Saturday, March 2, 2013
Bittern fact sheet by Emma Williams
Australasian Bittern Botaurus poiciloptilus (Wagler, 1827) Order: Ciconiiformes Nationally Endangered/Endangered Family: Ardeidae Other names: Matuku, Boomer, Bunyip, Black-backed or Brown bittern, Bull-bird (English), Avetoro Australiano (Spanish), Butor d’Australie (French), Australische rohrdommel (German) Geographical variation: One species. Plumage does vary but this is not thought to be geographical. Bitterns are extremely cryptic and rarely seen. This is due to a combination of factors, including their secretive/evasive behaviours, crepuscular habits, inconspicuous plumage and the inaccessibility of their habitat. As a result, their presence is most commonly discerned through hearing the distinctive ‘booming’ call of the males during the breeding season. Despite this, many an enthusiast has been treated to a sighting in areas where birds are more conspicuous, such as wetland/farmland edges, dykes, drains and even roads. Such sightings never disappoint, as bitterns can be behaviourally fascinating; with interesting copulation rituals, booming postures and their infamous ‘freeze’ stance. Identification [74 cm, weight: males 1400g and females 900g] Bitterns are large, stocky birds, with streaky dark-brown and beige plumage on their thighs, chest, breast, and throat; with dark brown along the back and neck. The head is dark except for pale beige around the cheek, forming either a pale superciliary- or eye-stripe. Plumage varies greatly but does not appear to be related to region, season or sex. Some authors have suggested age may explain some variation (Heather & Robertson,1996); with juveniles possessing a more ‘buffy’ plumage, which darkens as they mature, and then may pale again as the bird ages (Teal, 1989). However, this still hasn’t been confirmed and there have been so few opportunities to age birds that it is currently not relied upon in the field. Voice: Bitterns produce a sequence of distinct ‘Wooom’ calls known as ‘booms’. These calls are thought to be produced by males only. Each call sequence consists of several individual booms, ranging from 1-10 booms per sequence (bps), with a mean of 3 bps . These sequences are often repeated at regular intervals that appear consistent to individual birds. The function of these calls is 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 but are said to produce a bubbling noise when they return to the nest (Marchant, 1990; Soper, 1958; Teal, 1989) and a nasal ‘kau’ call when alarmed (Teal, 1989). Birds in flight have been reported to produce profound, resonant ‘kau kau’ calls or a more nasal single ‘kau’ call (Teal, 1989) Distribution and habitat Found throughout New Zealand, as well as parts of Australia and New Caledonia (BirdLife International, 2011). Studies in Whangamarino wetland, New Zealand suggest bittern mostly inhabit mineralised and semi-mineralised wetlands (Ogle and Cheyne, 1981), although they are often spotted foraging in drains and wetland/farmland edges. Population The New Zealand population is thought to have <900 individuals (Heather and Robertson, 1996) but accurate numbers are not available. Australia is thought to have < 1000 individuals and New Caledonia <50 (BirdLife International, 2012) Threats and conservation As 90% of wetlands are now gone in New Zealand (Cromarty and Scott, 1996; Ausseil et al., 2010), habitat loss is expected to be the largest threat to bittern populations. A review of related bittern species by O’Donnell (2011) suggested that predators, poor water quality, and reduced food availability may also be contributing to a population decline. Despite the likelihood that bittern are in decline, there is currently no information regarding the extent of this decline. More research is required to understand the relationship between bittern population changes and potential threats. Breeding The cryptic nature of bitterns has resulted in a deficit of information regarding breeding behaviour. Egg laying is thought to take place from August to December, peaking in November. Nestlings are usually reported between September and February, and expected to fledge from November to May (O’Donnell, 2011). Behaviour and ecology Bitterns are extremely sensitive to disturbance. When approached they either a) try to steal away silently before they have been detected or b) 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, effectively disappearing from view in the blink-of-an-eye. When foraging, bitterns often stalk along wetland/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. Food Australasian bitterns are opportunistic feeders. They are thought to feed predominantly on fish; but are also known to forage on eels, spiders, insects, molluscs, annelids, crustaceans, reptiles and amphibians (Teal, 1989; Whiteside, 1989). Web links http://www.nzbirds.com/birds/matuku.html http://www.birdlife.org/datazone/speciesfactsheet.php?id=3767 http://www.doc.govt.nz/conservation/native-animals/birds/wetland-birds/australasian-bittern-matuku/facts/ http://www.iucnredlist.org/apps/redlist/details/106003767/0 http://bird.net.au/bird/index.php?title=Australasian_Bittern http://www.teara.govt.nz/en/wetland-birds/3 http://www.doc.govt.nz/conservation/land-and-freshwater/wetlands/arawai-kakariki-wetland-restoration-programme/ http://en.wikipedia.org/wiki/Australasian_Bittern References Ausseil, A.-G. E., Chadderton, W. L., Gerbeaux, P., Stephens, R. T. T., & Leathwick, J. R. (2010). Applying systematic conservation planning principles to palustrine and inland saline wetlands of New Zealand. Freshwater Biology, 9999(9999). 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, 395. Heather, B., & Robertson, H. (1996). The Field Guide to the Birds of New Zealand: Penguin Books (NZ) Ltd. Marchant, S., and Higgins, P. J. (Eds) (1990). ‘Handbook of Australian,New Zealand and Antarctic Birds. Vol. 1: Ratites to Ducks.’ (Oxford University Press: Melbourne.) 1056 - 1061 O Donnell (2011). Breeding of Australasian Bittern (Botaurus poiciloptilus) in New Zealand. Short Communication. Emu. 111, 197-201 Ogle, C. C., & Cheyne, J. (1981). The Wildlife and Wildlife Values of the Whangamarino Wetlands (No. Fauna Survey Unit Report No. 28). Wellington: New Zealand Wildlife Service. 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. Breeding ecology facts tab Social structure: Mostly solitary. Female-only incubation and brood-care (Heather and Robertson, 1996)) Breeding season: August – May, peaking in November and December (O’Donnell, 2011) Maximum number of successful broods per season Observations so far suggest Australasian bittern produce only one brood per season. However rare successful second clutches may be possible, as they have been observed in more well-studied, closely-related species (e.g. Botaurus stellaris, Mallord et al., 2000). Nest type: raised or floating platform Nest description: A platform usually surrounded by water and made from broken raupo, reeds, rushes and/or woody vegetation (O’ Donnell, 2011). Nest height above water Mean of 5 nests = 0.43m (Teal, 1989) Clutch size Minimum: 3 Mean: 4 Maximum: 6 Mean egg dimensions (mm) 51 x 37mm Egg colour: Olive-brown Egg laying dates August- December (with median records in November (O’Donnell, 2011)) Interval between eggs in a clutch (days) 24 - 48 hours (Soper, 1958) Incubation behaviour: female only (Heather and Robertson, 1996) Incubation length (days) c. 25 (Heather and Robertson, 1996) Nestling type: Altricial (Kushlan and Handcock, 2005) Nestling period: Length unknown but nestlings have been recorded from September – February (O Donnell, 2011) Age at fledging approximately 7 weeks (Heather and Robertson, 1996) Age at independence (days) unknown Age at first breeding (years) unknown Maximum longevity (years) unknown Maximum dispersal (km) unknown References Heather, B., & Robertson, H. (1996). The Field Guide to the Birds of New Zealand: Penguin Books (NZ) Ltd. Kushlan, J. A., and Hancock, J. A. (2005). Herons. (Oxford University Press:New York.) Mallord, J. W., Tyler, G. A., Gilbert, G., & Smith, K. W. (2000). The first case of successful double brooding in the Great Bittern Botaurus stellaris. Ibis, 142(4), 672-675. Ogle, C. C., & Cheyne, J. (1981). The Wildlife and Wildlife Values of the Whangamarino Wetlands (No. Fauna Survey Unit Report No. 28). Wellington: New Zealand Wildlife Service. O Donnell (2011). Breeding of Australasian Bittern (Botaurus poiciloptilus) in New Zealand. Short Communication. Emu. 111, 197-201 Soper, M. F. (1958). A Bittern’s nest. Notornis 8, 50–51.