GREY-CROWNED BABBLER (Pomatostomus temporalis). Order PASSERINE - Perching birds (Paper 2, attachment)
summation of, and suggested reason for, the decline in numbers of
Grey-crowned Babbler within the Shannondale population, South of
Grafton, New South Wales
Clarence Valley Regional Water Scheme
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The Grey-crowned babbler is a member of the Apostle bird family, which makes them mainly a ground-foraging species that gleans insects, spiders and small lizards from debris and litter across forest floors and the edges of pastureland where it meets with natural bushland.
Mainly their very noticeable activities within the vicinity of country and village homes have given them a dubious reputation of being fringe-dwellers, so believed to depend on cleared areas as well as natural forests to sustain their family groups and scattered population numbers. This species, however, is not renowned for surviving around urban development, and cleared land preferences mainly relate to stream edges and natural glades and grasslands within native woodlands.
The Grey-crowned Babbler mainly maintains family groups of from 5 to up to 12 birds, consisting of the mated adult pair, fledged offspring, and maturing birds from the previous season's breeding. These numbers alter through eventual dispersal of older adult birds to form their own colonies.
The Shannondale population
Being an apparently well-organised species, with its larger group numbers to aid in defence against predators, it would be expected that the Grey-crowned Babbler should be surviving and thriving happily across its entire preferred range. This, however, is far from the case. Records of public sightings of parties of these birds, which provide amusement to humans by their foraging antics and excited burbling communication, are becoming worryingly infrequent. In the case of the Shannondale population it seems correct to say that it is virtually on the brink of extinction.
For ease of reference we can describe this study area as comprising 4 separate compartments :
(1) A 2,000 acre tract of natural, mostly dry sclerophyll bushland with ephemeral creeks and a significant wetland directly adjoining the southern Shannondale boundary (Lot 119/59, Toothill Parish, Map 9438-11-N)
(2) A 2,000 acre tract of mainly rising sandstone ridges (Lot 90, Blaxland Parish)
(3) A similar sized previous cattle station utilising Shannon Creek to the west, with significant dry rainforest gullies, sandstone outcrops and regenerating forest around cleared grazing land
(4) The Chambigne Nature Reserve to the north-west.
The first three parcels of land now belong to the Clarence Valley Council. All land to the east is taken up by 100 acre subdivided farmlets, with established residences, domestic animals and mainly cleared paddocks. There appears to be no record of Grey-crowned Babblers visiting these properties. Similarly the Chambigne Nature Reserve provides suitable habitat only in confined areas around its eastern boundaries with Shannon Creek, and in an approximately 1x1½ km of red gum woodland on the eastern flats of the Eastern section. To date no Babblers have been recorded within any of this habitat.
The broad acre grazing land across the old cattle property can also be discounted as habitat. Although groups of Babbler may well utilise the forest fringes some distance from the solitary homestead, this area has been recently researched by a number of visiting ecologists, with no records of the species being noted. Similarly, apart from the small known and recorded Shannondale population, these same expansive ecological studies have failed to identify any Babbler families anywhere within either of the 2,000 acre bushland properties.
An unrelated family group is known on occasions to excite the interest of residents around the village of Coutts Crossing, some 6km to the south-east. However, by the size of a family's territory, the considerable areas of grazing land between the two groups, and with no other groups known between the village and Shannondale Estate, it seems safe to say there is no contact between these two known families.
Therefore, concentrating solely on the suitable habitat for Babbler across the 2,000 acre bushland directly adjoining the southern Shannondale boundary, there seems an estimated potential for 80 family groups to be supported in safety and in health by this property. Nevertheless there are no others known to be there. Recent ecological studies brought about by a proposal for a roadway to slice through this natural bushland have identified no members of this very vocal and easily identified species.
In around 1990 the first Grey-crowned Babbler family was first identified on the Shannondale property - a section of the then newly-cleared 1,000 acres Shannondale Estate, a satellite development subdivided into 100 acre lots. At this date there were 7 birds within the group, which was apparently healthy and productive and often seen avidly building nests, seemingly for the sake of activity rather than for any constructive breeding purposes.
Over the last 15 years the territory habitat of this family group has remained virtually unchanged, and possibly is even improved by a prolonged tree-planting program and introduction of garden ponds. The adjoining forest remains healthy, undeveloped and without roadways, and the ephemeral creeklines also remain the same. Yet over this time the Grey-crowned Babblers have been gradually disappearing. Their numbers have never been known to increase above 7 birds, and for a number of years remained static at just 5 individuals. More recently, in a now infrequent visit to the garden, the group was found alarmingly to have dropped to just 4 individuals.
So why, in this seemingly safe haven, which is preserved for wildlife, without cats or dogs, cattle or kids on trail bikes, are the Babblers failing to reproduce and thrive?
To find an answer to this question we must first look at the disadvantages of being a Grey-crowned Babbler in a rapidly changing world.
During the 4-week breeding season (July to February) only the single mated pair within the group will nest and lay eggs, despite other adult birds being capable of reproducing.
Being mainly ground-foragers, they are susceptible to cat or fox attack. However although this may well be the situation in the Coutts Crossing vicinity, within the Shannondale area cats, which were known to be a problem in the first years of human occupancy, have for some years been virtually, if not totally, eliminated by like-minded residents. Similarly foxes are unknown within the so far undeveloped forest and sandstone ridges, which also support a viable population of pure-strain dingo (NPWS).
Babblers' foraging activities are concentrated on leaf-litter and bark of living and dead trees, apparently with more success from trees of rough-barked species. Attempting to catch insects in flight is apparently more often than not unsuccessful, by which the indication is that older, rough-barked trees could possibly be crucial to the Babblers' survival, along with fallen logs and a natural breakdown of vegetation by a biodiverse ecosystem across a forest floor. The forest tract was originally owned by a succession of graziers, who, knowing the tough native grasses were unpalatable to cattle, nevertheless adhered to the stockman's mantra that grazing had to involve regular burning. By this protracted activity old dead and dying trees and fallen logs frequently fell victim to fire. Insect numbers would have fallen in unison, including from among the scorched rough bark of the ironbark trees. With this property now owned by the Clarence Valley Council, this practice of regular burning has ceased.
The Babblers' breeding season corresponds with the rains, which should naturally equate to an increase in abundance of insects to sustain the house-bound family. However the now very clear and acknowledged change in natural weather patterns, with shorter and more erratic wet periods, places considerable pressure on the insect fodder supplies. Being non-aggressive despite their lethal-looking beaks, Babblers are more likely to stand down in the queue of aggressive honey-eaters chasing protein during low nectar periods. Similarly the larger family-related birds, White-winged Choughs, compete in the same forage habitat and will take preference over babblers.
Babblers can breed opportunistically during unseasonable warm wet autumns and winters. However although the changing weather has led to warmer autumns and winters, these periods are also generally accompanied by increased spells of drought.
With the entire family group occupying and competing within the same compacted area, any reduction in nourishment during critical high-energy activities, such as dormitory construction and breeding, must inevitably serve to weaken the family defence, so a possible cause of the number of breeding attempts that end in failure.
The large dormitories these birds construct may successfully protect a larger family group, as presumably would be the normal situation. However a conspicuous roost must disadvantage a smaller family, making them attractive and vulnerable to a night-time raid when they lack ability to become actively or passively mobile.
Likely causes of the decline
As with most threatened species the recognised threat to the babblers' viability is land clearing and predation by feral animals. Despite this broad understanding, and protection by law, such knowledge and law is rarely transferred to determent or deferment of any proposed development activity.
What is not so widely understood or researched is the ecological imbalance created by such land-clearing and development activities, and the effect of this on any natural predators.
Under normal circumstances hawks and kestrels would prey on Grey-crowned Babblers. However such attacks are not often successful within the preferred woodland habitat of this species, and would in the main serve only to keep their numbers in check. The true threat,therefore, must come at night, when the birds are at their most vulnerable and unprotected by their small communities.
The common brush-tailed possum is a known predator of eggs and nestling chicks. It is a savage hunter, equipped with ferocious teeth and claws despite its prevalence to eating nectar and blossoms, and has been known to take adult species of small birds. Possums are tough survivors, able to adapt to change and disturbance, and known to be able to sleep and gain relative rest even in odd exposed places while under daytime attack by birds shocked at finding their space invaded by a displaced animal. The main predators of brush-tailed possum are other possums, which are fiercely territorial and will engage in vicious fights with any unknown outsider. Where individuals are known within the territory, however, then the larger forest owls become the main predator of young brush-tailed possums.
Uncontrolled, unsympathetic land clearing has served to concentrate the previously widely scattered and solitary brush-tailed possums into quite small pockets, where they are forced to eke out a precarious existence by any method available to them. This often involves getting along to a certain degree with other possums, which they are able to do provided each animal keeps a respectful distance. The other predators, the owls, unfortunately are proving to be not quite so adaptable.
Loss of hollow-bearing trees and dead standing stags by development, fire, and natural attrition are ongoing, and our previous careless land-management practices such as rural subdivision and over-logging has ensured a critical shortage of mature early-hollow stage trees to take their place. Large owls require large territories, and a decrease in viable territories is creating a corresponding decline in owls.
Of the 3 portions of bushland habitat described, none in entirety form any large tracts of suitable habitat for Babbler other than the ex Lot 119/59 Toothill Parish, while the joint portions together are surrounded by cleared farmland and human occupancy. By today's habitat depleted standards the many species of wildlife within this relatively large pocket possibly fare considerably better than those in most other areas. However, with a lack of predators the concentration of possums is now believed by the author to be the major threat not only to the Grey-crowned Babbler but to most species of smaller birds.
During the 2003 drought it was not uncommon to see 8 possums scavenging together on the lawn of Lot 2 Shannondale Estate, in search of scraps and overlooked seed tossed out for the birds during the daytime. Mainly these animals kept a discreet distance from one another, but inevitably fights often erupted. Despite their antisocial nature their numbers only began to dwindle when the practice of tossing out edible snacks was stopped and the animals dispersed. In the interim, though, other small birds that made a habit of nesting within the safe environs of the house, such as willie wagtail and blue fairy-wren, had made three efforts at breeding before being disrupted by possum invasion.
The domino effect
In the case of the Shannondale Babbler population, if the brush-tailed possum is in fact the major threat, then this threat is about to be further compounded by clearing for a road, planned to dissect the natural bushland for construction of a proposed dam planned to provide water to the Clarence Valley's and Coffs Harbour's joint growing communities.
Ecological studies for the proposed access road to the site conveniently found no babblers, nor any other type of threatened bird species directly within the proposed corridor to be cleared. Reports ensuing from these surveys noted dead stags and hollow-bearing trees to be removed, but depicted considerably smaller numbers than in fact will be the case.
In truth 74 stags, including 3 in the early hollow-forming stage, are to be cleared by the roadway. 21 of these were not counted or recorded in the final reports. 25 old-growth habitat trees containing well-formed and/or early-forming hollows and pipes are also to be destroyed, and, possibly even more worrying, so are 112 mature trees in the late stages of growth, just forming, or already bearing, some small hollows and pipes. These trees are considered insignificant by the proponents as they provide no immediate habitat, their removal apparently needing no justification.
The dam and pipeline construction is also set to remove further considerable acres of suitable habitat for large owl species known in the area, and will inevitably further compact the brush-tailed possum habitat. The clearing and access roadwork alone could prove a final straw to drive this tenuous Grey-crowned Babbler family to extinction.
The common Brush-tailed Possum appears to be the only feasible cause for the Grey-crowned Babbler's decline from the otherwise healthy-seeming Shannondale bushland.
The possums' natural predators, the large forest owls, are already compacted by surrounding clearing and subdivision into small, quite possibly unsustainable territories.
Removal of any numbers of current hollow-bearing or maturing recruitment trees from this bushland habitat is likely to have a further possibly devastating impact on the owls. In any case it will almost certainly remove further numbers of natural predators from the possums' territory.
It seems quite feasible, therefore, that should any development be allowed to occur within this remaining natural forest, then the domino effect will fall too heavily on the Grey-crowned Babbler, quite likely driving it to extinction.
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Patricia Edwards (Scientific Licence S11209) Page