VOL. 13 (3) SEPTEMBER 1989 73 AUSTRALIAN BIRD WATCHER 1989, 13, 73-77 Regular Double-brooding by Clamorous Reed-Warblers Acrocephalus stentoreus in the Canberra Region by M. LENZ, 8 Suttor Street, Ainslie, A.C.T. 2602 Summary During territory-mapping of the breeding bird community on the campus of the Australian National University, Canberra, in the 1978-79 season 36 territories of the Clamorous Reed-Warbler Acrocephalus stentoreus were recorded. In 29 territories dependent young were heard and/or seen (a search for nests was not undertaken) between early November and early March; in all 39 broods were recorded. Of these 11 were found between early November and late December, 28 between early January and early March. In 19 of the territories only one brood was noted, however in 10 both a first and a second brood were recorded. Additional observations in other years in the same area and elsewhere in the Canberra region have confirmed that at least part of the Reed-Warbler population raises two broods in a season. Introduction A number of Australian songbirds have quite extended breeding seasons of six or more months, whereby the breeding season is defined as the period over which nests with eggs have been recorded (e.g. Morris et al. 1981). In some species the nesting period may not be of uniform length throughout the entire range, with populations farther south or at higher altitudes breeding for a shorter time than birds in northern or lowland regions (Beruldsen 1980; Canberra Ornithologists Group 1985; Frith 1984; Morris et al. 1981; Schodde & Tidemann 1986). The duration of the breeding season may also differ from one year to the next depending on rainfall patterns and other environmental variables (Davies 1979). However, individuals in many species with a prolonged breeding season regularly raise more than one brood within that time (Beruldsen 1980; Schodde & Tidemann 1986). Other species are assumed to be only single-brooded despite confirmed egg-laying over periods of six months or more. An interesting species in this context is the Clamorous Reed-Warbler Acrocephalus stentoreus. This species is generally thought to breed only once per season even though the nesting period can last up to six months. Only in a few publications has the possibility of double-brooding been mentioned. Frith (1977) and Schodde & Tidemann (1986) stated that nesting takes place between September and February. Similarly Serventy & Whittell (1976) and Morris et al. (1981) reported dates of egg-laying from September to February for south-western Australia and New South Wales respectively. Beruldsen (1980) gave a nesting season from September to November or December with a breeding frequency of once, perhaps twice on occasion. Schodde & Tidemann (1986) indicated that in south-western Australia 'there may even be two immigrant waves of breeding birds ... , in October-November and December-January'. The source for this comment is probably Brown & Brown's work at the Middlesex Field Study Centre south of Perth. Van Delft (1988) repeated the statement of Schodde & Tidemann and cited Brown & Brown (1983) as the reference; the correct one is, however, Brown & Brown (1985). In the latter paper Brown & Brown summarised some aspects of their work on a colour-banded population of Reed-Warblers. They actually said: 'The arrivals are in two waves, the first to nest in Oct-Nov, and the second to nest in Dec­ Jan, making a peak in December, as second broods of earlier nesters coincide with the first nests of the second wave' (emphasis not in original). R. Brown also confirmed AUSTRALIAN 74 LENZ BIRD WATCHER double-brooding by the same pair, at least by one partner, in recent correspondence to me. Likewise, Sue Gordon has observed double-brooding in a population near Perth (pers. comm.). Campbell (1900) quoted a Mr Lau from South Queensland as '[The Reed-Warbler] ... comes to Queensland in the latter end of August, and leaves, after rearing two broods, in February'. It is surprising that the observations on two successive broods by the same bird(s) within a season have not been included in the general accounts of the biology of A. stentoreus cited above. Double-brooding in this species should not be such an unusual event when one considers that in several other species of Acrocephalus from the northern hemisphere with shorter breeding periods than in the Clamorous Reed-Warbler, individuals in some populations raise two broods regularly (Beier 1981; Bibby 1978; Bibby & Thomas 1985; Nisbet & Medway 1972; Olschlegel 1981; Springer 1960). I report here some indirect evidence from the Canberra region indicating that double­ brooding in A. stentoreus is not restricted to south-western Australia and perhaps Queensland, but occurs quite commonly in this south-eastern part of the species' range. Plate 25 Sullivan's Creek, ANU Campus, Canberra, A.C.T. Photo: M. Lenz Study area and methods In the 1978-79 breeding season I surveyed the breeding bird community of the campus of the Australian National University in Canberra (35 "17 'S, 149 "fJ1 'E) with the modified territory mapping method (Lenz 1978). Part of the stuqy site, the mouth of Sullivan's Creek (c. 16 ha) and a few adjoining ponds, provided nesting habitat for Clamorous Reed-Warblers. Descriptions of the Sullivan's Creek area are given in Garnett (1978, 1980) and Martin et al. (1979). During surveys I mapped the locations of singing males, of birds carrying nest material or food, and any other activities indicating breeding. During the previous breeding season I had noticed some Reed-Warbler fledglings in late February. As singing birds are first noticed at the site by late September/early October and there are signs of breeding by mid October, the late fledglings could well have originated from second broods. I checked territories as often as possible between October and early March and looked for fledglings (by call and/or sight) on two well-separated occasions. In practice, I could monitor the area several times from October to the middle of December. On these days I walked along the shores of the creek and around the ponds, stopping only briefly at each territory. During later visits until early March I was able to spend more time at each territory, searching for signs of Reed-Warbler chicks (mostly begging calls). VOL. 13 (3) SEPTEMBER 1989 Clamorous Reed-Warblers: Regular Double-brooding 75 Results A total of 36 Reed-Warbler territories was established in the Sullivan's Creek area. In 29 of them dependent young were heard or seen at least once between early November and late February (early March) (Table 1). From the distribution of dates with records of chicks, early and late broods could be identified. However, a clear separation of early (first) and late (second) broods, excluding replacement clutches, is only possible with the data from 10 territories in which two successive sets of young were recorded at different times well apart from each other: young from first clutches between 6 November and 29 December and from second clutches between 4 January and 24 February (Table 1). Table 1 Dates (split into early (E), middle (M) or late (L) part of the month) on which young (mostly fledglings) of the Clamorous Reed-Warbler were recorded in territories at Sullivan's Creek on the campus of the Australian National University, Canberra. Territory November December January February March number E M L E M L E M L E M L E 4 3* 8 20 7 4* 24 7 5* 6 24 20 7 6 30 8* 18 29 21 9 5 10* 18 22 30 21 11* 18 20 5 12* 30 20 13* 30 30 24 14 5 15 4 30 16 4 17 5 4 18 4 21 4 22 4 26 6 27 18 4 28 21 29 30 30 21 31 30 32 30 33 30 34* 18 21 35* 18 30 36 30 *Territories with two successive broods Discussion In the strict sense I have not proved double-brooding in the Clamorous Reed-Warbler because the observations stem from a population of unmarked birds and hardly any details of the actual breeding chronology for each territory were known, except for dates when young could be either heard or seen, i.e. were close to leaving or after AUSTRALIAN 76 LENZ BIRD WATCHER having left the nest. Again, the exact age of the chicks at the time could not be established. However, out of the 28 territories with young, 10 of them or 34% had young noted on two occasions with the time interval sufficiently long to assume two successsive broods had been raised in the same territory. In populations of the Sedge Warbler A. scirpaceus from Germany the minimum time from nest building to independence of fledglings took 45-46 days (Beier 1981; t5lschlegel 1981). In a British one the time needed from eggs to independence of hatchlings was 40 days (Bibby 1978). For the Great Reed-Warbler A. arundinaceus in Japan the shortest interval between successive broods was 44 days (Ezaki 1987), in southern Germany 47-50 days (Beier 1981). The periods for incubation (14-15 days) and raising the young to the fledgling stage (14-16 days) as reported for A. stentoreus (Schodde & Tidemann 1986) correspond to those of other species of Acrocephalus. One can assume therefore that the time interval between successive broods of about 45 days may also apply for the Australian species. Dates between observations of young in the 10 territories, even accounting for the fact that the age of the chicks (mostly fledgling stage) was not known at the time of recording, are sufficiently spaced out as would be typical for successive broods in other species of Acrocephalus.