The Australasian bittern (Botaurus poiciloptilus), or in Māori, is a stocky, sizeable and elusive heron-like bird native to the wetlands of Australia, New Zealand, and New Caledonia.Heather, B. D., & Robertson, H. A. (1996). The Field Guide to the Birds of New Zealand. Penguin Books (NZ) Ltd. It belongs to the bittern subfamily of the heron family Ardeidae. The Australasian bittern is best known for its cryptic plumage and behaviours, which allows it to blend into the rushes and reeds of its wetland habitats, making it particularly difficult to spot.O'Donnell, C. F. J., Williams, E. M., & Cheyne, J. (2013). Close approaches and acoustic triangulation: techniques for mapping the distribution of booming Australasian bittern (Botaurus poiciloptilus) on small wetlands. Notornis, 60, 279–284. Despite being rarely seen, Australasian bittern males have a distinct "booming" call that can carry long distances. This call is said to resemble that of a Bunyip, which is why the Australasian bittern is often known as "the Bunyip Bird" in parts of Australia.
Physically, the Australasian bittern stands around 66-76 cm tall with a wingspan of around a metre long.McKilligan, N. (2005). Herons, egrets and bitterns: their biology and conservation in Australia. CSIRO publishing. New Zealand Birds Online. (2024). Australasian bittern. Retrieved September 2, 2024. Its streaky, beige, mottled plumage allows it to camouflage itself among the reeds and rushes of its habitat to avoid predators and hunt for prey, undetected.Williams, E. M. (2016). Developing monitoring methods for cryptic species: A case study for the Australasian bittern, Botaurus poiciloptilus [Thesis, Massey University]. The species primarily feeds on fish, amphibians, and invertebrates, which it hunts by quietly stalking through shallow waters or remaining "frozen" before ambushing its prey.Menkhorst, P. (2012). The food and foraging rate of an Australasian Bittern. Australian Field Ornithology, 29(3), 133–142.
Historically, the Australasian bittern was described in the early 19th century and has since been of significant interest due to its cryptic nature. The species is classified as Vulnerable on the IUCN Red List, and continues to experience population declines resulting from habitat reduction and disturbance from wetland drainage, climate change and agricultural expansion.International Union for Conservation of Nature and Natural Resources. (2024). Australasian Bittern. The IUCN Red List of Threatened Species. Retrieved October 10, 2024.Herring, M. W., Robinson, W., Zander, K. K., & Garnett, S. T. (2019). Rice fields support the global stronghold for an endangered waterbird. Agriculture Ecosystems & Environment, 284, 106599. Conservation efforts are therefore becoming increasingly necessary for its survival.
Researchers continue to attempt to understand this birds' daily patterns, ecology, and breeding behaviour to improve current conservation strategies, although their cryptic nature makes this particularly difficult.
Taxonomy The taxonomic classification of the Australasian bittern remains unresolved, and debated among ornithologists.Olliver, N. (2023). Matuku, Australasian bittern. New Zealand Birds.
Its order, in particular, is ambiguous amongst the literature with some sources placing it in Ciconiiformes and others in Pelecaniformes.
Physical description The Australasian bittern is a relatively large, stocky bird with noticeable size dimorphism between the sexes. Males can weigh up to , while females are smaller, typically weighing around .Kiwi Coast Northland Project. (2024). Bittern – Matuku hūrepo. Retrieved September 10, 2024.
Beyond size, the species exhibits minimal sexual dimorphism, with males and females displaying similar plumage. Their beige, streaked, and mottled feathers provide them with camouflage in the reeds and rushes of their wetland environments. The bird is characterised by a long yellow beak, broad neck, and short legs, standing around 75 cm tall, though some individuals may reach up to one metre in height.
Juvenile Australasian bitterns have lighter outer wing mottling and yellowish irises, which darken as they mature. Specific details about the timing and progression of these plumage changes are limited.
Habitat & distribution The Australasian bittern is found in various fresh water wetland environments across New Zealand, south-eastern Australia, south-western Australia, southern Australia and New Caledonia. The biogeographical origins of the species and the dispersal events between Australia, New Zealand, and New Caledonia remain largely unexplored. However, it is known that the Australasian bittern shows a strong preference for densely vegetated habitats rich in rushes, reeds, and sedges, thriving in both temperate and subtropical climates. These birds travel seasonally by flight for long distances (up to 600 km) to exploit coastal wetlands .
Australia's population was estimated at 2,500 mature individuals before around 2010. However, a severe drought around then led to a sharp decline, and current estimates suggest that there are now less than 1,000 individuals, prompting its classification as Endangered under the Australian Environment Protection and Biodiversity Conservation Act 1999.Department of Climate Change, Energy, the Environment and Water. (2022). National Recovery Plan for the Australasian Bittern. In Department of Climate Change, Energy, the Environment and Water. Australian Government Department of Climate Change, Energy, the Environment and Water. Retrieved October 5, 2024. Today, over half of Australia's Australasian bittern population is thought to reside in New South Wales (NSW). Recent research indicates that during the breeding season, rice fields in the Riverina region of NSW hold between 500–1,000 bittern individuals, the largest known breeding population for this species. Before this research, the significance of agricultural wetlands, especially rice fields, as vital habitats for this cryptic species, had been overlooked.
In New Zealand, earlier population estimates of the Australasian bittern are likely inflated, as the degradation of their habitats and fewer recorded booming males at essential wetland sites, such as Whangamarino (the most significant NZ habitat for the species), suggest a more precarious status.Department of Conservation. Australasian bittern/matuku-hūrepo.
The species' reliance on specific wetland habitats makes it a key indicator for assessing wetland health, as these birds favour particular habitat locations that provide stable water levels and abundant food sources.O'Donnell, C. F. J., & Robertson, H. A. (2016). Changes in the status and distribution of Australasian bittern (Botaurus poiciloptilus) in New Zealand, 1800s−2011. Notornis, 63, 152–166.
Diet The Australasian bittern exhibits a diverse, opportunistic diet primarily comprising freshwater aquatic organisms, including fish, frogs, and eels.Whiteside, A. J. (1989). The Behaviour Of Bitterns and Their Use of Habitat. Notornis, 36, 89–95. In addition, they are known to consume some insects and small mammals, such as mice.Cork, S. C., & Stockdale, P. H. G. (1995). Carcinoma with concurrent haemosiderosis in an Australian bittern(Botaurus poiciloptilus). Avian Pathology, 24(1), 207–213. Research indicates that these birds often hunt for larger prey while foraging. While bitterns primarily consume fish, their opportunistic diet suggests they can adapt to changes in food availability, boosting their resilience to fluctuating food availability.Williams, E. M., & Brady, M. W. (2014). Monitoring of an Australasian bittern (Botaurus poiciloptilus) following release into the wild. Notornis, 61, 208–212. Nevertheless, recent studies reveal that Australasian bitterns face increased nutritional stress due to irregularities in their diet.Kennard, V. (2022). Anthropogenic environmental changes and stress in Australasian bitterns (Botaurus poiciloptilus). [MA Thesis, University of Canterbury]. These irregularities are driven by periods of insufficient access to high-quality food sources, which can negatively impact their growth and reproductive success and deplete their fat reserves. The bittern's barriers to accessing food have been linked to climate change effects, including global warming, urban expansion, habitat destruction, over-exploitation, the introduction of non-native species, and other anthropogenic-driven environmental changes.
Breeding Australasian bitterns share several reproductive traits with other species in the Botaurus genus, and has a breeding season that spans from June to February.O'Donnell, C. F. J. (2011). Breeding of the Australasian Bittern (Botaurus poiciloptilus) in New Zealand. Emu - Austral Ornithology, 111(3), 197–201.
A key characteristic of Australasian bitterns is the male's distinctive "booming" vocalisations, which is thought to attract mates and assist in defending their territories against other bittern males. These booms can be heard up to away from the bird.Marchant, S., & Higgins, P. J. (1990). Handbook of Australian, New Zealand & Antarctic birds. 1. Oxford University Press. In contrast, females are generally quieter but may occasionally produce "bubbling" sounds or louder calls when alarmed. Females take on the primary responsibilities of incubating the eggs in their nests and later feeding the hatchlings.
Information and data on nesting and reproductive biology of Australasian bitterns is limited.
Cryptic behaviours The Australasian bittern exhibits various complex and cryptic behaviours, underscoring why it has been a challenging species to detect and study. This species demonstrates all four types of cryptic features found in members of the Botaurus genus: visual crypsis, behavioural crypsis, spatial crypsis (due to its habitat selection of dense vegetation), and temporal crypsis. When disturbed by humans or predators, bitterns often freeze in a rigid pose. This behaviour, combined with their beige, striated and mottled plumage, makes them difficult to detect among the surrounding vegetation. Upon approach, they may exhibit a characteristic "freeze response", silently retreat into the vegetation or flatten their bodies against the ground. They may also slowly take flight as a form of retreat, though this behaviour has been observed only a few times, typically occurring when they are more than away from a disturbance or perceived threat.Teal, P.J. (1989). Movement, habitat use and behaviour of Australasian bittern (Botaurus poiciloptilus) in the lower Waikato wetlands. [MA Thesis, Waikato University].