VOL. 18 (1) MARCH1999 3 AUSTRALIAN WATCHER 1999, 18, 3-7 Notes on the Breeding Biology of the Tropical Mangrove­ dwelling Yellow White-eye Zosterops luteus

by RICHARD A. NOSKE, School of Biological & Environmental Sciences, Northern Territory University, Darwin, Northern Territory 0909

Summary The Yellow White-eye Zosterops /uteus is endemic to the monsoonal tropics of Australia, where it occurs chiefly in mangrove communities. Opportunistic observations in. the Darwin region indicate that the breeds all year round, and that nests are mostl:,r butlt m the top third of short mangroves. The hm1ted data suggest that the mcubat10n and nesthng penods each occupy between 9 and 11 d~ys, similar to those reported for the closely related Z. latera/is. If these nests are typ1cal, wh1te-eyes may well have the shortest known mcubat10n penods for Australian .

Introduction Very little is known about the life history of most species of tropical Australia, and the endemic Yellow White-eye Zosterops luteus is no exception. This species has a wide distribution across coastal northern Australia from Cape York, Queensland, to Shark Bay, Western Australia, and occurs mainly in mangrove communities (Blakers et al.1984, Johnstone 1990). In the Darwin region, Northern Territory, Crawford (1972) suggested that the Yellow White-eye was nomadic, with numbers fluctuating irregularly, but a long-term study in Darwin Harbour showed that breeding are sedentary within large overlapping home ranges, while non-breeding birds apparently form mobile flocks (Noske 1996). Frith & Davies (1961) and Storr (1977) gave the breeding season in the Northern Territory as October to January. Within Western Australia, however, the breeding season has been described as November to March, May to November, and July to February, in the Kimberley, Pilbara and Shark Bay regions, respectively (Storr 1980, 1984, 1985), suggesting marked geographical variation in the timing of breeding. This note provides information on the breeding biology of the Yellow White-eye from 24 breeding records in the Darwin region, which suggest that the species breeds year-round, and that, like the closely related Silvereye Z. latera/is, it has a remarkably short incubation period. Marchant (1980) has shown that incubation and nestling periods of Australian passerines vary little when data are collected precisely, yet surprisingly few studies of nesting biology provide actual data (time and date of events) or explanations of how such periods were determined.

Method Twenty-four breeding records for the Yellow White-eye in the Darwin region were examined. I follow Courtney & Marchant (1971) and Marchant (1980) in calculating the incubation and nestling periods as the mid-point (or median) between the minimum and maximum possible intervals, plus or minus half the range in hours between the difference of these two intervals. Of the 21 Yellow White-eye nests found, 11 were found at the Palmerston mangrove study site (13 km east-south-east of Darwin Post Office; Noske 1996), another nine in mangals (mangrove communities) within Darwin, and the last on the campus of the Northern Territory University, about 500 m from the nearest mangal. All were found by the author, except one, the data for which were supplied by H. Thompson (fers. comm.). The Darwin region experiences strong seasonality in rainfall, with about 90% o the rainfall (1700 mm) falling in the wet season from November to April. Egg-laying dates were extrapolated from the estimated date of hatching, or AUSTRALIAN 4 NOSKE BIRD WATCHER

the month in which eg~s were first discovered in the nest; and in three additional records of fledglings, from the estimated date of fledging (see Noske 1996).

Results The Yellow White-eye showed no obvious reproductive seasonality, eggs being laid in every month except November (Figure 1). The eight records (33% of total) in September-October suggest a peak just before the wet season, but there was no significant difference in the frequency of nests during the wet (8) and dry (16) seasons overall (Yates' corrected X2 = 2.04, 1 df, P>0.05). The clutch-size was two at eight nests, and three at two other nests, giving a mean clutch-size ( ±sd) of 2.2 (±0.4). Of 19 nests in mangals, 12 (63%) were in Grey MangroveAvicennia marina, three each in Spurred Mangrove Ceriops australis and Stilt Mangrove Rhizophora stylosa, and one in Black Mangrove Lumnitzera racemosa. Except for two of the nests in Stilt Mangrove (which were built at 3.3 and 3.8 m), nests were less than 2.5 m from the ground. The mean ( ±sd) height of 16 nests (including those in Stilt Mangrove)was 1.79 ( ±0.78) m. Of the nine nest-plants for which plant height was recorded, the mean distance between the nest and the top of the plant was 0.47 ( ±0.31) m, and seven were in the top third. The urban Darwin nest was built at approximately 6 min a 10m high Black Wattle Acacia auriculiformis, 5 m from a small power substation building. The high (3.8 m) Stilt Mangrove nest was built only 1.5 m above a nest of the Lemon-bellied Flycatcher Microeca flavigaster, the owners of which repeatedly attacked the White-eyes when they were building the nest. Owing to the generally opportunistic nature of my nest observations, and predation at some nests (seep. 6), only four nests yielded useful data on incubation and nestling periods. One nest was three-quarters built when discovered on 31

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0 J F M A M J J A s 0 N D Month Figure 1. Months of egg-laying for Yellow White-eyes in the Darwin region. VOL. 18 (1) MARCH1999 Breeding Biology of Yellow White-eye 5

Yellow White-eye nest, egg and nestling, Darwin suburbs, June 1998. Plate 2 Photo: R.A. Noske March 1998, and was still empty at 1845 h on 6 April. One egg was present the following day at both 0715 and 2200 h, and was unattended on both occasions. An adult was sitting at 0630 h on 8 April, and two eggs were seen at 0800 h when I reluctantly decided to flush the bird. These data suggest that the eggs were laid in the early morning. One egg had hatched by 0700 h on 20 April but the other egg failed to hatch, and was still present when the nestling was photographed at 1200 h on 27 April. At 0800 h on the following day the young bird was found perched about 20 em above the nest (which, at the time, was 40 em above the water level); at 1500 h, on the low tide, it was perched about 10 em to the side, while an adult sat on the presumably infertile egg. Neither adults nor young were found the following morning. The nest period from start of incubation to fledging of the single fledgling was thus 19 days 19 h ± 15 h, though given that the young bird was still very close to the nest in the afternoon, it is conceivable it was forced to depart somewhat early, owing to the risk of inundation by the high spring tide. Another nest was found at 0700 h on 15 January 1997 with two eggs, which were still present at 0800 h on 24 January, giving a minimum incubation period of 9 days 1 h. Two nestlings had hatched by 1000 h on 25 January but, during that evening, high (7.2 m) spring tides and stormy weather caused choppy waves which apparently soaked the nest. As only one nestling remained the following morning, it is possible that the other was washed away. The surviving nestling was still in the nest at 0800 h on 3 February 1997, and was last seen perched 30 em above the nest at 0800 h on 5 February, when it was presumed to have fledged. The median nestling period was thus 10 days 11 h ( ±37 h). The third nest had two eggs at 1030 h on 19 September 1998, one of which had hatched before 1330 h on the next day. By 0730 h on 21 September the other egg had hatched. Both nestlings were still in the nest at 1130 h on 30 September, but by 0930 h on the following morning, both were perched 3 em above the nest. Thus, the median nestling period was 10 days 0 h ( ±20 h . AUS1RALIAN 6 NOSKE BIRD WATCHER

The fourth nest had two eggs when discovered at 1130 h on 26 January 1988, and there were still two eggs at 1130 h on 3 February, giving a minimum incubation period of 8 days 0 h. One egg had hatched by 0900 h on 5 February, and it is possible that the second egg hatched the next day, as when the nest was checked at 0900 h on 9 February one nestling was distinctly larger than the other. Both young were still present.on 13 February, but the nest was not checked again until 16 days later, when it was empty and undamaged. The difference in size of the two nestlings, only four days after the first had hatched, suggests that incubation at this nest may have begun with the laying of the first egg, with resultant asynchronous hatching. Observations at one nest with a colour-banded attendant (Blue over Yellow, hereafter BY) demonstrated that both sexes participate in incubation, brooding and feeding of young. At one nest, both BY and its unbanded partner were seen incubating the eggs, one for 60 minutes, and the other for 5 + minutes after a 20-minute non-incubating gap. During 150 minutes of continuous observation 3-4 days after the young had hatched, the parents brooded for 72 minutes, of which 45 minutes involved BY (bouts of 28, 2 and 15 + minutes), and the remaining 27 minutes by the unbanded bird (21 and 6 minutes). During the same period BY and the unbanded bird fed the young three times each. At another nest, the single nestling was still being brooded (at 1200 h) 7 days after it had hatched. Nest predation definitely occurred twice. At 1130 h on 30 January 1988 one hatchling and one egg were present in one nest, but by 0900 h on 3 February (less than 4 days later) the nest was empty, albeit undamaged. Another nest had two eggs on 25 September 1988, but by 2 October was empty and damaged.

Discussion My data on laying dates greatly extend the breeding season as stated by Storr (1977), and contradict the claims of Lane (1986) and Longmore (1991) that breeding of the species occurs from October (November) to March. Though additional data are clearly required, it appears that the Yellow White-eye breeds year-round in the Darwin region, possibly with a peak in the late dry season. The relative lack of nests (one) in November-December, compared with the average of 3.7 for any other two consecutive months, may indicate an avoidance of the early wet season and consequent heavy storms. The mean clutch-size for Yellow White-eyes may be lower than that of its primarily temperate Australian congener, the Silvereye. Clutch-size for the Silvereye ranges from two to four, but is usually three, eggs (Beruldsen 1980, Lane 1986), and on Heron Island, Queensland, 97% of 89 nests had clutches of three eggs (Kikkawa & Wilson 1983). In south-eastern New South Wales Marchant (1980) determined the incubation and nestling periods at one nest of the Silvereye as 9 days 17 h ( ± 15.5 h) and 11 days 9.5 h ( ± 15.5 h) respectively, giving an approximate combined nest period of 21 days. These periods are only slightly longer than the estimates given here for the Yellow White-eye. The minimum incubation period for the second nest was about 9 days, and the nestling period for the second and third nests ranged from about 9 (both) to 11 and 12 days, respectively. The maximum combined period for the first nest was 20 days 10 h which, given the above range for the nestling periods, suggests an incubation period of 9 to 11 days (although the young bird may have fledged prematurely). On the other hand, Kikkawa & Wilson (1983) and Wilson & Kikkawa 1988 ave both the incubation and nestlin eriods of VOL.18 (1) MARCH1999 Breeding Biology of Yellow White-eye 7

Heron Island as 12 to 14 days (modal nestling period, 11 days), though they did not present supporting data. Subsequently, Wilson & Kikkawa (1988) determined a modal nestling period of 11 days. Notwithstanding possibly longer nest periods for the Heron Island population, those reported for the Silvereye in south-eastern Australia (Marchant 1980) and Yellow White-eye (data herein) are surely among the shortest known for Australian passerines. Of the 23 passerines studied by Marchant (1980), the Silvereye was the only species with an incubation period shorter than 13 days, though several species had nestling periods as short as, or shorter than, 11 days. It may be significant that the Silvereye was the only studied species belonging to the parvorder Muscicapae (see Sibley & Ahlquist 1985), whereas the remainder were representatives of the ancient endemic Australian passerine assemblage (parvorder Corvi). Some species of the latter have been shown to have substantially longer nest periods than their North Temperate counterparts in the former group (Woinarski 1985), though Rowley & Russell (1991) suggest that this is not a general trend.

References Beruldsen, G. (1980), A Field Guide to Nests and Eggs ofAustralian Birds, Rigby, Adelaide. Blakers, M., Davies, S.J.J.F. & Reilly, P.N. (1984), The Atlas of Australian Birds, Melbourne University Press, Melbourne. Courtney, J. & Marchant, S. (1971), 'Breeding details of some common birds in south-eastern Australia', Emu 71, 121-133. Crawford, D.N. (1972), 'Birds of the Darwin area', Emu 72, 131-148. Frith, H.J. & Davies, S.J.J.F. (1961), 'Breeding seasons of birds in subcoastal Northern Territory', Emu 61, 97-111. Johnstone, R. E. (1990), 'Mangroves and mangrove birds of Western Australia', Rec. West. A ust. Mus. Suppl. 32, Western Australian Museum, Perth. Kikkawa, J. & Wilson, J.M. (1983), 'Breeding and dominance among the Heron Island Silvereyes Zosterops latera/is chlorocephala', Emu 83, 181-198. Lane, S.G. (1986), 'Yellow White-eye', in Schodde, R. & Tidemann, S.C. (Eds), Reader's Digest Complete Book ofAustralian Birds, 556, Reader's Digest Services, Sydney. Longmore, W. (1991), Honeyeaters & their Allies of Australia, National Photographic Index of Australian Wildhfe, Angus & Robertson, Sydney. Marchant, S. (1980), 'Incubation and nestling penods of some Australian birds', Corella 4, 30-32. Noske, R.A. (1996), 'Abundance, zonation and foraging ecology of birds in mangroves of Darwin Harbour, Northern Territory', Wildlife Research 23, 443-474. Rowley, I. & Russell, E. R. (1991), 'Demography of passerines in the temperate southern hemisphere', in Perrins, C.M., Lebreton, J.D., & Hirons, G.J.M. (Eds), Bird Population Studies: Relevance to Conservation and Management, 22-44, Oxford University Press, Oxford. Sibley, C. G. & Ahlquist, J.E. (1985), 'The phylogeny and classification of the Australo-Papuan passerine birds', Emu 85, l-14. Storr, G.M. (1977), 'Birds of the Northern Territory', Spec. Pubis West. Aust. Mus. 7, Western Australian Museum, Perth. --(1980), 'Birds of the Kimberley Division, Western Australia', Spec. Pubis West. Aust. Mus. 1i, Western Australian Museum, Perth. --- (1984), 'Birds of the Pilbara Region, Western Australia', Rec. West. Aust. Mus. Suppl. 16, Western Australian Museum, Perth. ---(1985), 'Birds of the Gascoyne Region', Rec. West. Aust. Mus. Suppl. 21, Western Australian Museum, Perth. Wilson, S.M. & Kikkawa, J. (1988), 'Post-fledging parental investment in the Capricorn Silvereye', Emu 88, 81-97. Woinarski, J.C.Z. (1985), 'Breeding biology and life history of small insectivorous birds in Australian forests: response to a stable environment?', Proc. Ecological Soc. Aust. 14, 159-168. Received 24 March 1998 •