribute to discussions on the evaluation of the IUCN Red List status of Globally Threatened Birds, please visit BirdLife's Globally Threatened Bird Forums.
Additional resources for this species