Australian Field Ornithology 2012, 29, 169–181

Contributions to the reproductive effort in a group of plural-breeding Pied nigrogularis

D.G. Gosper

39 Azure Avenue, Balnarring VIC 3926, Email: [email protected]

Summary. Concurrent nesting by two females from a single social group of Pied Butcherbirds Cracticus nigrogularis is described. Young fledged from two nests in the first 2 years, but breeding failed in the following two seasons after Australian C. tibicen displaced the group from its original nest-tree. The two breeding females in the Pied group constructed their nests and incubated synchronously in the same tree without conflict. Only the females constructed the nest and incubated the eggs. Other group members fed the females before laying and during incubation. Females begged for food using a display resembling that of juvenile Butcherbirds. Most (and probably all) members of the group fed the nestlings and fledglings, with multiple members delivering food to the young in both nests and removing or eating faecal sacs. Immatures at the beginning of their second year, with no previous experience, performed the full range of helping tasks, including the provisioning of females from the pre-laying stage. During the nestling stage, the group fed on nectar from Silky Oaks , but did not feed this to the young.

Introduction The Cracticus nigrogularis () may breed co-operatively (Rowley 1976; Dow 1980; Clarke 1995). Higgins et al. (2006) considered it to be an occasional co-operative breeder, although the only detailed study to date (i.e. Robinson 1994) suggested that, despite not being an obligate co-operative breeder, the species usually breeds in territorial co-operative groups. Most studies on Pied Butcherbirds mention ‘helpers’ (individuals other than the breeding female and male) feeding the nestlings and fledglings (Thomas 1951; Courtney & Marchant 1971; Higgins et al. 2006), behaviours that have been photographed by C.A. Webster (see Boles 1995; Debus 1996). Co-operative breeders, including some species in the Artamidae, vary greatly in their social organisation, mating systems, and extent of co-operation (Hughes & Mather 1991; Hughes et al. 1996; du Plessis & Emlin 1999; Rowley 1999; Russell & Rowley 2009). One variant is plural breeding, in which more than one female in the group lays eggs, either in the same nest or with each female building her own nest (Curry 1988). Robinson (1994), in a 3-year study of helpers at the nest in 11 Pied Butcherbird territories in farmland in south-eastern Queensland, recorded plural breeding in four groups in which a second (beta) female built a separate nest and produced eggs and sometimes young. Fleay (1953) and Nielsen (1965), also in south-eastern Queensland, referred to what was probably plural breeding. 170 Australian Field Ornithology D.G. Gosper

Fleay (1953) described two females simultaneously constructing separate nests in trees ~30 m apart. A single male associated with both females during construction but fed only one. Young fledged from both nests. Nielsen (1965) described a nest containing seven eggs, probably laid by more than one female (based on patterning of the eggs), which was attended by four adults, two of which incubated. In this paper, I describe the breeding activities of a social group of Pied Butcherbirds over consecutive years. The Butcherbirds were found attending two active nests in the same tree, suggesting plural breeding. Behaviour of the territorial group at and near the nest(s) was monitored over four breeding seasons (1994–97) to elucidate the roles and contributions of individual members in the group’s reproductive effort. The territory was again monitored in 2007 to see what changes, if any, had occurred over 10 years.

Study area and methods The study area was at Goolmangar (28°42′S, 153°13′E), near Lismore in north-eastern New South Wales. The Pied Butcherbird breeding territory was on the floodplain of Goolmangar Creek, among open pasture used for grazing cattle. The area was flat, mostly treeless, with fence-lines ~300 m apart. The creek was lined by remnant gallery dry rainforest, dominated by introduced Camphor Laurel Cinnamomum camphora, and featuring scattered emergent Forest Red Gums Eucalyptus tereticornis, River She-oaks Casuarina cunninghamiana and Silky Oaks Grevillea robusta. It ran north–south through the territory, which lay primarily on its eastern side. Scattered trees, mainly Forest Red Gums, Hoop Pines Araucaria cunninghamii and Swamp Turpentines Syncarpia suaveolens 300–350 m north, east and south of the nest-sites were rarely visited during nesting.

Monitoring of nests In 1994, the nests contained young when discovered in early October. In subsequent years, monitoring began in July, and continued weekly until nest-building began. Thereafter, nests were monitored every 1–4 days (occasionally twice daily) until fledging in early November. A total of 72.2 h of observation was made over four seasons (range 8.4–35.4 h). Nest-watch sessions averaged 1 h (range 0.25–2.2 h). Watches were conducted from 51 minutes before sunrise until 33 minutes after sunset, but most fell in the periods 0530–0930 h and 1630– 1815 h. In 2007, ten 1-hour sessions were logged during late incubation and nestling stages. Nest-watches were made from below a tree 60–70 m from the nest-tree, with good surrounding vision allowing away from the nest to be followed. I recorded time of day, identity (if possible) of individual(s) arriving at the nests, the response of the occupant, the activity which followed (e.g. added nest material, fed nestlings, passed food to brooder, settled to brood, removed faecal sacs), and time of departure from the nest. Individual Butcherbirds were tracked for up to 35 minutes, during which their direction and/or flight- path to and from the nest(s) were recorded, as was their subsequent destination. Other behaviours at or near the nest-tree, such as nest-defence and foraging, were also recorded.

Results All data refer to the period 1994–97 unless otherwise indicated. The Pied Butcherbird group bred, or attempted to breed, in each of the four consecutive breeding seasons (Table 1). The territory boundaries remained unchanged, and no intraspecific confrontations were observed, although Plural breeding in Pied Butcherbirds 171

Table 1. Overview of breeding by a social group of Pied Butcherbirds at Goolmangar, NSW, in 1994–97 and 2007.

Year Social group Nests Breeding success Comment

1994 3 adults + 2 (plural) 3 young fledged 2 immatures (helpers) 1995 3 adults + 2 (plural) 3 young fledged 2 adults (helpers) 1996 3 adults + 2 (plural) 2 breeding Butcherbirds 2 immatures 2 (plural) attempts; all nests displaced from (helpers) failed/abandoned original nest-tree before breeding by nesting Magpies 1997 2 adults + 1 Nest failed/ Nesting Magpies 1 near-adult- abandoned again occupied plumaged, original nest-tree 1 immature (helpers) 2007 2 adults 1 2 young fledged Original nest-tree collapsed; Magpies nesting nearby

Butcherbirds in a neighbouring territory were heard calling. In 2007, the nest was in a tree 100 m farther north-east, and the breeding pair clashed with other Butcherbirds inside the southern edge of the original territory.

Locations of nests In 1994 and 1995, the group nested in an isolated senescent River She-oak, in a depression 60 m from the edge of the riparian zone. Both nests (A and B) were in forks 3.8 m apart, on opposite sides of the trunk. Nest A was 6.5 m and Nest B 8.3 m above ground, the latter being at approximately two-thirds the height of the tree. The Butcherbirds used three dead trees 60 m south-west, standing well above the adjacent riparian zone canopy, as perches for singing, preening and vigilance. In 1996, Australian Magpies Cracticus tibicen nested in the original nest-tree before the Butcherbirds started breeding, thus displacing the Butcherbirds, which subsequently built two nests (X1 and Y1) in adjoining she-oaks on the eastern bank of the creek, 70 m north-west of the original nest-tree. These nests were abandoned, as were two further nests (X2 and Y2) in non-adjacent she-oaks on the opposite bank. In 1997 Magpies re-occupied the original nest-tree, and the Butcherbirds built a single nest low in the isolated Hoop Pine 160 m south-east of their original nest-tree. By 2007, the original nest-tree had collapsed, and the Butcherbirds built a nest 100 m north-east of it in an isolated Rough-leaved Elm Aphananthe philippinensis 172 Australian Field Ornithology D.G. Gosper sapling (3.5 m high) beside a fence-post. Magpies were nesting in a Camphor Laurel 70 m east of the original nest-tree.

Composition of social group and the identity of individuals visiting the nest(s) In the first three seasons the breeding group consisted of five birds—two females, a male, and two helpers or auxiliaries (Table 1). In the fourth year, the group comprised a female, a male, and two helpers. Following the failed breeding attempt in 1997, a fifth bird (in adult ) associated with the group. In the 1994 and 1996 seasons, both helpers were immature (brown-plumaged) birds; in 1995, both helpers had adult plumage; and in 1997, one helper was immature and the other had near-adult plumage, separable from adults by its blackish-brown (not black) back and generally less neat appearance. No birds were marked, thus limiting data collection on the roles and contributions of members of the group. Immatures were distinguishable from others in the group, but not from each other. In years when there were two immature helpers, nest attendance by one could be monitored if both were simultaneously in view, and the second was inactive. Play behaviour (see p. 178) distinguished brown and adult-plumaged helpers from breeding adults, but this behaviour was seldom useful to identify individuals visiting nests. Adults were generally indistinguishable, but individuals could be tracked by temporary alterations to plumage (e.g. wet and/or soiled feathers). In 1995, the nest-builders, presumably females, were distinguishable by the shape of their black ‘bibs’. Extended daytime brooding early in the nestling stage and night brooding of nestlings were also attributed to the females. Nest attendance by the adult male was confirmed, but not quantified, in years when there were immature helpers only.

Nest-site selection Presumed prospecting for nest-sites was observed on 1 August 1996. An adult Butcherbird flew to the nest-tree and moved to the approximate location of the previous season’s Nest A. Here it sat in a fork, manipulating live twigs within reach with its bill. It also examined another fork, close to where a second adult had perched, before returning to the first site and repeating the previously observed behaviour. During this period, an immature arrived and perched nearby, appearing to watch the activity. The Butcherbirds did not re-occupy their nest-site(s) in 1996 because Magpies had begun building in the tree. On 4 August, at least four Butcherbirds, including the two immatures, interacted with two Magpies in the nest-tree. By 17 August, the Magpies were constructing a second nest, and the Butcherbirds were building in the riparian zone.

Nest-construction Nest-construction was carried out by one bird, presumably female. In the years when plural breeding occurred, both females were often present simultaneously at their respective nests during construction, but neither was seen to attend the Plural breeding in Pied Butcherbirds 173 other’s nest. No other member of the group was seen to bring material to the nest(s), either when the builder was present or absent. Occasionally another Butcherbird appeared to accompany the builder to or from the nest-tree, but none approached the nest. Brief confrontations between a building female and another adult around the nest were observed three times. The aggressor flew in, clashed briefly with the female and then flew off. Both females were at their nests during one incident, and each was targeted once on separate occasions. Nest-construction commenced in August each year (inferred in 1994). In 1995, building was in progress at both nests on 19 August, but at an early stage. Subsequently, Female B (less experienced?) took longer than Female A to complete construction, as sticks placed in the nest often fell through the fork into foliage or on to the ground. In addition to procuring material elsewhere, Female A collected sticks lost below Nest B and added them to her nest (five times in one 9-minute period on 20 August), even when Female B was present. Female B was observed retrieving fallen sticks from both the foliage and the ground below her own nest. She also started building at a second site, ~1.5 m away, but was similarly unsuccessful and returned to the original site. Significant progress toward a nest-platform was not evident until 2 September, after which nest-formation accelerated. Nest A was apparently complete by 31 August (building last observed on 27 August), whereas Female B did not complete her nest until at least 9 September, more than 9 days after Female A finished building and having taken more than 21 days in total. Nest-material was collected from the riparian zone, and once from nearby paddocks. Building birds consistently collected nest-material from the ground in the riparian zone. In 1995, both females visited a vine thicket on the edge of the riparian zone and attempted unsuccessfully to break twigs off a living plant. For the 1997 nest, the builder repeatedly visited an eroded steep bank to collect grass roots. She clung to clods of soil or the protruding roots of larger plants, jumping up to seize fine roots exposed by erosion, tugging at them until they broke off, and carrying rootlets (about 5 cm long) to the nest; the procuring and carrying of multiple rootlets (up to five at a time) to the nest was observed over consecutive visits to the creek bank by the female. Building was observed between 0745 and 1705 h, mostly before 1000 h, with 8–12 visits per hour to the nest. It was interspersed with foraging, for ~10–15 minutes, mostly near the nest-tree. At least one other member of the group was usually nearby. Occasionally the builder joined others, usually in the dead trees, and briefly called and displayed. In the late afternoon, when building activity appeared to cease, all of the Butcherbirds foraged as a loose group in the paddocks up to several hundred metres from the nest-tree.

Post-nest-construction interval Incubation commenced 7 to 12 days (mean 10 days) after the nest was completed. There was little activity at the nest just after it was finished, and incubation was inferred from the date when the female began to occupy the nest during the day (i.e. extended periods on the nest interspersed with absences of <10 minutes) and/or went onto the nest at dusk. 174 Australian Field Ornithology D.G. Gosper

Provisioning of females Nesting females solicited food from other members of the group from 3 to 11 days (mean 6 days) pre-incubation, and during incubation. Females ceased begging once eggs had hatched, and were no longer fed. In 1995, Female B (still incubating) continued to solicit and receive food for at least 3 days after the eggs in Nest A had hatched. Females solicited food using a begging display resembling that of juvenile Butcherbirds. Begging started with one or more short squeaks, before escalating into frenzied squawking as the food carrier neared, and was accompanied by crouching and vigorous quivering of partly spread wings. Females appeared to respond after seeing another member of the group (provider) approaching with food or pouncing on prey up to 70 m from the nest. Butcherbirds arriving in the nest-tree without food did not induce a begging response. The female was fed on or near the nest, elsewhere in the nest-tree and, occasionally, in an adjacent tree. During incubation, food was mostly delivered to the females at the nests. Females often begged simultaneously from the nest. Sometimes the unfed female immediately left her nest and foraged on the ground, or displayed nearby with others of the group. Once, Female B, having been ignored by the provider (which had visited Female A several times in succession), left her nest and followed the provider to the ground below, but was not fed. Sometimes, on finding a nest unoccupied, a provider responded to begging from the other incubating female and fed her. In the absence of a female from her nest, the other female sometimes left her own nest to perch beside the other’s nest to take food from a provider arriving at the unattended nest; both females showed this behaviour. Sometimes the owner returned immediately, and settled on her nest, with the other female still present. When a provider did not arrive directly at a nest, a begging female flew or hopped to the provider to take the food. Females were never observed to compete for food. Feeding of females was intermittent, with long periods without provisioning interspersed with short bursts of activity when up to four items were delivered in 10 minutes. In 1995, Female A received 61% of food items delivered (n = 36) at a rate of 1.8 items/hour, whereas Female B received 0.9 item/hour. Food was delivered from 13 minutes after sunrise until 13 minutes after sunset. Females were fed by multiple (probably all) members of the group. In 1995, during pre-laying and laying, both females were fed simultaneously in the adjacent dead trees, with all five birds present. During incubation, the same provider in turn fed Female A, then Female B. On different occasions during laying and incubation, each female was fed in close succession by two different individuals. In 1996, at the first breeding attempt, the male fed each female at her nest during the pre-laying and laying periods. At least one of the two immatures provisioned the female at one nest from about the time incubation commenced (the second nest failed around the start of incubation). Of the ten provisioning acts observed, three were performed by the male and seven by the immature(s). In 1997, all three members of the group fed the single female. The immature provisioned before and Plural breeding in Pied Butcherbirds 175 during incubation; and the male and second (near-adult-plumaged) helper did so during incubation. Five provisioning acts were contributed by the male, three by the near-adult-plumaged helper, and seven by the immature helper.

Copulation Copulation was brief and took place on or near the nest, preceded by a crouching postural display (accompanied by wing quivering and tail wagging) by the female, and calling by the male as he flew in from some distance (>70 m) away. Two matings were witnessed, one apparently involving each female, between 0720 and 0745 h on 11 September 1995. Identity of the male(s) involved was not determined.

Incubation I never saw another member of the group relieving an incubating Butcherbird, nor visiting the nest and attempting to incubate the eggs when the bird that incubated was absent. Therefore, I assumed that only females incubated. Incubating birds frequently left the nest for short periods to forage, defend the nest area (either solo or with other group members), or to join group displays. Females finally settled on the nest for the night up to 29 minutes after sunset, and first left the nest up to 56 minutes after sunrise. In 1995, Female A incubated for 362 of 447 minutes (81%) and Female B incubated for 508 of 606 minutes (84%). Mean duration of incubation sessions was 18.1 minutes (range 1–64 min.) for Female A and 19.5 minutes (range 1–77 min.) for Female B. Mean duration of absences from the nest was 5.3 minutes (range 1–11 min.) for Female A and 4.5 minutes (1–12 min.) for Female B. In 1997, the year in which incubation failed, the female incubated for 52% of the time (240 of 465 minutes). In this year, mean duration of incubation sessions was 6.9 minutes (range 1–26 min.) and absences 5.9 minutes (1–13 min.).

Predation of eggs On 26 September 1996, a suspected instance of egg-predation was observed. At 0618 h, the incubating female on Nest X1 left, swooping down across the creek and out of view, uttering an alarm call as she flew. At 0626 h, an adult Pied Butcherbird (identity unknown) arrived at the nest, reached into the cup and, on raising its head, appeared to be holding an egg in its bill. The bird immediately dived below and was lost from sight, obscured by vegetation. Soon after, the same (?) individual flew up into an adjacent tree and proceeded to wipe its bill on the branch.At 0636 h, the female returned from across the creek, resumed incubating, and was fed at the nest by an immature at 0638 h. The nest was subsequently abandoned.

Nestling stage Multiple members of the group brought food to one or both nests. In 1994, Female A, at least one other adult and at least one immature fed the nestlings in Nest A. Female B, at least one other adult and both immature helpers fed nestlings at Nest B. In 1995, at least three individuals (but probably more), comprising the respective females and at least two other adult-plumaged individuals, delivered 176 Australian Field Ornithology D.G. Gosper food to each nest. Frequently one or more individuals fed young in one nest, and then on their next visit brought food to the other nest, sometimes visiting nests alternately. Adult(s) also performed short bouts of brooding (<13 minutes) at one nest, while alternating between delivering food to both nests. Occasionally a bird fed nestlings in one nest, and then moved directly to settle on and brood at the other nest. In 1995, a bird delivered food to Nest A, and remained beside the nest for 7 minutes, periodically probing the nest-cup (see Nest-hygiene below). It then perched midway between the nests and, after a pause, hopped to Nest B, which was being brooded, and twice made brief attempts to brood at that nest, first by settling on top of the sitting bird, and then by pushing in beside it. A bird visiting a nest without food, probably a female that had been foraging for herself, almost always settled to brood. Brooding of chicks was irregular, and became progressively less frequent, but brief spells were noted up to fledging. Food brought to the nest was usually fed directly to the nestling(s). Brooding birds did not vacate the nest on the arrival of the provider, which passed food to the brooder, which then fed it to the nestling(s). This occurred at both nests for up to 8 days after hatching. The feeding rate was higher at the nest which fledged more young. In 1994, in 350 minutes (before 0925 h or after 1620 h) Nest A (one nestling fledged) received 7.1 visits per hour, and Nest B (two nestlings fledged) 11.4 visits per hour, a mean group feeding rate of 18.4 visits per hour (range 8.0–28.0 visits/h, n = 107). In 1995, in 523 minutes (before 1140 h or after 1415 h), Nest A (two nestlings fledged) received 7.2 visits per hour and Nest B (one nestling fledged) 4.8 visits per hour, a mean of 12.0 visits per hour (range 5.6–20.7 visits/h, n = 105). The intensity of provisioning fluctuated, with several members of the group (up to four) often involved in servicing one or both nests for short periods. Sometimes a nest went unvisited for up to 45 minutes. Nestlings were fed from 15 minutes after sunrise to 21 minutes after sunset.

Nest-hygiene After bringing food to the nest, providers regularly removed faecal sacs expelled by the nestlings. Faecal sacs were either eaten at the nest or dropped from the outer branches of the nest-tree, a fence-post or dead trees. Both females consumed faecal sacs, but this behaviour was not confirmed for other members of the group. Immature(s) took faecal sacs from both nests in the 1994 season in the last few days before fledging. Of faecal sacs removed during nest-watches n( = 36), 28% were eaten and 72% were discarded away from the nest. In 1994, faecal sacs were consumed up to the last day before fledging, but in 1995 none were seen to be eaten after the ninth day of the nestling period. An adult that fed nestlings at Nest A in 1995 sometimes perched on the rim of the nest for 7–8 minutes, periodically engaging in bouts of vigorous probing into the bottom of the nest-cup. This behaviour was interpreted as nest-hygiene, either cleaning the nest-floor or inducing the nestlings to defaecate (cf. Wood 2001; Noske & Carlson 2011). Chicks were not seen to void out of the nest. Plural breeding in Pied Butcherbirds 177

Table 2. Responses by Pied Butcherbird group at Goolmangar, NSW, to intrusions (total = 84) into the nest(s) area by other birds during the breeding cycle. DF = defensive response, NR = no response, * = response by immature (brown) birds only.

Species No. of intrusions DF NR

White-headed Pigeon Columba leucomela 1* 4 Crested Pigeon Ocyphaps lophotes 3* 6 Bar-shouldered Dove Geopelia humeralis 1 Australian Pelican Pelecanus conspicillatus 1 Goshawk/sparrowhawk Accipiter sp. 1 Nankeen Kestrel Falco cenchroides 1 Eastern Rosella Platycercus eximius 4 17 Laughing Kookaburra Dacelo novaeguineae 6 Noisy Miner Manorina melanocephala 1 2 Black-faced Cuckoo- Coracina novaehollandiae 2* 1 Australasian Figbird Sphecotheres vieilloti 2* 9 Australian Cracticus tibicen 11 2 Pied Strepera graculina 2 Corvus orru 4 1 Magpie-lark Grallina cyanoleuca 1* Unknown 1 Total 41 43

Fledgling stage In 1994, the first nestling fledged from Nest B, and was found on the ground 40 m from the nest-tree 3 days before its sibling, and the nestling from Nest A, left their nests. Both fledglings from Nest B were less developed than the fledgling from Nest A, and spent at least the first 6 days after leaving the nest on the ground or on fallen branches. The fledgling from Nest A was able to fly well, and perched high in the nest-tree for 2 days before flying 60 m to the dead trees. In 1995, the nestling from Nest B fledged at least 7 days after the two nestlings from Nest A, both of which left the nest on the same day. Multiple members of the group, including immatures, fed the fledgings. At least some birds fed fledglings from both nests. In 1995, one bird alternated between feeding the nestling in Nest B (and removing faecal sacs) and a fledgling from Nest A perched low in the nest-tree or in nearby dead trees. Both free-flying fledglings from Nest A were also seen on or beside Nest B when food was being delivered, but neither was seen to be fed.

Defence of nest All members of the group defended the nest-tree and its immediate vicinity from the nest-building to the immediate post-fledging period (Table 2). Most 178 Australian Field Ornithology D.G. Gosper defences were mounted by one or two members of the group, but on occasions three birds were involved. Defenders regularly targeted intruders landing in the nest-tree, but Laughing Kookaburras Dacelo novaeguineae, Pied Strepera graculina and Torresian Crows Corvus orru were always challenged when flying within 70 m or when they landed in the nearby dead trees. Brooding female Butcherbirds were seen to leave the nest to pursue a raptor (Accipiter sp.), Kookaburras, Currawongs, and sometimes Magpies, but tolerated other species in the nest-tree. Immatures were responsible for most frivolous attacks targeting non-threatening species, such as pigeons, Australasian Figbirds Sphecotheres vieilloti and Eastern Rosellas Platycercus eximius. During nest-building in 1995, the group tolerated two immature Magpies that sometimes foraged under the nest-tree.

Play behaviour Helpers frequently engaged in ‘play-chasing’ behaviour during incubation and, to a lesser extent, during nest-building and nestling stages. One Butcherbird leisurely pursued another through the branches of dead trees, the riparian canopy, and across paddocks. This often lasted for >5 minutes, sometimes for up to 15 minutes. Chases sometimes concluded with a brief display of posturing and duetting. Once, two immatures engaged in a ‘tug of war’, but whether this involved an object or the locking of bills was not recorded in field notes at the time. In 1997, the immature, while playing in the top of the nest-tree, hung motionless by its bill from a twig, wings folded and legs pulled into the body. Play-chasing was most frequent in the late afternoon, when up to three bouts were observed in 1 hour.

Foraging The Pied Butcherbirds foraged primarily in the paddocks. Most food was taken on the ground, birds frequently pouncing from fence-posts and trees onto prey. Other techniques exploited temporarily abundant food, including swarms of flying ants (Formicidae) emerging from underground nests. The Butcherbirds gathered at such swarms, hopping on the ground and clinging, robin-like, to lower trunks or sides of posts, picking ants off these surfaces. Swarming attracted up to four Butcherbirds, with all individuals in the group taking part at some time. Once, two immature Magpies commenced feeding at an ant swarm at the base of the nest- tree. They were joined by three Butcherbirds, which perched on the side of the trunk and lowest branch, and foraged beside the Magpies for at least 10 minutes. The Butcherbirds also hawked for flying insects (probably ants) from the nest-tree and the canopy of the riparian zone, sometimes with Figbirds (cf. Gosper 1996), and took the prey to nestlings. Pied Butcherbirds, including immatures, also foraged at the flowers of Silky Oaks in the riparian zone, from late September to late October each year. They probed flowers, working along a raceme with mandible and tongue movements. Periodically, each bird tilted its head back, raising the bill above the horizontal, with more movement of the mandibles and tongue, indicating swallowing. When the bill was raised, its wet tip was visible glistening in the sunlight, and against Plural breeding in Pied Butcherbirds 179 a clear sky it was evident that only nectar was being taken. Individuals, which foraged at flowers for up to 10 minutes at a time, repeatedly wiped the billon a branch. Sometimes nectar feeding was interrupted to pick prey (presumably insects) from flowers and/or foliage elsewhere on the tree. Pied Butcherbirds interspersed delivering food to nestlings with nectar feeding, but they were not observed to go directly from feeding at flowers to the nest and feed young. Brooding females left the nest to visit flowers for short periods. Eastern Rosellas, Lewin’s Honeyeaters Meliphaga lewinii, Australasian Figbirds, Torresian Crows, Zosterops lateralis and Common Starlings Sturnus vulgaris feeding at Silky Oak flowers were all ignored, but a was driven away by one of the Butcherbirds, which then resumed feeding at the flowers.

Discussion Concurrent nesting in the same tree by two females from a single social group of Pied Butcherbirds has not been previously reported (Higgins et al. 2006), although it is known in Australian Magpies (Fulton 2006, 2007; DGG pers. obs.). Robinson (1994) found plural-breeding female Butcherbirds nesting within 10 m but did not detail nest locations, and Fleay (1953) reported nests in separate trees 27 m apart. Two pairs of Butcherbirds reportedly nesting in the same tree simultaneously (Royal Australasian Ornithologists Union Nest Record Scheme, cited in Higgins et al. 2006), were probably plural breeding, similar to that described here. In the present study, and in instances described by Robinson (1994), there was negligible conflict between the two breeding females, though nesting was synchronous rather than sequential (cf. Hughes et al. 1996). This raises the possibility that the two females were related, e.g. female and female offspring. The observations of unidentified individuals brooding young in one nest and delivering food to both nests suggest that female(s) may also feed each other’s young. As the extent of the help provided by the territorial male to the individual females was not quantified, an understanding of his relationship with the second female is lacking. The breeding success and stability of the Butcherbird group in this study was affected by Australian Magpies taking over their original nest-tree. Pied Butcherbirds appear to prefer a tree and nest-site that allow good visibility and unimpeded flight-paths to and from the nest. They prefer isolated, open-crowned or sparsely foliaged trees or saplings (Higgins et al. 2006; DGG pers. obs.). When displaced from the original nest-tree, with no suitable trees in the core of their territory, the Butcherbirds nested in the dense riparian zone. This reduced the females’ ability to monitor the movements of group members, obtain support from them, and defend their nests against brood parasitism (Robinson 1994) and egg-predation. Changes in the composition of the Butcherbird group between breeding seasons imply that dispersal of immatures from the natal territory, and the movement of older birds between groups, may be variable. Robinson (1994) concluded that immatures remain in their natal territory until adult plumage is attained at 15–18 months, with their first opportunity to disperse occurring prior to the second breeding season after they fledged. However, in 1997 in the present study, after a 180 Australian Field Ornithology D.G. Gosper failed previous breeding season, the two helpers were a brown bird and a near- adult-plumaged bird, suggesting that at least the former had joined the group as an immature. After breeding again failed, a fifth bird (in adult plumage) associated with the group. In 1995, the two helpers were adult-plumaged, suggesting that the offspring of the 1994 season had either died or left the natal territory, with the helpers of the previous season staying on. How provisioning of nesting females by others in the Butcherbird group is initiated and maintained is unclear. From pre-laying to hatching, females appeared to monitor the activities of others, and gave begging displays to individuals carrying food. Provisioning by helpers may be maintained by the female’s begging, which mimics that of juveniles (cf. Dow 1980). It is noteworthy that immatures at the beginning of their second year, having had no experience as helpers, commenced to feed the females before laying. Overall, the contributions of Pied Butcherbird group members to nestlings and fledglings were consistent with the findings of Robinson (1994). However, Robinson (1994) reported competition between members of the group for faecal sacs, but (in contrast with the present study) made no mention of Pied Butcherbirds eating faecal sacs, stating only that sacs were removed and dumped away from the nest. Regular (annual) use of nectar as a food by Pied Butcherbirds has not been previously reported (Higgins et al. 2006). In studies in monsoonal northern Australia, Franklin (1999) found the Cracticus torquatus to be a prominent opportunistic nectarivore, but recorded nectar feeding by Pied Butcherbirds only once.

Acknowledgements I am grateful to the following people for their assistance: Carl Gosper and Glenn Holmes for comments on early drafts; Carl Gosper with accessing references; Sue Murphy with proofreading; and referees H.F. Recher and Hugh A. Ford, and editor James Fitzsimons for advice that greatly improved the manuscript.

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Received 26 January 2012

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