Thick-billed Grasswren (Eastern Subspecies) (Amytornis textilis modestus) (North, 1902) Recovery Plan

NSW National Parks and Wildlife Service, July 2002
ISBN 0 7313 65143

4 Ecology

4.1 Breeding and Life Cycle

The breeding biology of this species is poorly known. Most of the information available on the life cycle of the Thick-billed Grasswren is from studies of the western subspecies, A. t. textilis, in particular a limited study of a marked population at Monkey Mia, WA (Brooker 1988). Pairs are probably monogamous, but this is not known for certain. Observations of groups of more than two adults, and of an additional adult feeding young and performing a distraction display (Brooker 1988), suggest that cooperative breeding may occur. Schodde (1982a) suspects that pairs remain in the same 4-5 ha of scrubland throughout the year, and suggests that territories may be contiguous in 20-40 ha pockets that are separated by many kilometres of seemingly suitable habitat. During one season at Monkey Mia four nests were found within 100 m of another nest, suggesting that breeding pairs, or groups, may occupy territories of about 1 ha during 'good' seasons (Brooker 1988). The Thick-billed Grasswren probably breeds throughout its range (Blakers et al. 1984; Birds Australia Nest Record Scheme [BANRS]).

4.1.1 Breeding Season

Active nests have been recorded from late June to September (n = 28), with most clutches reported during August (Whitlock 1910, 1924; Schodde 1982a; Storr 1985a, 1985b; Brooker 1988; BANRS). Among central Australian populations, breeding is said to also occur between January and April in response to sufficient rainfall (North 1901-14; Schodde 1982a; Rowley & Russell 1997). At Monkey Mia fewer than usual nesting attempts were recorded during a dry year (Brooker 1988). Adequate rainfall may allow pairs to raise a second brood during a year (Schodde 1982a).

4.1.2 Nest and Eggs

Nests are usually found in low shrubs. In western populations nests are found in a wide variety of shrubs. At Monkey Mia, 25 nests were found in at least eight species of plants, often built amongst more than one species of plant. The eastern population tends to nest in either saltbush or bluebush (Jackson 1910; Whitlock 1924; Chapman 1996; BANRS) with one record of a nest being found in Sandhill Canegrass (Whitlock 1924). Nests have also been reported amongst flood debris (Whitlock 1924). The Monkey Mia population were found to have nests that were situated in the centre of a plant, at a median height of 0.3 m above the ground, and with 0.15-1.10 m of foliage above them (Brooker 1988). Nest structure varied from an open cup to a full dome, entrances usually faced south (17 of 24 nests faced between 90-270°), and invariably faced towards the densest and tallest part of the bush (Brooker 1988). Nests are usually solid structures with thick walls composed of dry grass, twigs of saltbush, and narrow strips of bark. They are usually lined with fine grass, stems and downy plant material, but occasionally also fur and feathers (Jackson 1910; Whitlock 1910, 1924; Gibson 1911; Brooker 1988). It is thought that only the female constructs the nest, but the male may escort her during this period (Rowley & Russell 1997). It is said that nests may be complete for a week or more before laying occurs (Schodde 1982a). Nests are 90-150 mm tall and 80-120 mm wide, with openings 40-70 mm wide at the top (Schodde 1982a).

There are one to three eggs per clutch, although two is average (Campbell 1900; North 1901-14; Whitlock 1910; 1924; Storr 1985b, 1986; Brooker 1988) and they may be laid on either consecutive (Schodde 1982a) or alternate days (Rowley & Russell 1997; Brooker 1998). In WA, recorded egg dimensions are 22-25 mm ´ 16-17 mm (n = 8) (Brooker 1988). In central and eastern Australia, dimensions are 20-22.5 mm ´ 14.5-16 mm (n = 32), and on the Eyre Peninsula, SA, 21-23 mm ´ 15-16.5 mm (n = 15) (Schodde 1982a). Reports on the length of the incubation period vary from 13-14 days (Schodde 1982a) to 15-17 days (Rowley & Russell 1997). Males are thought to share incubation and brooding (Whitlock, 1910, 1924; Brooker 1988). Nesting frequency is generally once per year, although a second attempt may be made during good seasons (Beruldsen 1980).

4.1.3 Young

Young are remain in the nest after hatching (Brooker & Brooker 1987). Males have been reported assisting females in the brooding and feeding of young (Whitlock 1910; Schodde 1982a; Brooker 1988; Rowley & Russell 1997). Young are said to fledge from the nest after 10-12 days (Rowley & Russell 1997). For the first 1-2 weeks after fledging the young are thought to remain huddled together in dense cover for much of the time (Schodde 1982a; Rowley & Russell 1997). Nestlings produce high-pitched alarm notes when removed from a nest (Whitlock 1924) and adults perform 'rodent-run distraction displays' when nests or young are threatened (Carter 1917; Brooker 1988). It is said that juveniles remain with their parents until they attain foraging independence, but disperse soon thereafter (Schodde 1982a).

4.1.4 Breeding Success

There are no measures of breeding success available. Nests are parasitised by Horsfield's Bronze-cuckoo (Chrysococcyx basalis) (Brooker & Brooker 1988). Potential predators include goannas and snakes, Little Crows (Corvus bennetti), feral Cats (Felis catus) and Foxes (Vulpes vulpes) (Carter 1917; Storr 1985a; Brooker 1988).

4.1.5 Social Organisation and Behaviour

The social organisation of this species is not well known, although there is some information from the study of marked individuals at Monkey Mia, WA (Brooker 1988). Birds occur singly, in pairs, or in small groups of up to six (Gould 1865; Whitlock 1910, 1924; Carter 1917, 1924; Brooker 1988; Rowley & Russell 1997, NPWS Wildlife Atlas). Schodde (1982a) suggests that family groups do not stay together for long after the breeding season. Unlike other members of the genus, such as the Grey Grasswren, individuals do not appear to band together in groups during the non-breeding season, but this requires confirmation (Schodde 1882a).

Like other members of the genus, the Thick-billed Grasswren's behaviour is poorly known as they are secretive and elusive (Whitlock 1910; Morgan et al. 1926; Badman 1979; Schodde 1982a; Brooker 1988). Thick-billed Grasswrens have several calls but are thought to vocalize less frequently than any other species in the genus. When alarmed, birds produce a high-pitched alarm note (Whitlock 1924), and they may perch on exposed branches as intruders approach (Carter 1917; Brooker 1988). When disturbed they run to cover at speed, with the head hunched low and the tail erect. Only rarely do they fly above the shrubs (Gould 1865; Campbell 1900; North 1901-14; Carter 1910; Morgan et al. 1926; Schodde 1982a). Once in cover the birds are reluctant to leave and have been known to remain hidden in a bush until the bush was kicked by an observer (Ragless 1998). When an observer remains silent and motionless they may become inquisitive and approach (Carter 1917; Whitlock 1924). In response to a 'chirping' call by an observer, birds may run from, turn to face, or even approach an observer with outspread wings and tail, the head inclined forward, body feathers puffed up, and sometimes issuing a low note (Carter 1917). The function of such behaviour is unclear. Both juveniles and adults may retreat into Rabbit or similar burrows to hide (Carter 1924; Whitlock 1924; Morgan et al. 1926; Coate 1994).

4.2 Habitat

A. t. modestus was formerly recorded in dry, sandy watercourses strewn with flood debris, amongst Sandhill Canegrass (Zygochloa paradoxa) on sand dunes and in areas of tall, dense saltbush and bluebush (Carter 1908, 1910, 1924; Gibson 1909; Serventy & Whittell 1976; Storr 1985b, 1987; Saunders & Ingram 1995; Ragless 1998). This subspecies also inhabited gibber plains with chenopod shrubs growing along watercourses (McAllan 1987, 2000). Furthermore, this subspecies possibly occurred in areas of Nitre Bush (Nitraria schoberi) (Parker 1972), although this historical record could also have been obtained in areas of Grey Saltbush (Atriplex cinerea) or Prickly Glasswort (Salsola kali) (McAllan 1987).

A. t. modestus is now restricted to chenopod shrublands, especially those supporting saltbush and bluebush (Morgan et al. 1961; Parker 1972; Ford 1974, 1987; Badman 1979; Schodde 1982a, 1982b; Garnett 1993; Coate 1994; Gee et al. 1996; Rowley & Russell 1997). It may favour areas along drainage lines where saltbush and bluebush shrubs are taller and thicker (Chapman 1996).

4.2.1 Critical Habitat Components

Nesting: Both A. t. modestus and A. t. myall nest in clumps of Sandhill Canegrass growing on sand dunes, and in saltbush and bluebush. A. t. textilis is known to nest in at least eight species of shrub (Whitlock 1910, 1924; Bryant 1937; Parker 1972; Serventy & Whittell 1976; Brooker 1988; Garstone 1990; Chapman 1996; Schodde 1982a; Rowley & Russell 1997), and will sometimes nest near heavily disturbed areas (Brooker 1989).
Foraging: Birds forage on the ground below shrubs but sometimes in open areas (Whitlock 1910; Schodde 1982a; Chapman 1996; Rowley & Russell 1997). A. t. textilis was once recorded foraging on a beach (Brooker 1989).
Shelter: Thick-billed Grasswrens shelter at ground level within the bases of large, dense shrubs (White 1913; Nicholls 1924; Whitlock 1924; Parker 1972; Schodde 1982a; Blakers et al. 1984; Garstone 1990; Coate 1994). A. t. modestus also shelters in goanna and Rabbit (Oryctolagus cuniculus) burrows, and other natural crevices, or amongst flood debris within dry creek beds (Whitlock 1924; Parker 1972; Schodde 1982a; Coate 1994).

4.3 Diet and Foraging Ecology

To date, there have been no detailed studies of the Thick-billed Grasswren diet. Their diet is known to consist of seeds (from grasses, herbs and occasionally acacias), insects and occasionally fruits (Hall 1910; Hill 1913; Lea 1915; Carter 1917; Whitlock 1924; Lea & Gray 1935; Barker & Vestjens 1991). Insect food mostly consists of small beetles, bugs, grasshoppers, ants and termites (Gould 1865; Keartland 1904; Hall 1910; Hill 1913; Lea 1915; Whitlock 1924; Lea & Gray 1935; Barker & Vestjens 1991; Chapman 1996).

The birds forage throughout early and mid-morning, and sometimes in late afternoon (Schodde 1982a). They forage singly, in pairs, or small groups with individuals usually keeping within 5-20 m of each other (Rowley & Russell 1997). Birds move with rapid zigzagging runs from one bush to another, slowing to search amongst accumulated litter on the ground beneath and around each shrub (Schodde 1982a; Rowley & Russell 1997). There are occasional reports of birds having been observed foraging in the open (Keartland 1904), with one bird seen turning over stones in search of prey (Chapman 1996).