Bitterns are extremely cryptic and rarely seen. This is due to their secretive behaviour, inconspicuous plumage and the inaccessibility of their habitat. Their presence is most commonly discerned through hearing the distinctive ‘booming’ call of the males during the breeding season. Bittern occasionally show themselves in the open along wetland edges, dykes, drains, flooded paddocks or roadsides, often adopting their infamous ‘freeze’ stance, with the bill pointing skyward, even when caught out in the open.
Bitterns are large, stocky birds, with streaky dark-brown and beige plumage on their throat, breast, belly and thighs, with dark brown on the neck and back. The head is dark except for pale beige around the cheek, forming either a pale eyebrow or eye-stripe. Plumage varies greatly but this does not appear to be related to region, season or sex. Age may explain some variation, with juveniles possessing a more ‘buffy’ plumage, which darkens as they mature, becoming paler again as the bird ages. However, there have been few observations of known-age birds to confirm this.
Voice: male bitterns produce a sequence of distinctive ‘wooom’ calls known as ‘booms’. Each call sequence consists of several individual booms, ranging from 1-10 booms per sequence, with a mean of 3. These sequences are repeated at regular intervals that appear consistent to individual birds. The calls are assumed to be related to breeding as their production is almost entirely limited to the breeding season. Booms are commonly preceded or interceded with gasps or inhalations.
Females are largely silent, apart from producing a bubbling noise when they return to the nest, and a nasal ‘kau’ when alarmed. Birds in flight may produce a resonant or nasal ‘kau’ or ‘kau kau’.
Distribution and habitat
Found throughout New Zealand, as well as parts of Australia and New Caledonia. In Whangamarino wetland, bittern mostly inhabited mineralised and semi-mineralised wetlands, although they also foraged in drains and wetland/farmland edges.
The New Zealand population is thought to be fewer than 900 individuals, but few data are available. Australia is thought to have fewer than 1000 birds, and New Caledonia fewer than 50.
Threats and conservation
Bittern numbers in New Zealand declined greatly following destruction of 90% of their wetland habitat to create farmland and towns. Ongoing habitat loss is still their greatest threat, although predators, poor water quality, and reduced food availability may have contributed to population declines. As bittern are difficult to study or count, there are few data available to determine whether the population is currently stable or still in decline.
Few bittern nests have been found or studied. The nest is a platform of reeds built by the female among dense wetland vegetation, usually 20-30 cm above the water. The 3-5 eggs are laid between August and December (peaking in November), and are incubated by the female alone for about 25 days till hatch. Chicks remain in the nest for about 7 weeks. Nestlings are usually reported between September and February, and fledge from November to May.
Behaviour and ecology
Bitterns are extremely sensitive to disturbance. When approached they either try to steal away silently before they have been detected, or adopt a still posture, commonly known as the ‘freeze’ or ‘surveillance’ pose. This stance involves standing with an erect neck and bill. It allows the bird to blend perfectly into most wetland environments, whilst maintaining good surveillance of its surroundings. If an observer continues to approach a bittern that is in freeze stance, the bird will eventually take flight laboriously, or occasionally will flatten its chest completely to the ground, using its superbly camouflaged plumage to blend into the background.
Foraging bitterns often stalk along wetland or water edges slowly, taking prey opportunistically. In areas where prey are abundant they have been observed slowly moving towards prey items until they are close enough to take their prey in a discreet, steady manner without needing to lunge or swipe.
Australasian bitterns feed mainly on fish, including eels, but they also take spiders, insects, molluscs, worms, freshwater crayfish, frogs and lizards.
BirdLife International. 2012. Species factsheet: Botaurus poiciloptilus.Available at http://www.birdlife.org/datazone/speciesfactsheet.php?id=3767 [Accessed 11 July 2012].
Cromarty, P.; Scott, D. 1996. A directory of wetlands in New Zealand. Department of Conservation, Wellington.
Heather, B.D.; Robertson, H.A. 1996. The field guide to the birds of New Zealand. Penguin Books (NZ) Ltd.
Marchant, S.; Higgins, P.J. (eds) 1990. Handbook of Australian, New Zealand and Antarctic birds. Vol. 1, ratites to ducks. Oxford University Press, Melbourne.
O’Donnell, C.F.J. 2011. Breeding of Australasian bittern (Botaurus poiciloptilus) in New Zealand. Emu 111: 197-201.
Ogle, C.C.; Cheyne, J. 1981. The wildlife and wildlife values of the Whangamarino Wetlands. Fauna Survey Unit Report No. 28. New Zealand Wildlife Service, Wellington.
Robertson, H.A; Baird, K.; Dowding, J.E.; Elliott, G.P.; Hitchmough, R.A.; Miskelly, C.M.; McArthur, N.; O’Donnell, C.F.J.; Sagar, P.M.; Scofield, R.P.; Taylor, G.A. 2017. Conservation status of New Zealand birds, 2016. New Zealand Threat Classification Series 19. Wellington, Department of Conservation. 27p.
Soper, M.F. 1958. A bittern’s nest. Notornis 8: 50-51.
Teal, P.J. 1989. Movement, habitat use and behaviour of Australasian bittern (Botaurus stellaris poiciloptilus) in the lower Waikato wetlands. Masters thesis, Waikato University.
Whiteside, A.J. 1989. The behaviour of bitterns and their use of habitat. Notornis 36: 89-95.
Williams, E. 2013 [updated 2017]. Australasian bittern. In Miskelly, C.M. (ed.) New Zealand Birds Online. www.nzbirdsonline.org.nz
- Social structure
- female-only incubation and brood-care
- Breeding season
- Nest type
- floating platform, raised platform
- Nest description
- Platform usually surrounded by water made from raupo, reeds, rushes and or woody vegetation.
- Nest height (mean)
- 0.43 m
- Maximum number of successful broods
- Clutch size (mean)
- Clutch size (min)
- Clutch size (max)
- Mean egg dimensions (length)
- 51 mm
- Mean egg dimensions (width)
- 37 mm
- Egg colour
- Egg laying dates
- Interval between eggs in a clutch
- 24-48 hours days
- Incubation behaviour
- female only
- Incubation length (mean)
- Approximately 23 days
- Nestling type
- Nestling period (mean)
- Age at fledging (mean)
- Approximately 49 days
- Age at independence (mean)
- Age at first breeding (typical)
- Maximum longevity
- Maximum dispersal