VOL. 17 (2) JUNE 1997 71 AUSTRALIAN BIRD WATCHER 1997, 17, 71 -80 Male Superb Lyrebird, with Eastern Yellow Robin watching. Plate 6 Photo: L.H. Smith Building a Viable Lyrebird Population by L.H. SMITH, 36 Duke Street, Kew, Victoria 3101 Summary Available data are used to prepare a model showing how a Superb Lyrebird Menurp novaehol/ondiae population grows, under prescribed conditions, as the annual breeding proceeds from an initial population of breeding Lyrebirds. On the assumption that breeding of the initial stock ceases after their first progeny begin to breed (in their eighth year), the model shows that an egg-to-breeder success rate of about 22% would suffice to establish a viable Lyrebird population. For growth to occur, a higher success rate would be necessary. If the success rate for the egg-to-chick stage is x % and that for the chick-to-fledgling stage is y%, the value of z, the fledgling-to-breeder success rate required to establish a viable population, may be calculated from the formula z = 22/xy%. The effects of predation and forest conditions are discussed. The strong growth of the Lyrebird population in Tasmania, where there are no Red Foxes Vulpes vulpes, is used to demonstrate what is required in mainland Australia to achieve the higher values of x, y and z which are prevalent in Tasmania: namely, vigorous and efficient fox-control programs and appropriate management of Lyrebird habitats. Introduction The decline in the population of the Superb Lyrebird Menura novaehollandiae in Sherbrooke Forest, Victoria, has been attributed to the combined effect of predation and the deterioration of the forest environment caused by the unimpeded accumulation of forest litter and the uncontrolled proliferation of native plants such as ground-ferns, wire-grass, shrubs, trees, etc. The loss of access to its food , which the Lyrebird derives AUSTRALIAN 72 SMITH BIRD WATCHER from the forest floor, has been exacerbated by invasions of exotic species such as English ivy, wandering jew, violets, holly, sycamores, blackberries, etc. The closure of walking and fire-protection tracks has precluded the adoption of appropriate management practices designed to ameliorate conditions, thereby compounding the hazards confronting the Lyrebird (Smith 1994). The Lyrebird lays only one egg each year and there is only one clutch (Lill1986, Reilly 1988, Smith 1988). Lack of data on the events preceding egg-laying precludes further consideration of that aspect here, but Reilly (1970) and Lill (1979a,b, 1980) have provided data on the incubation of the egg and the growth of the chick into a fledgling, and Smith (1982) has described the development of the fledgling into a mature Lyrebird. It is thus possible to construct a model to demonstrate what is involved in the development of a viable Lyrebird population, under certain conditions listed below. Some of the assumptions made may not be realised in practice, but an analysis of the factors involved in the production and interpretation of the model will provide useful guidance in the formulation of any plan to manage an area reserved (not necessarily exclusively) for the preservation of the Lyrebird. There are at present no such 'guidelines'. Lyrebird populations At any given time, the Lyrebird population in a particular area will consist of breeding females, breeding males, immature male and female birds of various ages and, possibly, mature males and females not directly involved in breeding. The status of such birds within the community changes from year to year, but the total number of birds can increase only through the annual breeding success of the mature females (provided that the number of breeding males is adequate), on a sustainable basis. There are no reliable statistics on Lyrebird populations (Smith 1994), but there is abundant evidence that, before Australian forests gave way to various forms of 'development', Lyrebirds were numerous. Over 150 years ago, Gould (1848) found himself 'surrounded by these birds'. Haydon (1846) was woken 'by the singing of several pheasants'; Aflalo (1896) described how, within a few months, two enterprising brothers in New South Wales obtained '500 dozen pairs of tails for sale', by shooting. Campbell (1900) provided further evidence of the abundance ofLyrebirds in earlier days. Conditions within the forests must then h'ave been more favourable to the Lyrebird; predation was apparently not a major problem before the wide dispersal of the introduced Red Fox Vulpes vulpes (O'Donoghue 1931, Rolls 1984). Keast (1969) stated that the indigenous mammalian fauna was low in predators, and Lill (1980) concluded that 'predation was unlikely to have been a significant mortality factor ancestrally'. Food must also have been abundant and readily accessible to sustain strong Lyrebird populations throughout the bird's range. Studies on avian population densities in other countries show that, in general, the richer and more varied the food resources of an environment the denser its bird population (Welty 1982). Food requirements The Lyrebird's diet consists of worms, grubs, centipedes, scorpions, land yabbies, 'hoppers' (Talitrus sp.), spiders, etc., which are obtained from the forest litter and soil (Robinson & Frith 1981, Smith 1988, Barker & Vestjens 1990). The lack of data on the Lyrebird's daily food requirements, yield of food per hectare and the significance of habitat quality in the economy of the Lyrebird, render it difficult to formulate firm conclusions regarding the survival of the Lyrebird in the wild. VOL. 17 (2) JUNE 1997 Building a Lyrebird Population 73 The range of the Superb Lyrebird embraces a wide variety of habitats including rainforests and eucalypt forests (Blakers et al. 1984). I have observed Lyrebirds in the dry open forest of Central Gippsland and north-eastern Victoria, in the dry sandstone areas around Sydney and in the dry ridges of the of the Blue Mountains in New South Wales. In the Mount Buffalo area, some mounds where male birds display had the fine tilth of those of Sherbrooke Forest, whereas others consisted of coarse gravel. Bell (1976) described Lyrebird habitats in which mounds (reflecting the nature of the terrain) varied in composition from soil and humus to pebbles and quartzite. In such cases, it is more difficult for Lyrebirds to obtain sufficient food to meet their needs than it is in the moister soils of the fern gullies of other areas. Lyrebirds must be able to obtain sufficient food to sustain themselves, but there is a limit to the amount of energy which can thus be expended without detriment to breeding efficiency. Female Lyrebirds need extra food to compensate for the energy consumed in supporting their young. Longevity Few data exist on the longevity of birds. It is necessary to distinguish between 'potential longevity' as realised by captive birds and actual longevity achieved by birds in the wild. Welty (1982) listed ages attained by many species of banded wild birds, ranging from 18 to 42 years, whereas others lived 5 to 16 years. In the wild, birds are exposed to a wide range of environmental factors including predation, the absence of which favours captive birds, which may live for 50 years. Birds having delayed breeding are long-lived (Ford 1989). The fact that the Lyrebird requires about 7-8 years to mature and the female lays only one egg each year suggests that this species may have a propensity for longevity and that, under the conditions which prevailed before white settlement, a lifespan in excess of20 years was not uncommon for Lyrebirds, in the wild. There is evidence to support this view. For example, a free-ranging 'pet' Lyrebird lived on a farm in Gippsland for about 20 years before being shot (Godfrey 1905). The celebrated 'Timothy' lived in Sherbrooke Forest for about 25-26 years; his successor 'Spotty' was about 21-22 years old when he died. Two female Lyrebirds exhibited in the Adelaide Zoological Gardens died at the age of 19-20 years (Smith 1988). One of my 'friendly females' nested regularly near Sherbrooke Falls from 1947 to 1960; she was therefore about 20 years old (possibly older) when I last saw her. Chisholm (1960) described a pair of captive Lyrebirds which died as a result of an accident, at the age of 15 years. On the other hand, many of Sherbrooke's Lyrebirds died at an earlier age, almost certainly the result of predation. 'Red' (1958-67), R/R(1959-68), W/W(1960-68) and R/B(1959-68) were among the casualties. Age at maturity The male Superb Lyrebird acquires his adult tail plumage at the age of7-9 years, but the age at which he first mates is not precisely known (Smith 1988). A captive female Lyrebird in New South Wales successfully reared her first chick in her ninth year (Chisholm 1960), and a female Lyrebird in Sherbrooke Forest, banded as a nestling in 1963, produced her first chick in 1970. For the purposes of this exercise, it is assumed that both sexes begin to breed in their eighth year. Sex ratios Information on sex ratios for Lyrebirds is not available. The demonstration by Kenyon (1972) that one male Superb Lyrebird mated with several females during one breeding season suggests that there might have been an excess of females in that -.1 ""'" I Table 1 Theoretical growth of Lyrebird population as breeding proceeds (see text) Year No. breeding Total no. No. chicks Total no. Total no. No. immature Lyrebirds of different ages Lyrebirds mature produced immature Lyrebirds at end at end of breeding season (F + M) Lyrebirds Lyrebirds of breeding . season 1st yr 2nd yr 3rd