Brown Thornbills Research Articles

Tiger talk

Tiger talk

From the Pages of Aesop's Fables.

The article discusses research published in the journal "Proceedings of the Royal Society B" which suggests that the Australian birds known as brown thornbills use their hawk alarm calls and mimic those of other species in order to discourage the predator bird known as the pied currawong from attacking brown thornbill nestlings. According to biologist Branislav Igic, the mimicked calls may help to persuade the currawongs that a hawk is really present.

Crying wolf to a predator: deceptive vocal mimicry by a bird protecting young.

Animals often mimic dangerous or toxic species to deter predators; however, mimicry of such species may not always be possible and mimicry of benign species seems unlikely to confer anti-predator benefits. We reveal a system in which a bird mimics the alarm calls of harmless species to fool a predator 40 times its size and protect its offspring against attack. Our experiments revealed that brown thornbills (Acanthiza pusilla) mimic a chorus of other species' aerial alarm calls, a cue of an Accipiter hawk in flight, when predators attack their nest. The absence of any flying predators in this context implies that these alarms convey deceptive information about the type of danger present. Experiments on the primary nest predators of thornbills, pied currawongs (Strepera graculina), revealed that the predators treat these alarms as if they themselves are threatened by flying hawks, either by scanning the sky for danger or fleeing, confirming a deceptive function. In turn, these distractions delay attack and provide thornbill nestlings with an opportunity to escape. This sophisticated defence strategy exploits the complex web of interactions among multiple species across several trophic levels, and in particular exploits a predator's ability to eavesdrop on and respond appropriately to heterospecific alarm calls. Our findings demonstrate that prey can fool predators by deceptively mimicking alarm calls of harmless species, suggesting that defensive mimicry could be more widespread because of indirect effects on predators within a web of eavesdropping.

Bird population responses to wildfire and planned burns in foothill forests of Victoria, Australia

Fires and people have helped shape forest ecosystems. Planned burns can be used to reduce social or ecological risks from wildfires, but we need a better understanding of the ecological effects of planned burns and wildfire over various temporal and spatial scales. Here, two approaches were used to answer various questions about responses of diurnal birds to fire regimes in mixed eucalypt forests in the foothills of Victoria, Australia: what are the effects of planned burns (at two different frequencies and seasons) when applied systematically over many years, and what are the short-term effects of severe wildfire? A long-term experiment examined planned burns in two seasons and at two frequencies on 25 sites in Wombat State Forest in 1984. An opportunistic study compared bird abundance in 33 burnt and 33 unburnt sites in the year after an extensive severe wildfire in Bunyip State Park in 2009. Brief reference is also made to a retrospective study, which provided a longer-term perspective by examining bird abundances at 113 sites in eastern Victoria that had been subject to different regimes over the past 40+ years, addressing questions about the impacts of different fire frequencies and times since fire. This paper considers data from snapshot surveys in 2009–2012. Data from Wombat State Forest showed significant effects of fire treatments for just four of the 27 species analysed (p < 0.05). In frequently burnt sites, Brown Thornbills Acanthiza pusilla (shrub foraging insectivores) and Scarlet Robins Petroica boodang (open-ground insectivores) were least common, and Rufous Whistlers Pachycephala rufiventris (canopy-foraging insectivores) and Australian Magpies Cracticus tibicen (open-ground insectivores) were most common. There was weak evidence that three species were advantaged by burning in autumn rather than spring, and one of them (White-winged Chough Corcorax melanorhamphos, an open-ground insectivore) was rarely recorded except in sites burnt in autumn. Data from Bunyip State Park showed that birds were 77 % less numerous on burnt than unburnt sites in the winter after severe bushfire, but some redistribution had occurred by the next spring, along with an unprecedented spring influx of White-browed Woodswallows Artamus superciliosus (aerial insectivores) to burnt sites from inland Australia. The study shows that planned burns can be conducted in spring or autumn every 3–10 years with only small effects on common forest birds, at least when they are conducted on small areas in a forest with a history of disturbance. Wildfires can have much more dramatic short-term effects on bird populations. The retrospective study suggested that these effects may be short-lived, as such dramatic contrasts were not found between sites burned at different times beyond 3 years. Multiple approaches will be needed to answer key questions about longer-term effects of planned burns and wildfires in order to help reduce risk and conserve biodiversity.

The alarm call system of breeding Brown Thornbills (Acanthiza pusilla): self-defence or nest defence?

Many species produce alarm calls during predator encounters that are directed at either conspecifics or the predator. Although many studies have investigated alarm calls in foraging individuals, antipredator communication during reproduction is rarely studied. In birds, where nest predation is a key cause of reproductive failure, some species have evolved sophisticated alarm calls to inform nestlings of danger from nest predators. However, different predator species differ in their primary prey type (brood predators, predators of adults), and accordingly, birds could have evolved different alarm calls depending on who is in danger. We experimentally investigated parental antipredator communication in Brown Thornbills (Acanthiza pusilla), where more than 50 % of all nests are depredated. We presented breeding pairs the models of a brood predator, a predator of adult birds and a harmless control species. Parents gave two different alarm calls (short and intense tzzt calls), which were not predator-specific, but responded to the predator of adults with more alarm calls than for the brood predator. Parental alarm calls did not affect nestling begging levels, which were influenced only by parental feeding rate. Our results suggest that alarm calls of breeding Brown Thornbills are mainly used as a form of self-defence by the individuals of the breeding pair rather than being directed at the nestlings. This fits with the predictions of life-history theory that parents in long-lived species are more concerned about their own survival.

A songbird mimics different heterospecific alarm calls in response to different types of threat

Birds frequently mimic other species’ alarm calls, but the type of information conveyed to receivers, and therefore the function of mimetic alarm calls, is often unknown. Alarm calls can convey predator-specific information that influences how receivers respond: aerial alarms signal the presence of flying predators and provoke receivers to flee, whereas mobbing alarm calls signal the presence of less dangerous predators and provoke receivers to mob. The function of mimetic alarm calls may therefore depend on the type of heterospecific alarm calls mimicked. We examined the use of alarm call mimicry by brown thornbills (Acanthiza pusilla) across different contexts of danger: terrestrial threat, aerial threat, when captured by a predator and during nest attack. Thornbills were most likely to include mimetic alarm calls among their vocalizations when captured and during nest attack, less likely in response to terrestrial and aerial predator threats, and least likely in the absence of any threat. Furthermore, the type of danger affected the type of mimetic alarm calls used: thornbills mimicked mobbing alarm calls in response to terrestrial threat and aerial alarm calls in response to aerial threat but also during capture and nest attack where no aerial danger was present. We suggest that mimicking predator-appropriate heterospecific alarm calls in response to aerial and terrestrial threats may facilitate interspecific alarm communication with other prey species, whereas mimicking heterospecific aerial alarm calls in the absence of aerial predators might deceptively startle predators to release captured callers or offspring when attacked.

Fidelity of vocal mimicry: identification and accuracy of mimicry of heterospecific alarm calls by the brown thornbill

Avian vocal mimicry has been studied for decades, but little is known about its function or requirements for accurate imitation. Furthermore, progress is hampered by the difficulty in identifying which vocalizations are indeed mimetic. We tested historical claims of vocal mimicry in the brown thornbill, Acanthiza pusilla, using a combination of human and computer methods to identify mimicry, followed by comparisons of acoustic similarity with model vocalizations. We recorded vocalizations of brown thornbills and sympatric heterospecifics while undisturbed and during mist net capture or the presence of natural or model predators. We then cross-validated human classification of mimicry with computer classification based on spectrographic measurements and spectral cross-correlation. Finally, we quantified the accuracy of the most common imitations. Brown thornbills predominantly imitated alarm calls given by heterospecifics towards aerial predators, which function in these models to provoke immediate flight by receivers. Human and computer-based methods produced consistent results when identifying and classifying mimicry. Mimicked aerial alarms were not perfect imitations of their corresponding model alarms, but did retain specific acoustic properties previously shown to be important for provoking immediate alarm responses. Although less accurate mimicry may reflect physiological constraints, we suggest that mimetic function, perhaps startling predators, only requires mimicry to retain features of model alarms that provoke immediate alarm responses by receivers. Understanding what factors influence the acoustic structure of mimetic vocalizations is essential in understanding the evolution of vocal mimicry, particularly with accumulating evidence that mimetic function does not always require perfect resemblance in other sensory modalities.

Causes of Ring-Related Leg Injuries in Birds – Evidence and Recommendations from Four Field Studies

One of the main techniques for recognizing individuals in avian field research is marking birds with plastic and metal leg rings. However, in some species individuals may react negatively to rings, causing leg injuries and, in extreme cases, the loss of a foot or limb. Here, we report problems that arise from ringing and illustrate solutions based on field data from Brown Thornbills (Acanthiza pusilla) (2 populations), Siberian Jays (Perisoreus infaustus) and Purple-crowned Fairy-wrens (Malurus coronatus). We encountered three problems caused by plastic rings: inflammations triggered by material accumulating under the ring (Purple-crowned Fairy-wrens), contact inflammations as a consequence of plastic rings touching the foot or tibio-tarsal joint (Brown Thornbills), and toes or the foot getting trapped in partly unwrapped flat-band colour rings (Siberian Jays). Metal rings caused two problems: the edges of aluminium rings bent inwards if mounted on top of each other (Brown Thornbills), and too small a ring size led to inflammation (Purple-crowned Fairy-wrens). We overcame these problems by changing the ringing technique (using different ring types or larger rings), or using different adhesive. Additionally, we developed and tested a novel, simple technique of gluing plastic rings onto metal rings in Brown Thornbills. A review of studies reporting ring injuries (N = 23) showed that small birds (<55 g body weight) are more prone to leg infections while larger birds (>35 g) tend to get rings stuck over their feet. We give methodological advice on how these problems can be avoided, and suggest a ringing hazard index to compare the impact of ringing in terms of injury on different bird species. Finally, to facilitate improvements in ringing techniques, we encourage online deposition of information regarding ringing injuries of birds at a website hosted by the European Union for Bird Ringing (EURING).

Learning to listen? Nestling response to heterospecific alarm calls

Many nestling birds go silent in response to parental alarm calls, potentially lowering their risk of being overheard by predators. Parents are not always nearby, however, and so offspring could also benefit if they respond to the alarm calls of other species. Response could be innate, particularly if heterospecific alarm calls are acoustically similar to conspecific alarms, or learned through experience or association with parental behaviour. We investigated the responses of both young (5–6 days old) and older (10–11 days old, close to fledging) nestling white-browed scrubwrens, Sericornis frontalis , to the mobbing alarm calls of three heterospecifics whose nests are vulnerable to similar predators. Brown thornbills, Acanthiza pusilla , produce the most similar alarm calls to scrubwrens, while superb fairy-wren, Malurus cyaneus , and New Holland honeyeater, Phylidonyris novaehollandiae , produce alarm calls that are distinct. Heterospecific mobbing assays demonstrated that nestlings were likely to overhear the alarm calls of all three species. In support of innate response, even young nestlings suppressed calling after hearing both thornbill and parental alarms. However, young nestlings ignored or increased calling to fairy-wren and honeyeater alarms. Older nestlings continued to suppress calling to thornbill and parental alarm calls, but also suppressed calling to honeyeater and fairy-wren alarms, suggesting that they could have learnt to recognize those calls. This study thus demonstrates that nestlings can respond to the alarm calls of other species, and that these responses are likely to be enabled through both innate mechanisms and learning.

When more is less: Urban remnants support high bird abundance but diversity varies

Urban remnant vegetation, especially where it occurs in public parks, allows for relatively easy access for ongoing biodiversity monitoring. However, relatively little baseline information on bird species distribution and abundance across a range of identifiable urban remnants appears in the published literature. We surveyed the relative abundance and distribution of birds across urban and suburban remnant vegetation in Melbourne, Australia. One hundred and six species were recorded, of which 98 were indigenous. Red wattlebirds had the highest mean relative abundance with 2.94 birds/ ha, followed by rainbow lorikeets (2.51), noisy miners (1.93), brown thornbills (1.75) and spotted doves (0.96). There was no obvious trend between overall relative abundance and the size of the remnant, in contrast to species richness which was positively correlated with remnant size. The data revealed that some species were either totally restricted to, or more abundant in, larger remnants and generally absent from smaller remnants. Some of the more common birds (crimson rosella, superb fairy-wren, spotted pardalote and black-faced cuckoo-shrike) recorded during this study were detected at similar densities to those found in comparable vegetation to the east of Melbourne within a largely forested landscape. Other species occurred at much lower densities (e.g., white-browed scrubwren, brown thornbill, eastern yellow robin and grey fantail) or had habitat requirements or ecological characteristics that could place them at risk of further decline or local extinction in the urban area. We identify a suite of bird species of potential conservation concern within Melbourne’s urban landscape. The establishment of repeatable, fixed-point, and long-term monitoring sites will allow for repeat surveying over time and provide an early warning of population declines, or conversely an indication of population increase for other species.

Mate choice in the brown thornbill ( Acanthiza pusilla ): are settlement decisions, divorce and extrapair mating complementary strategies?

In socially monogamous birds, females may express mate preferences when they first select a breeding partner, through divorce and subsequent breeding dispersal to a new partner and through extrapair mating. We examined settlement patterns, divorce and breeding dispersal in a sedentary Australian passerine, the brown thornbill (Acanthiza pusilla), in relation to two traits known to influence extrapair paternity (male age and male size). Settlement decisions, divorce and territory switching behaviour were all female strategies that reduced their likelihood of breeding with 1-year-old males. Females preferred to settle in territories with 2+ -year-old males, were more likely to divorce 1-year-old males, and only switched territories if they had an opportunity to form a new pair bond with an old male. In contrast, female settlement and divorce decisions were not influenced by male size. Female thornbills obtain a direct benefit from preferring older males as social mates because breeding success improves with male age in brown thornbills. Nevertheless, divorce rates in this species were low (14% of pair bonds were terminated by divorce), and individuals rarely switched territories following the death of a mate. Both of these mating strategies appeared to be primarily constrained by the distance adults moved to initiate a new pair bond (1–2 territories) and by the limited availability of unpaired older males in the immediate neighbourhood.

Pair bond duration influences paternal provisioning and the primary sex ratio of brown thornbill broods

Parents should vary their level of investment in sons and daughters in response to the fitness costs and benefits accrued through male and female offspring. I investigated brood sex ratio biases and parental provisioning behaviour in the brown thornbill, Acanthiza pusilla, a sexually dimorphic Australian passserine. Parents delivered more food to male-biased than female-biased broods. However, factors determining parental provisioning rates differed between the sexes. Female provisioning rates were related to brood sex ratio in both natural and experimental broods with manipulated sex ratios. In contrast, male provisioning rates were not affected by brood sex ratio in either natural or experimental broods. However, males in established pairs provisioned at a higher rate than males in new pairs. Data on the sex ratio of 109 broods suggest that female brown thornbills adjust their primary sex ratio in response to pair bond duration. Females in new pairs produced broods with significantly fewer sons than females in established pairs. This pattern would be beneficial to females if the costs of rearing sons were higher for females in new than established pairs. This may be the case since females in new pairs provisioned experimental all-male broods at elevated rates. The condition of nestlings also tended to decline more in these all-male broods than in other experimental broods. This will have additional fitness consequences because nestling mass influences recruitment in thornbills. Female thornbills may therefore obtain significant fitness benefits from adjusting their brood sex ratio in response to the status of their pair bond. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

Extra-pair paternity and mate-guarding behaviour in the brown thornbill

We used multilocus DNA fingerprinting to assess parentage in the brown thornbill, Acanthiza pusilla, a socially monogamous Australian passerine. Extra-pair paternity was uncommon (6.2% of 178 offspring; 11.9% of 67 broods) and there was no evidence of intra-specific brood parasitism. Extra-pair paternity was limited because pairs spent more time together when females were fertile and males were able to evict intruding males before they could approach the female. Males were responsible for the close proximity of partners during the fertile period. Mate guarding therefore appears to be a male tactic aimed at preventing female infidelity rather than a cooperative behaviour of the pair aimed at preventing extra-pair copulations and/or female harassment. Females did not attempt to escape male guarding and were rarely observed to solicit copulations from intruding males. Nevertheless, females paired to smaller and younger males were more likely to cuckold their mates than females paired to larger and older males. This suggests that females may be more likely to seek or accept extra-pair matings when paired to small, young males or that old, large males are better at preventing their mates from engaging in extra-pair copulations. We found that male age but not male size influences mate-guarding behaviour. Older males tended to respond more aggressively to intruders. We therefore speculate that the relationship between male size/age and extra-pair paternity in brown thornbills may arise because female thornbills prefer large males as mates but are unable to express this preference as easily when paired to older males.

Post‐fledging care, philopatry and recruitment in brown thornbills

Summary We describe patterns of post‐fledging care, dispersal and recruitment in four cohorts of brown thornbillsAcanthiza pusilla. We examine what factors influence post‐fledging survival and determine how post‐fledging care and the timing of dispersal influence the probability of recruitment in this small, pair breeding, Australian passerine. Fledgling thornbills were dependent on their parents for approximately 6 weeks. Male fledglings were more likely than female fledglings to survive until independence. For both sexes, the probability of reaching independence increased as nestling weight increased and was higher for nestlings that fledged later in the season. The timing of dispersal by juvenile thornbills was bimodal. Juveniles either dispersed by the end of the breeding season or remained on their natal territory into the autumn and winter. Juveniles that delayed dispersal were four times more likely to recruit into the local breeding population than juveniles that dispersed early. Delayed dispersal was advantageous because individuals that remained on their natal territory suffered little mortality and tended to disperse only when a local vacancy was available. Consequently, the risk of mortality associated with obtaining a breeding vacancy using this dispersal strategy was low. Males, the more philopatric sex, were far more likely than females to delay dispersal. Despite the apparent advantages of prolonged natal philopatry, however, only 54% of pairs that raised male fledglings to independence had sons that postponed dispersal, and most of these philopatric sons gained vacancies before their parents bred again. Consequently, few sons have the opportunity to help their parents. Constraints on delayed dispersal therefore appear to play a major role in the evolution of pair‐breeding in the brown thornbill.

The influence of age on reproductive performance in the Brown Thornbill

I examined age effects on reproduction in the Brown Thornbill Acanthiza pusilla in Canberra, Australia. I found that the reproductive performance of both males and females improved with age, although only age‐related improvement in male performance had a significant effect on annual reproductive success. Reproductive success improved with male age as a result of improved performance during two stages of the breeding cycle: first‐year males were less likely to fledge young than those aged two or more, while both first and second‐year males were less successful at raising fledglings to independence than males of three or more. Male performance appears to improve over three years as they gain experience at provisioning nestlings and caring for fledglings without attracting predators, rather than as a direct result of improved foraging skills. In contrast, reproductive success only improved slightly with female age, although females of two or more years initiated their first clutch earlier in the season than one‐year‐old females, and tended to be more likely to re‐nest if a breeding attempt failed. The poor performance of young females appears unlikely to be related to their foraging ability but may be associated with costs imposed by dispersing to a breeding vacancy earlier in the year. Although the reproductive performance of Brown Thornbills improves considerably with age I found no evidence that performance improved as a result of repeated breeding attempts with the same partner.

Life history and demography of an uncooperative Australian passerine, the brown thornbill

The genus Acanthiza may be important in understanding the evolution of avian mating systems because while brown thornbills, Acanthiza pusilla, are thought to breed only in pairs, a recent phylogenetic analysis suggests that cooperative breeding is the ancestral trait within this genus. We provide a detailed account of the breeding biology of the brown thornbill, confirm that they breed exclusively in pairs, and compare their population demography with what is known for other members of the Pardalotidae. We found that brown thornbills produced small clutches (3 eggs) with a two-day laying interval, had a long incubation period (declining from 19 to 16 days through the season), and had a long breeding season (4.0 months) that allowed females to occasionally raise two successful broods. Brown thornbills, in our study, produced an average of 1.57 fledglings per pair and had relatively high annual survival rates (c. 63%). We found no evidence to suggest that the evolution of pair-breeding within the Pardalotidae is associated with a reduction in annual survival rates, a short breeding season with reduced productivity, or high levels of predation post-fledging. Since there also appear to be no ecological correlates with mating system in the Pardalotidae we suggest that examination of reproductive conflict between parents and young may shed light on the evolution of pairbreeding in this family.

Winter Ecological Energetics of 2 Passerine Bird Species in Temperate Wet Forest

Autumn and winter time-energy budgets were constructed for brown thornbills, Acanthiza pusilla, and eastern yellow robins, Eopsaltria australis, inhabiting a temperate wet forest in south-eastern Australia. Birds spent 84-88% of daylight hours foraging in both seasons, but decreased the metabolic cost of other activity in winter by spending more time on energetically inexpensive behaviours. Estimated daily energy expenditures were either seasonally constant or increased (thornbill) or decreased (robin) in winter by no nore than l0%, depending on the assumed degree of substitution for the thermoregulatory requirement. Thornbills increased foraging efficiency in winter to compensate for the reduction in absolute foraging time. Less dramatic changes in behavioural strategies were required to achieve energy balance than have been recorded for many small north temperate birds. Brown thornbills used an energetically expensive, active search foraging technique to capture small, cryptic prey at a fast rate. Yellow robins employed an inexpensive, 'sit-and-wait' strategy to capture larger, more conspicuous prey at a slower rate. Both species had similar time investments in foraging, but allocated greatly differing proportions of energy to active foraging and resting alert. These contrasting strategies offer the potential for performing several activities simultaneously in the yellow robin and for reducing foraging and vigilance investments through exploiting gregariousness in the brown thornbill.

Ecology of Brown and Striated Thornbills in Forests of South-eastern New South Wales, with Comments on Forest Management

Acanthiza pusilla and A. lineata often occur together, but striated thornbill forages mainly in the subcanopy and canopy (>75% of foraging observations) where it specialises in taking food from the foliage of eucalypts (>90% of observations). Brown thornbill forages mainly in the shrub layer (>70% of observations) and takes prey from bark, debris and the leaves of a large variety of plants including eucalypts. Both thornbills forage mainly by gleaning but striated thornbill commonly hang gleans (>20% of observations), a behaviour rarely used by brown thornbills. Striated thornbills are disadvantaged by logging, which reduces the amount of canopy and subcanopy vegetation, but brown thornbills benefit from the increased amount of shrub and ground vegetation that results. Conversely, brown thornbills are adversely affected by fires that reduce the amount of debris and low vegetation. In the absence of eucalypts, striated thornbills are absent from pine plantations, but brown thornbills may be abundant.

Field Notes on the Fan-Tailed Cuckoo

In the field a distinction can usually be made between those Fan-tailed Cuckoos Cuculus pyrrhophanus that are darkly rufous below, perch prominently and call much and others that are paler below and unobtrusive and quiet. Five specimens of the first sort were all males and four of the second all females. Skins show that in general males are rather more dark rufous on the underparts than females. The trilling and whistling calls ofthe species seem to be given only by males. The females apparently produce only a 'chireee' call. These Cuckoos are often found in groups of three or more. While the males perch prominently and persistently trill and whistle, the females tend to skulk in thickets, giving the chireee call from time to time, often in response to the chireee of another bird. It is doubtful whether this is connected with territorial activity. Birds collected in 1977 were coming into breeding condition in late October and early November but in 1978 fresh eggs were found in the middle of October. In any case, the Cuckoos seem to come into breeding condition too late for the first broods of their chief biological hosts, Brown Thornbills Acanthiza pusilla and White-browed Scrubwrens Sericornis frontalis.