Threatened species & ecological communities

National recovery plan for the Christmas Island Hawk-Owl Ninox natalis

Prepared by Richard Hill
Birds Australia
Department of the Environment and Heritage, 2004
ISBN0 642 55010 7

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Background information

Conservation status

The Christmas Island Hawk-Owl Ninox natalis is currently listed as Vulnerable under the EPBC Act due to its small population size. This was based on estimates provided by van Tets (1975) of 10 - 100 pairs and subsequently by Stokes (1988) of 100 pairs. Following the outbreak of Crazy Ant supercolonies, Garnett & Crowley (2000) recommended the Christmas Island Hawk-owl be upgraded to Critically Endangered.

Approximately 25% of the island’s forests have been cleared since settlement and all but the small areas of regrowth vegetation are currently unsuitable and are unoccupied by Christmas Island Hawk-Owls. Hill & Lill (1998a) estimated that the island prior to settlement had a carrying capacity in the order of 740 ± 135 owl territories. Between 1994-1996 Hill & Lill (1998a) estimated the population at 556 ± 101 occupied owl territories in Primary Rainforest and 6 ± 4 occupied owl territories in regrowth vegetation, and a total population size of 562 ± 105 occupied territories. The total Christmas Island Hawk-Owl population has probably decreased by at least 25% since settlement (Stokes 1988, Hill & Lill 1998a).

Taxonomic status

The Christmas Island Hawk-Owls is now considered a full species after Norman et al. (1998). Formerly a subspecies of N. squamipila, Norman et al. (1998) concluded, using molecular data, that the three subspecies for which material was available, including the Christmas Island Hawk- Owl, each represented separate species in the genus Ninox.

Distribution and location

The Christmas Island Hawk-Owl is restricted to Christmas Island in the Indian Ocean (10o0'S; 105o40' E), approximately 1400km northwest of Australia. The nearest land is Java in the Republic of Indonesia, 360km to the north. Christmas Island is 135km2 in area and 75% is covered with original vegetation. During 1994-1995, Christmas Island Hawk-Owls were widespread and occurred in all Primary, Marginal and secondary regrowth rainforests. The highest densities were found in Primary rainforests (Hill & Lill 1998a). Hill and Lill (1998a) concluded that the total population size was probably limited primarily by the area of remaining rainforest and that the observed owl densities within available habitat had probably not changed markedly since settlement (Hill & Lill 1998a). The lack of change in density of birds in the habitat available combined with the reduction in 25% of habitat area leads to the conclusion that the population has declined by 25%.

Using data from radio-tracking, territory mapping and census by playback of calls, Hill & Lill (1998a) found that owls were uniformly distributed throughout the two Primary and Marginal of rainforest at a density of 5.5 ± 1.0 territories per square kilometre. They speculated that densities might be lower in some Marginal rainforests because of the more seasonal microclimates in areas exposed to the prevailing southeast winds. In regrowth vegetation they found that owls were present at significantly lower densities (1.7 ± 1.1 territories/km2).

Approximately 75% of Christmas Island is still covered with natural vegetation and 84% of this (63% of the island) is within National Park (Figure 1). “Primary Rainforests” (du Puy 1993) outside the park are currently protected from clearance by a Federal Government moratorium, and Primary rainforest, Marginal rainforest and second-growth rainforest or regrowth vegetation may be removed only within the constraints of the EPBC Act and EPBC Regulations.

Rainforest rehabilitation has been occurring on Christmas Island for several decades. Currently, the main aim is to revegetate mined areas adjacent to Abbott's Booby nesting habitat in an attempt to reduce wind turbulence caused by the clearings and implicated in reduced nesting success of the booby (Reville et al. 1990). There are approximately 70 clearings covering 3200ha or 24% of the island area (Carew-Reid 1987). It is not known how long it will take for rehabilitation plantings to develop into forest which can be used by Christmas Island Hawk- Owls: it would not be long before these areas might be used for foraging, however, it is likely to be centuries before mature trees with nesting holes are available.

Ranging Habits

The ranging habits of adults in Primary rainforest have been studied by radio-tracking and territory mapping (Hill & Lill 1998a). Male and female pairs ranged within an apparently exclusive territory and two females radio-tracked used a smaller area than the two males tracked. One female of a pair occupied a smaller area that was wholly contained within that used by the male. Both females were preparing breeding at the time of being tracked and presumably range further when not breeding. Pairs defend their territory year round. Territorial boundaries appeared to be sharply delineated and crossings into neighbouring territories appeared to be rare. Territories appeared to be contiguous and were estimated to be 18ha in size on average. There is evidence that territories are contiguous in areas of Marginal rainforest (Hill & Lill 1998a). There are no data on the ranging habits of independent young.

Diet

Christmas Island Hawk-Owls are primarily insectivorous eating a wide variety of medium to large insects, especially Orthoptera, Lepidoptera, and Coleoptera. They also are recorded eating a range of vertebrates including Black Rats Rattus rattus, geckos, and Christmas Island Whiteeyes Zosterops natalis (Hill & Lill 1998b).

Habitat

Primary Rainforest (after Du Puy 1993) is restricted to the central plateau and is an evergreen closed forest 30 - 40 m high with emergent trees up to 45 m tall. Common canopy trees include Syzygium nervosum, Planchonella nitida, and Hernandia ovigera. The canopy formed by these trees is irregular, especially on the western side of the island where it is sheltered from strong southeast trade winds which blow throughout the dry season. The understorey comprises dense thickets of Pandanus elatus up to 4m tall, and more open areas which commonly include the plants Aidia aff. racemosa, Arenga listeri, Leea angulata, Ochrosia ackeringae and Pisonia umbellifera (Environment Australia 1994). Primary Rainforest can be further divided into two types after Mitchell (1974): Deep-soil Tall Closed Forest and Shallow-soil Closed Forest (Type I and II forests in Orchard et al. 2002) where the limestone is close to or at the surface. Hill (1996) censused Christmas Island Hawk-Owls in homogeneous areas of both these Primary Rainforest types.

Marginal Rainforest (after Du Puy 1993) grows on generally shallower soils on the coastal terraces and scree slopes surrounding the island. Many Marginal Rainforest trees lose their leaves in the dry season. This forest is generally lower than Primary Rainforest, often less than 30 m tall, and may include many vines. The canopy height and shape is variable, determined primarily by the degree of exposure to the southeast trade winds. Marginal Rainforest facing south and east has a smooth, wind-pruned canopy and tends to increase in height with increasing distance from the sea cliff. The inland cliffs and scree slopes may have no vegetation or carry a closed forest, depending on the degree of the slope. Trees such as Ficus microcarpa and Dendrocnide sinuata are common there.

Old stockpiles and cleared areas that have not been mined may support a low secondary growth closed forest of colonising trees such as Macaranga tanarius and Claoxylon indicum and an introduced tree Leucaena leucocephala generally less than 10m high. Previously mined areas tend to have very little remaining soil and on them grow dense herblands of a fern Nephrolepis multiflora to 2m high along with introduced scramblers and occasional low trees. Hill & Lill (1998a) did not record owls using old minefields at all, however, they did observe owls hunting in grassy clearings.

Habitat Critical to Survival

All four nests recorded for this species, three in Marginal Rainforest and one in Primary Rainforest, have been in tree hollows in Syzygium nervosum (Hill & Young 1995) and all nest sites are likely to be in tree hollows (Hill & Young 1995). Syzygium nervosum is a common emergent tree particularly in Primary Rainforest where it forms approximately 18% of the forest canopy and 30% of the emergent species (Du Puy 1993). It is commonly greater than 35m high, has abundant tree hollows, unlike most other tree species in Primary Rainforest. Syzygium nervosum is, therefore, a very important tree species for Christmas Island Hawk-Owls and if its abundance was to change in the future, this might lead to a shortage of nest sites. Marginal Rainforest appears to have more species of trees with hollows and may have more potential nest sites for Christmas Island Hawk-Owls. Additional monitoring may show that owls can nest in a number of tree species.

Based on the available information and applying the EPBC Act criteria, habitat critical to the survival of the Christmas Island Hawk-Owl is defined as all Primary and Marginal rainforest, and all secondary growth rainforest that provides suitable habitat. The boundaries of Primary and Marginal rainforest are mapped and provided in Figure 1. Secondary growth rainforest suitable for Christmas Island Hawk-Owls has not been mapped, as there is currently insufficient data on this.

International Obligations

The Hawk-Owl is not listed under any international agreements.

Affected Interests

Commonwealth Parks Australia North, Shire of Christmas Island, Christmas Island Phosphates, Union of Christmas Island Workers, the Asia Pacific Space Centre Pty. Ltd, the Department of Transport and Regional Services (DOTARS), Department of Immigration, Multicultural and Indigenous Affairs (DIMIA).

Role and interests of indigenous people

Not applicable. Christmas Island does not have an indigenous population.

Social and economic impacts

The actions in this plan may have positive and negative social and economic impacts.

Positive social impacts will arise from community education actions that will increase Christmas Islanders knowledge and interest in their own environment. The rainforest rehabilitation program provides on-island jobs, as could the Hawk-Owl monitoring program. Christmas Island endemic birds attract specialist bird watching groups each year that is high value, low-impact tourism.

Negative social and economic impacts arising from implementation of the plan could include greater restrictions due to review of the quarantine barrier. The EPBC Act already provides a regulatory framework for the protection of rainforest on Christmas Island, and one element of this is assessing potential impacts of proposed developments on the listed threatened species. These provisions have the potential to impact on economic activity, for example by adding additional obligations for industry and other development on the island in order to minimise impacts on listed species. This arises from the listing of the species under the EPBC Act invoking a range of protective provisions and offences where a population is to be affected. The magnitude of this potential impact is unknown, as it will vary with the location, size and extent of the activity, proposed or current. However, the plan will aid in determining what could be significant impacts on the Christmas Island Hawk-owl (through defining habitat critical to survival and developing a management plan outside the national park).

Biodiversity Benefits:

Protection of the habitat of the Christmas Island Hawk-Owl provides protection for numerous other listed species (Table 1). Community education targeted at this species will promote awareness of all the endemic forest birds and their conservation needs.

Table 1: Native species listed under the EPBC Act that will benefit from recovery actions listed in this plan (after Environment Australia 2002).
Taxon National Status
Tectaria devexa var. minor E
Carmona retusa V
Christmas Island Pipistrelle (e) Pipistrellus murrayi E
Christmas Island Shrew 1 (e) Crocidura attenuata trichura E
Christmas Island Blind Snake (e) Ramphotyphlops exocoeti V
Christmas Island Gecko Lepidodactylus listeri V
Christmas Island Goshawk (e) Accipiter fasciatus natalis E MF
Abbott ’s Booby (e) Papasula abbotti E M S J
Red-footed Booby Sula sula rubripes M S C J
Christmas Island Frigatebird (e) Fregata andrewsi V M S C J
Great Frigatebird Fregata minor minor M S C J

Notes: (e)=species and subspecies endemic to the island
E Listed under the EPBC Act as Endangered
V Listed under the EPBC Act as Vulnerable
M Listed under the EPBC Act as a Migratory species
MF Within a family listed under the EPBC Act as Migratory
S Listed Marine species under the EPBC Act
C Listed under China-Australia Migratory Bird Agreement
J Listed under Japan-Australia Migratory Bird Agreement

Relationship to other plans

The Christmas Island National Park Management Plan is the strategic nature conservation document for the island. This recovery plan makes numerous recommendations in common with other recovery plans for Christmas Island threatened taxa. Opportunities for sharing resources and points shared in common with other recovery plans are identified in the implementation section of this plan.

Recovery Team

The Christmas Island Hawk-Owl Recovery Team should comprise on-island Parks Australia North staff, Shire of Christmas Island, an environment consultant, Department of Transport and Regional Services, and other members as thought appropriate.

Figure 1: Distribution of primary rainforest and marginal rainforest on Christmas Island and Crazy Ant supercolonies

Figure 1: Distribution of primary rainforest and marginal rainforest on Christmas Island and Crazy Ant supercolonies

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