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Sunday, 01 February 2015 07:01

A Threatening Species: The Noisy Miner

Residents of Sydney's suburbs cannot help but notice the abundance of the native honeyeater, the noisy miner (Manorina melanocephala) in their gardens and their aggressive defence of their territory against other birds of all sizes and almost anything else (eg bats, cats, koalas and cows). One wonders if they can cause damage to the ecological balance of areas where they dominate.

NoisyMinerThe answer is yes. The activity of this species was listed as a Key Threatening Process under the NSW Threatened Species Act in Sept 2013 and the federal EPBC Act in March 2013.

What is a Key Threatening Process?

A key threatening process is something that threatens, or could potentially threaten, the survival or evolutionary development of a species, population or ecological community. Examples are pest animals, weeds, diseases or human caused change such as land clearing or climate change.

What Happens when a Process is Listed?

Once a key threatening process is listed under either the state or federal act, a threat abatement plan can be prepared but is not obligatory. Currently there are no countrywide planned actions in place for dealing with the noisy miner as it is a native species that only becomes a threatening process in particular circumstances.

Noisy Miner Habitat

Noisy miners (a medium to large-sized honeyeater 24 to 27 cm; 60 to 90 g) live in sedentary colonies of up to several hundred birds and display a complex array of social behaviours and calls. They breed cooperatively, with non-breeding individuals assisting the breeding pair by feeding chicks. They feed primarily on nectar, lerp and insects.

They are native to the woodlands and open forests of eastern Australia from far north Queensland to Tasmania and west to South Australia in areas with more fertile soils. They prefer open structure at habitat edges but will penetrate large distances into the interior of forests if the habitat is suitable.

They have been advantaged by extensive fragmentation of woodland habitat into small patches, with high edge to interior ratio. In rural areas their prevalence has been exacerbated by land clearing that has left behind woodland remnants and narrow corridors of eucalypts and grazing that has modified the ground layer and mid-storey shrubby vegetation. In urban areas they favour open parklands and grassy yards.

They are typically thought of as a species that inhabits fragmented landscapes but they occur throughout the Brigalow Acacia bioregion of southern Queensland in contiguous remnant woodlands covering several million hectares. These forests have been disturbed by grazing, logging and burning that has simplified the habitat structure. They can dominate blocks several hundred thousand hectares in size and are often recorded more than 20 km from the nearest forest-agriculture edge.

Conversely the miners are not often found in the highly fragmented Buloke woodlands of Victoria’s Wimmera so that these remnants can support a diverse assemblage of small woodland birds. However when habitat restoration programs have used eucalypt and acacia species because they are fast growing the miners have moved in.

They spend most of their time gleaning insects from branches and leaves of eucalypts, they can often be seen feeding in parks and paddocks near eucalypts but they have a preference for short grass and avoid dense tall grass or shrubs. They usually restrict this ground feeding to within 25 m of a nearby eucalypt or fallen timber upon which they can perch looking out for intruders into their territory. It is rare for other honeyeaters to feed on the ground.

Why are Noisy Miners so Bad?

Birds within colonies cooperate to defend the area occupied by the colony against almost all other bird species through aggressive behaviour, physically attacking most other birds. They may break eggs and kill chicks of other birds. Noisy miners are able to exclude almost all passerine (perching) birds that are similar in size or smaller than themselves. Birds larger than miners can be repelled but are not always attacked and may even cooperate with the miners. Tim Low cites experience of butcherbirds, crows and magpies joining into the attacks of other birds and pied currawongs foregoing meals of miner chicks to win acceptance.

As a result of their aggression miners are a very common species (often comprising more than 50% of all birds present) and are increasing in abundance, but not necessarily in range. In the period between the publication of the first (1984) and second (2002) Australian bird atlases, the reporting rate of the noisy miner increased by 10 to 15% in some parts of their range Australia-wide. The evidence of increased prevalence was limited to sites close to the edges of forest and woodland.

The causal link between the presence of a noisy miner colony and decline in the remainder of the bird assemblage has been established through many separate experimental, statistical and observational studies. Conversely, removal of noisy miners from woodland patches generally results in influxes of small woodland birds, even without any change to habitat structure or condition. The removal of noisy miners from seven small (<10 ha) Box-Ironbark woodland remnants in north-eastern Victoria resulted in a major influx of small insectivorous birds such as the jacky winter, scarlet robin and the endangered regent honeyeater.

The loss of woodland habitat is a major cause of the overall decline in woodland birds. However the effect of noisy miner presence on the numbers of other birds is substantially greater than the effects of other recognized threats such as grazing or habitat removal in the surrounding landscape.

Noisy miners are also believed to be culprits in the degradation and dieback of woodlands because their feeding habits do not remove as many herbivore insects as other small birds. Their small colony range reduces the pollination distribution and seed dispersal services that would be provided by other birds.

Tim Low warns that as climate change occurs, noisy miners will handicap eucalypts by reducing the mobility of their pollen. To produce seedlings with a future, trees will need pollen from drier and hotter place, not pollen from the next tree. Droughts that thin forests will aid miners. Lorikeets, red wattlebirds and flying foxes will assume more importance in the future as they can spread pollen widely, little deterred by miners.

Threat Abatement Options

The options for managing noisy miners differ among regions and vegetation structure and types. Generally the need is for promotion of the growth of complex understorey vegetation with more shrubs and grasses.

However inappropriate habitat restoration such as occurred when eucalypts were planted in Buloke woodland remnants, may lead to noisy miners colonizing previously miner free areas.

There is evidence that direct control (culling with the necessary permit) of noisy miners could be relatively humane, low-cost, quickly effective, and long-lasting compared with trapping.

Further Research

The advice to the scientific committee that led to the declaration of the actions of the noisy miner to be a Key Threatening Process was informed by research carried out by a working group established under the auspices of the Australian Centre for Ecological Analysis and Systems. Their report is called Beating the Bullies: Managing Aggressive Manorinas to Restore Bird Assemblages. Research is continuing.

References

Low, T (2014) Where Song Began: Australia's Birds and How They Changed the World. Penguin Books Australia

The noisy miner: challenges in managing an overabundant species

Aggressive exclusion of birds from potential woodland and forest habitat by over-abundant noisy miners (Manorina melanocephala)

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