Data Zone
Justification of Red List category
This species is classified as Critically Endangered because, although it has increased, it still has only a small population, largely derived from captive-reared individuals. The annual release of substantial numbers of captive-reared birds, in combination with predator control, has almost certainly prevented it from becoming Extinct in the Wild, and the species's long-term survival remains dependent upon this intensive conservation management.
Population justification
The estimated adult population has increased from 61 adults in 2003 to 106 in 2017 (BRaid 2017, Department of Conservation n.d.). However, since the majority of the adult population is derived from captive-reared birds, the wild (according to the IUCN definition) population is considered to be smaller than the total adult count, and is placed in the band 1-49 mature individuals.
Trend justification
The adult population has increased from around 40-50 in the mid-90s to 106 in 2017 (BRaid 2017). However, the increase is largely thanks to the annual release of captive-bred subadults and juveniles (R. Maloney in litt. 2008).
Himantopus novaezelandiae was formerly widespread, breeding and wintering across the North and South Islands of New Zealand. Following a long-term decline it is now restricted during the breeding season to the upper Waitaki Valley in the South Island. The population is sedentary, with small numbers of birds being sighted in other regions of New Zealand. The population may have numbered 500-1,000 birds in the 1940s (Pierce 1984), by which time it had ceased to breed in the North Island and was rare as a breeding species in the lowlands. It continued to decline to a low of just 23 birds in 1981, when intensive management began (Keedwell 2005). In 1999-2000, the wild breeding population consisted of just four pairs (Keedwell et al. 2002) and a maximum of 31 adults. Since this point the population has increased primarily thanks to the annual release of 80-100 'fast-tracked' captive-reared or captive-bred subadults and juveniles. Since 2013, breeding pair numbers have fluctuated between 17 and 28 breeding pairs, and the total adult population has grown from 61 to 106 birds (Department of Conservation n.d., BRaid 2017), although the majority of the adult population has been captive reared and released from either wild laid or captive laid eggs. The 2017 population of 106 adults is the highest ever recorded since management begun. Recruitment rates from releases of juveniles or sub-adults remain low at 29%, with a recent increase to 49% in the Tasman River, where large-scale predator control is underway (Department of Conservation n.d.). Hybridisation with H. leucocephalus was exacerbated during periods of cross-fostering of H. novaezelandiae eggs to pied stilt parents in the 1980s and because low population size and the widely dispersed population meant that some H. novaezelandiae individuals were unable to find other H. novaezelandiae mates. Fewer than 5 dark H. novaezelandiae x H. leucocephalus hybrids are currently known (R. Maloney in litt. 2016), and through continued positive assortative mating, almost all H. novaezelandiae are pure bred. Cryptic hybrids are extremely rare (Steeves et al. 2010).
It breeds on braided riverbeds, but also occurs in wetlands, tarns, lake deltas and swamplands. A few individuals occasionally winter along the coastline in inter-tidal habitats and some make small local movements of a few kilometres to the river delta of their breeding valley (Pierce et al. 2015). It feeds primarily on insects, but also takes molluscs (Anon. 2009) and small fish (Pierce 1986a). Birds return to the nesting area in late winter-early spring, with breeding between September and January (Pierce et al. 2015). The species is monogamous and birds pair for life (Anon. 2009). It lays four eggs and will usually re-nest if the first clutch is lost early in the season. Both birds in a pair share incubation duties (Anon. 2009). Most breed for the first time at two or three years of age.
Predators, in particular introduced mammals such as cats, ferrets Mustela furo, stoats M. erminea, hedgehogs Erinaceus sp. and Brown Rats Rattus norvegicus, and the native Australasian Harrier Circus approximans and Kelp Gull Larus dominicanus (Pierce 1986b, Sanders and Maloney 2002) are today the primary threat, but the combined impact of habitat loss has exacerbated declines. Habitat has been lost through conversion to agriculture and hydroelectric developments (Maloney and Murray 2001). Nests are destroyed, and predation is potentially increased, by drainage and hydroelectric development, weed growth and flood-control programmes (Maloney and Murray 2001), and nesting birds are disturbed by recreational use of riverbeds. Adverse weather and natural flooding are additional, unpredictable threats (Maloney and Murray 2001).
Hybridisation with H. leucocephalus, which was allowed to continue under former management strategies, posed a threat because the crash in the Black Stilt population made it difficult for them to form conspecific pairs and a biased sex ratio resulted in single males mating with H. leucocephalus females or hybrids (Pierce 1984, R. Maloney in litt. 1999, Steeves et al. 2010), although hybridisation has been bidirectional (Steeves et al. 2010). Extensive bidirectional hybridisation appears to have been taking place since at least 1960 (Steeves et al. 2010). The sex ratio is now even and the frequency of hybridisation has decreased (R. Maloney in litt. 2008, Steeves et al. 2010). Adjustment of the sex ratio, low reproductive success in hybrid females and high mortality are the likely reasons for a lack of widespread introgression between the two species (Steeves et al. 2010). Adult mortality in the wild remains very high (Keedwell 2005). Despite the genetic bottleneck experienced by H. novaezelandiae, there is so far no evidence of inbreeding depression in the wild (Steeves et al. 2010). However, a negative relationship has been shown between inbreeding and fitness in the captive population; in light of this care should be taken to minimise the relatedness of pairs forming in captivity (Hagen et al. 2011).
Conservation Actions Underway
The species is listed as Nationally Critical in New Zealand (Robertson et al. 2016). Captive-rearing and release is ongoing. Active management involves double and triple clutching of parents by removing eggs to encourage re-laying. The removed eggs are reared in captivity. Playback calls are broadcast to juvenile birds during captive-rearing to equip them with the behavioural and auditory recognition skills necessary for survival (Galbraith et al. 2007). Advances in release methods appear to have enhanced the initial survival of released birds from 20-45% to 80-100%, but require further testing (van Heezik et al. 2009). A study was published on the influence of release age, size of release group and size of the wild population at release sites on the post-release movements of captive-reared Black Stilts, with implications for the future management of the programme (van Heezik et al. 2009). Research is ongoing into the causes of adult mortality, with the aim of determining methods to improve adult survival (O’Donnell et al. 2016).
Predator exclusion fencing was first installed at the site near Lake Tekapo in the late 1970s (Anon. 2009). Trapping for predators around wild nests has been on-going since 1997, and a large-scale predator control programme in the Tasman River Valley has been underway since 2005 (Keedwell et al. 2002, O’Donnell et al. 2016). Research has shown that predator control improved survival of adult Black Stilts (Cruz et al. 2013). Hybrids, now numbering fewer than 5 within the Black Stilt's range, are controlled and research is underway into the impacts of hybridisation on the Black Stilt's genetics (Forsdick 2017). Water-levels are manipulated in managed wetlands to attract birds to feed, and possibly breed, in areas where predators are controlled (R. Maloney in litt. 2016). Habitat restoration is on-going, and involves the removal of exotic weeds from riverbeds (Heather and Robertson 2015). The establishment of a second population is desirable and has undergone a feasibility study (Murray and Sanders 2000, R. Maloney in litt. 2008), but without intensive site management to control weeds and pests it is unlikely that a suitable release site will be found.
40 cm. Black, long-legged stilt. Adult, black with long, fine, black bill. Very long red legs. Juvenile, white breast, neck, head. Black patch around eyes. Similar spp. Hybridises with Pied Stilt H. himantopus. Resulting individuals highly variable. Compared to pure adults, darker hybrid adults have longer bills, shorter legs. Compared to pure juveniles, hybrid adults have some solid black on breast. Voice Loud, high-pitched, monotonous yapping.
Text account compilers
Bird, J., Symes, A., Taylor, J., Martin, R., Ashpole, J, Stringer, C., Wheatley, H., Butchart, S., Benstead, P., Khwaja, N., Pilgrim, J.
Contributors
Bayliss, M., Grant, A., Maloney, R., Murray, D.P., Steeves, T. & Cleland, S.
Recommended citation
BirdLife International (2024) Species factsheet: Black Stilt Himantopus novaezelandiae. Downloaded from
https://datazone.birdlife.org/species/factsheet/black-stilt-himantopus-novaezelandiae on 22/11/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from
https://datazone.birdlife.org/species/search on 22/11/2024.