National Multi-species Recovery plan for the Partridge Pigeon [eastern subspecies] Geophaps smithii smithii, Crested Shrike-tit [northern (sub)species] Falcunculus (frontatus) whitei, Masked Owl [north Australian mainland subspecies] Tyto novaehollandiae
A Recovery Plan prepared under the Commonwealth Environment Protection and Biodiversity Conservation Act 1999
Northern Territory Department of Infrastructure Planning and Environment, 2004
Part C: Known and potential threats
- Biology and ecology relevant to threatening processes
- Identification of threats
- Populations and Areas under threat
For the masked owl (both subspecies), the main ecological features relevant to management are (i) a large home range (and hence low population density); (ii) requirements for large trees with large hollows for nesting; and (iii) diet largely comprising mammals.
There are no data on population density nor home range size for either T. n. kimberli or T. n. melvillensis, but the subspecies T. n. novaehollandiae in coastal south-eastern Australia occupies home ranges of about 5-10 km2 (Kavanagh and Murray 1996). This may suggest that large areas of intact forest are needed to support viable populations, although T. n. kimberli is known to hunt over cleared areas and sugar cane plantations (Garnett and Crowley 2000).
There are no substantial data to suggest that large hollow-earing trees are limiting in northern Australia. However, one study in tall eucalypt forests and woodlands near Darwin (Pittman 2003) found that the population of common brushtail possums Trichosurus vulpecula and black-footed tree-rats Mesembriomys gouldii was near a carrying capacity imposed by hollow availability, and that possums monopolised hollows particularly in woodland fragments. Trends to increased frequency, intensity and scale of fires over the last 50 or so years, and in part more recently magnified by invasion of exotic pasture grasses, have probably resulted in declines in the number of the largest eucalypt trees, especially where these were extensively hollowed (Williams et al. 1999, 2003).
There is some evidence for a broad-scale decline in native small and medium-sized mammals across much of northern Australia over the last century (Winter and Allison 1980; Woinarski et al. 2001; Sattler and Creighton 2002; Pardon et al. 2003). If so, this may have reduced food availability for masked owls. More acutely, in parts of the Queensland range (particularly around horticultural areas) the broad-scale application of the rodenticide Klerat may have led directly to mortality of masked owls (Nielsen 1996; Young and De Lai 1997), although the link is unproven (Garnett and Crowley 2000). This pesticide is now banned (Garnett and Crowley 2000).
Many granivorous birds are known to have declined extensively across northern Australia (Franklin 1999), largely because this resource has been affected by vegetation change caused by altered fire regimes, grazing by livestock and incursions by exotic weed species (particularly pasture grasses). For granivorous animals, the most critical resource changes appear to be related to spatial heterogeneity, with seed resources most likely to be available year-round if the consumer can access a broader range of variability (such as that created or enhanced by fine-scale burning) (Fraser 2000ab; Fraser et al. 2003; Woinarski et al. in press, a). Seed availability is also affected by changes in phenology, vegetation structure and floristic composition, with each of these capable of being affected by imposed fire regimes, grazing and the occurrence of exotic plants (Crowley and Garnett 1999, 2001; Woinarski et al. in press, a).
For the partridge pigeon, the effects of changed fire regimes, invasion by exotic grasses, and prevalence of livestock and feral stock are compounded: not only do these factors change the abundance, timing and spatial pattern of food resources, but they may also directly affect reproductive output. The partridge pigeon nests on the ground, typically with simple scrape nests placed within or around clumps of grass. The nesting period is in the early dry season, a time when fires are prevalent, and the cured grass is highly flammable. Increased frequency or extent of fires in this season is likely to result in increased mortality of eggs and/or young.
But partridge pigeons are also disadvantaged by the absence of fire. Where unburnt, the very dense tall grass layer characteristic of open forests in northern Australia makes movement for ground-feeding vertebrates difficult, and reduces accessibility to food resources (Woinarski 1990).
Given these complex and partly contradictory responses to fire, the best fire regime for partridge pigeons appears to be one of fine-scale patchiness, where burnt and unburnt areas are juxtaposed at the scale of an individual pigeons home-range (Fraser 2000; Fraser et al. 2003).
Partridge pigeons spend almost all of their time on the ground, not only for foraging and nesting but also for roosting at night. This renders them particularly susceptible to introduced predators (dogs and cats), and it is highly likely that populations have suffered increased predation since the arrival of at least cats in northern Australia over the last 150 years. This predation impact may be compounded where fire regimes are now characterised by more extensive intense fires, that result in the absence of ground-layer vegetation over large areas. In the only study of such a situation in northern Australian open forests, Oakwood (2000) found that predation upon the ground-feeding northern quolls Dasyurus hallucatus was greatly increased in the months following extensive fire. Such impacts are likely to be more extreme for the partridge pigeon, given its night-time roosting on the ground, unprotected by such shelters as hollow logs or rock crevices used by northern quolls.
Recent studies in the Top End of the Northern Territory have documented the rapid spread of exotic pasture grasses away from their points of introduction into native open forests and woodlands, even including conservation reserves (e.g. Kean and Price 2003). These grasses replace native species (hence affecting food resources available for granivores such as partridge pigeons), but perhaps even more importantly they greatly increase the fuel load, typically leading to fire intensities about an order of magnitude greater than that typical with native grass understories (Rossiter et al. 2003).
These more catastrophic fires are likely to create a less heterogeneous understorey, with far more extensive uniform burnt areas, thereby reducing reproduction and survivorship of partridge pigeons.
In contrast to this generally dismal recent environmental change, partridge pigeons may derive some, perhaps minor, benefit from the invasion of cane toads (Bufo marinus) into their north Australian range. Cane toads are likely to reduce the numbers of goannas, snakes, quolls and some other predators, and hence reduce the total predation pressure on partridge pigeons. While this may represent some benefit to this particular threatened species, the detrimental impacts of cane toads on Australian fauna overall far outweigh such benefits, and a broad range of measures are in place or proposed to mitigate the spread of toads.
Other than a few notes based on brief observations (e.g. Sedgwick 1988; Holmes and Noske 1990; Robinson et al. 1992; Franklin et al. 1997), there is almost nothing known of the ecology of the northern shrike-tit. The crested shrike-tits of south-eastern Australia characteristically forage for invertebrates in and under the decorticating bark of eucalypt trees (Higgins and Peter 2002), but this microhabitat is not available in the eucalypt forests and savanna woodlands of northern Australia. Instead, the northern shrike-tit appears to forage, for invertebrates, mostly in foliage, branches, the trunk and bark across a range of eucalypt and other tree species. There are no data to suggest that the availability of foliage invertebrates has changed in northern Australia over the last century. The scarcity of records of northern shrike-tits suggests that populations are at very low density. Notes accompanying the brief observational records suggest that the species typically occurs in small groups (2-5 individuals), with these probably occupying a large home range (Robinson and Woinarski 1992). In other subspecies of crested shrike-tit, this large home range has led to it being particularly susceptible to habitat fragmentation, and unlikely to survive in smaller remnants (Higgins and Peter 2002). Nests are placed high in tree foliage (Higgins and Peter 2002), and are probably unaffected by all but the most intense of fires.
All four taxa are affected by broad-scale environmental changes, fueled by altered fire regimes, grazing by livestock and feral animals, and invasion of native woodlands by exotic plants, particularly introduced pasture grasses. The relative impacts and intensity of these factors vary across northern Australia, in relation to land use, land ownership, human population density and availability of management resources (e.g. Yibarbuk et al. 2001; Bowman et al. 2001; Woinarski and Ash 2002). In almost all parts of northern Australia, the very limited current management resources available are insufficient to halt this broad-scale degradation (e.g. Sattler and Creighton 2002).
These pervasive environmental changes affect the four taxa in various ways. Food resources have changed in abundance and/or year-round availability. For example, a decline in small mammals may have contributed to a decline in the abundance of masked owls; changes in grass species composition may have increased the seasonal nadir in seed availability characteristic of the wet-dry tropics (Crowley and Garnett 1999, 2001; Woinarski et al. in press), and hence contributed to a decline in the abundance of partridge pigeons.
In addition to these broad-scale threats, localised more acute factors affect some populations of some of these taxa. Increasing levels of land clearing may reduce the extent of suitable habitat for at least some of these four taxa on the Tiwi Islands, Darwin-Daly area, central Queensland, and possible east and south-west Kimberley.
Feral cats may prey on the almost entirely terrestrial partridge pigeon.
With the exception of acute changes associated with (i) land-clearing, particularly on the Tiwi Islands (affecting the Tiwi masked owl T. n. melvillensis and the partridge pigeon G. s. smithii), around Darwin and the Daly catchment (affecting T. n. kimberli, G. s. smithii and possibly F. (frontatus) whitei), and in north-eastern Queensland (affecting T. n. kimberli), and (ii) possible poisoning through local use of the rodenticide Klerat (affecting T. n. kimberli), most factors threatening these four taxa are pervasive and insidious. These broad-scale factors leading to environmental change (through altered fire regimes, invasion by weeds and grazing by livestock and feral stock) affect all four taxa across their entire range.