97 AUSTRALIAN FIELD ORNITHOLOGY 2011, 28, 97–113 The Breeding Biology of the Dusky Honeyeater Myzomela obscura in the Northern Territory, and the Importance of Nectar in the Diet of Nestling Honeyeaters RICHARD A. NOSKE1 and ASHLEY J. CARLSON2 1Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, Northern Territory 0909 (Email: [email protected]) 2P.O. Box 4074, Forster, New South Wales 2428 (Email: [email protected]) Summary The breeding biology of the Dusky Honeyeater Myzomela obscura is poorly known, despite the species’ wide distribution and use of a broad range of habitats. In the Northern Territory, breeding was recorded in all months, but just over 50% of estimated laying dates were in April and May, corresponding with the transition from the wet to the dry season. In Darwin, nests were placed 1.0 to 9.0 m above the ground in a variety of trees or tall shrubs, including exotics. The size of clutches or broods never exceeded two (n = 9). At two closely observed nests, the incubation period was 12.4 and 12.75 days, similar to another myzomeline honeyeater. Nest-attentiveness was ~65% over 2 days at one nest, similar to the few temperate-zone Australian honeyeaters for which such data are available. The nestling period at the only successful nest that we observed was 14.3 ± 0.7 days, consistent with slightly smaller species in other honeyeater genera. Unlike most honeyeater species, nestling Dusky Honeyeaters hatched with much down. Diurnal brooding represented 20–28% of the presumed female’s time during the first 4 days after hatching, but had ceased by Day 6, earlier than in temperate-zone honeyeater species. The mean rate of food provisioning was very low (8.9 feeds/h) compared with that of most other honeyeater species studied to date. Evidence suggests that Dusky Honeyeaters may feed their young largely on nectar. Introduction The Dusky Honeyeater Myzomela obscura ranges from North Maluku in eastern Indonesia through southern New Guinea to coastal and subcoastal Northern Territory (NT; the nominate subspecies, M.o. obscura) and Queensland (M.o. harterti) (Higgins et al. 2001) in Australia. It occurs in a variety of habitats, at recorded densities of 0.06–0.64 bird/ha (Higgins et al. 2001). Near Darwin, NT, it was most common in eucalypt forest with a dense understorey and semi-evergreen monsoon vine forest (0.4–0.6 bird/ha: Woinarski et al. 1988). In Kakadu National Park, NT, it was most abundant in monsoon rainforests, particularly in sandstone-spring monsoon rainforests (Brooker et al. 1990; Woinarski 1993). The species appears to be sedentary within large home-ranges. In heterogeneous native vegetation on a 470-ha plot 40 km south-west of Darwin, it was one of five species of honeyeaters that were present throughout a 39-month study (Franklin & Noske 1998). Recapture rates (13%) of banded birds (n = 149) were slightly lower than for the much commoner, slightly smaller, Brown Honeyeater Lichmera indistincta, and just over 50% of recaptured Dusky Honeyeaters were retrapped within 200 m of the banding site, with the maximum recorded movement being 1.2 km, and the maximum time elapsed to recapture 29 months (Franklin & Noske 1998). Despite this wide distribution and the broad habitat requirements, the breeding biology of this species is poorly known, with only six nest records in the Nest AUSTRALIAN 98 NOSKE & CARLSON FIELD ORNITHOLOGY Record Scheme (NRS) of Birds Australia up to 1998 (Higgins et al. 2001). Based on the gonadal condition of eight specimens collected in Darwin and Arnhem Land, NT, Deignan (1964) suggested that the ‘breeding season is of long duration in the [Northern] Territory’. Indeed, Noske & Franklin (1999) recorded breeding in almost all months of the year in the monsoon tropics of the NT. This is in contrast with the situation for the congeneric Red-headed Honeyeater Myzomela erythrocephala and several other species of honeyeater (Meliphagidae) in the region, which breed during the dry season (May–October), when nectar is most abundant (Noske & Franklin 1999; Franklin & Noske 2000). In this paper we describe the breeding season and clutch-size of the nominate subspecies of Dusky Honeyeater breeding in the NT from opportunistic observations and published sources, as well the incubation and nestling periods, and parental care at two nests in suburban Darwin. Methods We collated breeding records for the NT from two sources: (1) opportunistic records in Darwin (12°28′S, 130°52′E) since 1988 (by RAN), including those reported by Noske & Franklin (1999); and (2) the published literature, importantly including two records of breeding within 50 km of Darwin (Crawford 1972), three from Nhulunbuy (12°15′S, 136°45′E: Boekel 1976; Officer 1976), one from Kakadu National Park (12°33′S, 132°18′E: Brooker & Parker 1985) and one from Pine Creek (13°49′S, 131°50′E: Rix 1970). The NT lies within the Australian monsoonal tropics, which experiences little seasonal fluctuation in temperature and light availability. However, rainfall is highly seasonal, with >90% falling between November and April (the wet season). Mean annual rainfall at Darwin is ~1700 mm [1941–2009; http://www.bom.gov.au/nt (accessed 10 July 2010)], whereas at Nhulunbuy, farther to the east, mean annual rainfall is slightly lower and less seasonal, and the wet season begins and ends ~1 month later than in the west (Langkamp et al. 1979). Consistent with the literature, we define the breeding season as the months in which eggs are laid. As exact dates for laying were rarely determined, however, we have estimated the month of laying by extrapolation from dates on which eggs, nestlings or fledglings (which comprised most of our observations) were recorded. For example, Crawford’s (1972) breeding records comprise a bird sitting on a nest on 1 June 1968, and a dependent juvenile on 6 December 1970; we have estimated that laying for these birds occurred in May and November, respectively. Where only eggs were recorded, we have assumed that they were laid in the same month, unless the date fell within the first week, in which case half of the clutch was attributed to the previous month. Unless otherwise stated, records of building were excluded from the analyses because of the well-known tendency of many Australian birds to abandon nests before laying in them. In accordance with Marchant (1980), the duration of the incubation period at each nest was calculated from the midpoint between the maximum interval (i.e. from the last observation before the clutch was completed to the first observation of all eggs hatched) and minimum interval (i.e. from the first observation of the full clutch to the last observation of unhatched eggs), plus or minus half the range in hours between the difference of these intervals; the nestling period—i.e. from hatching to fledging—was calculated in a similar way. The contents of some nests were observed by using a mirror mounted on a pole. Detailed observations were made at two nests in Darwin, one in the suburb of Wagaman (12°23′08˝S, 130°53′08˝E) and the other in Jingili (12°23′21˝S, 130°52′18˝E). Both nests were in typical suburban yards consisting of a lawn with isolated exotic shrubs and palms (Wagaman) or garden beds of young native shrubs (Jingili). The Wagaman nest was discovered on 9 April 2008 during its construction, and was watched over 2 days during the incubation period (total 7.5 h) and 1 day during the nestling period (4.2 h) from a distance of 7 m, with the aid of 10 × 42 binoculars. The Jingili nest was built in a pot-plant on a concrete slab under the eaves of a house, beside a carport. It was found on 18 July 2009 during its construction, but observed only sporadically until the eggs hatched, after which nest activity was recorded for up to 6.6 h on each of 9 days during the nestling period (total 29.7 h), using a video camera set up near an open louvred window with a flyscreen inside VOL. 28 (3) SEPTEMBER 2011 Breeding Biology of Dusky Honeyeater, NT 99 a room of the adjacent house. Although the camera was only 1 m from the nest, the birds appeared not to be affected by it, or indeed by the frequent motion of, or sounds made by, human occupants of the room. Consistent with the literature (e.g. Conway & Martin 2000), we refer to bouts of incubating or brooding on the nest and absence from the nest as on- and off-bouts, respectively. Nest-attentiveness (the percentage of time spent by the sitting bird on the nest) was calculated from all video-recorded samples combined. Because video-recording sessions usually started when the incubating or brooding bird was already sitting or absent (incomplete bouts), only complete periods of absence from the nest were considered in calculating the total video-recording sample time, and hence feeding rates and brooding effort. The first or last incubation or brooding bout was discounted from the analysis so that the number of on- and off-bouts was equal. All means are given ± Standard Deviation. The species is sexually monomorphic in plumage and bare parts and, consequently, the sex of the birds attending the nest could not be determined. No more than two adults were observed at each nest, but no attempt was made to trap and colour-band adults. Given the frequency of co-operative breeding in honeyeaters (Higgins et al. 2008), the possibility of helpers cannot be ruled out completely, although we have no evidence for it in this species.