AUSTRALIAN 46 KLAU BIRD WATCHER

AUSTRALIAN BIRD WATCHER 1985, II, 46-48

Spotted Harriers Circus assimilis Nesting on the Nullarbor

By W.L. KLAU, Box 436, , W.A 6430

Summary After heavy autumn rains on the Nullarbor Plain (S.A) in 1983, Spotted Harriers Circus assimilis arrived and commenced breeding. They built their stick nests near telegraph poles; nest building took place in July and the young were almost fledged in mid September. There were at least 10 nests in a distance of 140 km, and the two closest nest were 180m apart. One nest was built on the ground and the eggs incubated, but it may have failed. By December most of the Harriers had left the area.

Introduction During the months of February, March and April of 1983 heavy rains fell on the Nullarbor Plain at Cook (30°38' S., 130°24' E.) and surrounding areas in South . The rains extended 140 km westward to about Deakin on the border. The rains also extended for an unknown distance in other directions from Cook, which received about 250 mm of rain during this period. Lush growth appeared soon after the rains, and by July the whole area was a green carpet. Later the wildflowers appeared. Several species of small birds became numerous after the rains, the most abundant being Richard's Pipit Anthus novaeseelandiae, Brown Songlark Cinclorhamphus crura/is, Rufous Songlark C. mathewsi, Crimson Chat Ephthianura tricolor and Orange Chat E. aurifrons. These were probably the main source of food for the Spotted Harriers Circus assimilis which arrived and commenced breeding. Australian Kestrels Falco cenchroides also appeared in great numbers. Wedge-tailed Eagles Aquila audax were observed, but their numbers were about normal. No other raptor species were observed. By December 1983 few Spotted Harriers remained, and they subsequently disappeared completely.

Spotted Harrier nests As is usual after rains on the Nullarbor, Little Crows Corvus bennetti started building nests on telegraph poles along the Trans Australian Railway. The Crows usually build their nests on the bottom cross-arm of the telegraph poles, and very rarely do they build on the top arm. While driving a train to Cook on 16 July 1983 I observed numerous active Crows' nests, and also some nests that looked clumsily built. At first I thought that these nests were corvid nests blown over by strong winds during construction, which happens quite often. Spotted Harriers were observed, but I didn't connect them with these nests until later. On the next trip on 22 July I observed Spotted Harriers sitting on these large, rather flat structures. It was then that I realised that these flimsy nests belonged to the Harriers. They apparently built the nests themselves instead of adding to old Crow nests, since the Harrier nests were built across the wires VOL. 11 (2) JUNE 1985 Spotted Harrier Nesting on Nullarbor Plain 47 adjacent to the cross-arms whereas the Crows' nests were usually built between the four insulators on the cross-arm. On the return trip on the following day, I counted 10 nests occupied by sitting Harriers, and there were still nests under construction. Some nests may have been overlooked. Most of the nests were adjacent to the top cross-arm and only two were near the bottom arm. On 4 August the nests were still occupied by sitting birds, but no nest count was made on this trip. On my next visit to Cook on 18 September I observed nearly fledged young in some of the nests. On the next trip on 18 October all nests had disappeared. The nests were most probably removed by the linesmen, who are instructed to remove all nests from the telegraph lines. Most linesmen do not remove nests until the young have left unless the nests interfere with the telephones, in which case they are removed immediately. The Harrier nests were within a distance of 140 km, and the two closest nests were only four telegraph poles apart. As the poles are approximately 60 m apart, these two nests were about 180m apart. The nests near the top arm were about 5 m and those near the bottom arm 4 m from the ground. At no time was I able to examine the contents of the nests; only when the young were nearly ready to fledge could they be seen from where I was sitting in the locomotive. On 4 August 1983 I found another unusual nest site for a Spotted Harrier. About 500 m south of the Cook townsite at 0900 h, I heard an unidentified noise coming from among dense grass against an earthen mound about 2 m high. On approaching the scene of the noise, a large bird flushed from the grass and it was identified as a Spotted Harrier. When I approached the site from which the bird was flushed, I found a nest containing three eggs. The nest was on the ground on trampled grass, against the mound amongst tall grass. It was a large, shallow nest, constructed of large twigs and lined with green leaves. As I was unsure that I had made a correct identification of the bird, I again approached the site at 1330 h and this proved my identification correct. This nest was again examined on 14 August, and it still contained three eggs which were warm. On 18 September the nest was empty, with only egg-shell fragments in the nest. This indicates that the eggs may have hatched, but it is doubtful that the young left successfully. This nest has been recorded on a nest record card for the RAOU Nest Record Scheme.

Discussion Recent field guides and handbooks, e.g. Macdonald (1973), Reader's Digest (1976) and Pizzey (1980) list the Spotted Harrier's nest sites as trees and bushes only, and make no mention of telegraph poles or the ground as nest sites. In fact, Morris (1976) stated the Spotted Harrier to be the only harrier that does not nest on the ground. My observations indicate that in good seasons in treeless areas, Spotted Harriers will nest across the wires adjacent to the cross-arms of telegraph poles and occasionally on the ground. In the only detailed study undertaken on Spotted Harriers, Baker-Gabb (1984a) found them to nest only in trees. He has argued convincingly (Baker­ Gabb 1984b) that the ancestral condition in the Spotted Harrier was ground nesting, and that this species evolved the tree-nesting habit because of high predation pressure in arid areas. It is significant that the ground nest at Cook was apparently unsuccessful. AUSTRALIAN 48 BAKER-GABB: Spotted Harrier: Autumn Breeding BIRD WATCHER

Acknowledgements I wish to thank Mr S. Debus for typing the manuscript and for suggesting some amendments to it.

References Baker-Gabb, D.J. (1984a), 'The breeding ecology of twelve species of diurnal raptor in north­ western Victoria', Aust. Wildt. Res. ll, 145-160. --(1984b ), The evolution of tree-nesting and the origin of the Spotted Harrier', Corella 8, 67-69. Macdonald, J.D. (1973), Birds of Australia, Reed, . Morris, F.T. (1976), Birds of Prey of Australia, A Field Guide, Lansdowne, Melbourne. Pizzey, G. (1980), A Field Guide to the Birds of Australia, Collins, Sydney. Reader's Digest (1976), Complete Book ofAustralian Birds, Reader's Digest Services, Sydney .

Short Note- • Autumn Breeding by the Spotted Harrier Circus assimilis Spotted Harriers Circus assimilis are unique among the harriers of the world for their well-known habit of nesting in trees. It is not generally recognised that they sometimes exploit another habit unique among harriers: that of having two breeding seasons per year. Whilst driving in the of New South Wales some 25 km south of the Lachlan River on 9 July 1984, Phil Maher and I observed two recently fledged Spotted Harriers perched in Callitris trees near the road. From subsequent observation of their inept flight, their inability to catch a mouse in the air which the female dropped to them, and my considerable field experience with this species (Baker-Gabb 1982), I estimated.that they had been out of the nest for less than a week. This observation is of interest because it means that the eggs must have been laid in late April. Throughout Australia most Spotted Harrier clutches are laid during July-October (Campbell 1900, North 1901, McGilp 1934, Cupper & Cupper 1981, Baker-Gabb 1982). However Storr (1973, 1980) and Cupper & Cupper (1981) have also reported a very few nests with eggs during March-April from both northern and southern Australia. This raises the quesJion as to whether Spo_tted Harriers sometimes breed twice per year (autumn and spring) as Black-shouldered Kites Elanus notatus do if food is abundant (Hollands 1984), or whether the autumn breeding would be the sole attempt for the birds concerned that year. House Mice Mus musculus were very numerous in the area of the Riverina in which our observation was made. References Baker-Gabb, D.J. (1982), Comparative ecology and behaviour of Swamp Harriers Circus approximans, Spotted Harriers C assimi/is and other raptors in Australia and New Zealand, Ph.D. thesis, -Monash University. Campbell, A.J. (1900), Nests and Eggs of Australian Birds, Wren, Melbourne. Cupper, J. & Cupper, L. (198 1), Hawks in Focus. Jaclin, Mildura. Hollands, D. (1984), Eagles, Hawks and Falcons of Australia, Nelson, Melbourne. McGilp, J.N. (1934), T he hawks of ', S. Aust Om. 12, 225-293. North. A.J. (190 1), Nests and Eggs of Birds Found Breeding in Australia and Tasmania, White, Sydney. Storr. G. (1973), List of Queensland Birds, W.A. Museum Spec. Pub!. No. 5. . -- ( 1980). Birds of the Kimberley Division, Western Australia, W.A. Museum Spec. Pub!. No. II. Perth. By D.J. BAKER-GABB, c/o RAOU. 21 Gladstone St., Moonr?e Ponds, Victoria ~9 •