DOI: 10.5061/dryad.xsj3tx9pf
title: Location and caller familiarity influence mobbing behaviour and the likely ecological impact of noisy miners around colony edges

description: Mobbing is a widespread, vocally coordinated behaviour where species
 approach and harass a threat. The noisy miner (Manorina melanocephala) is
 a notorious native Australian honeyeater, well-known for its
 hyperaggressive mobbing. Numerous studies have identified negative impacts
 of their mobbing behaviour, highlighting the exclusion of competitors from
 colony areas and the resulting loss of woodland-bird biodiversity. Despite
 this, few studies have investigated mobbing itself, and our understanding
 of the factors which influence its expression remains limited. Here, we
 use a field-based playback experiment to investigate whether mobbing
 responses vary in relation to colony borders and caller familiarity. Noisy
 miners were more likely to respond, reacted more quickly, and responded
 more strongly to mobbing calls broadcast inside as opposed to outside the
 colony. These behavioural differences likely arise from variation in the
 relative costs and benefits of responding. When noisy miners did mob
 outside the colony, more individuals joined in response to unfamiliar as
 opposed to familiar callers. Our results reveal that noisy miner mobbing
 may not be as indiscriminate as often assumed, with caller familiarity and
 location influencing this behaviour. We suggest there are benefits to
 greater consideration of the factors impacting noisy miner mobbing
 behaviour.
description: <strong>Study site and species</strong>
 Noisy miners are medium-sized (~ 70g) native Australian
 honeyeaters that live in persistent, year-round colonies across eastern
 and southern Australia (Higgins 2001). Consisting of up to several hundred
 birds, colonies occupy discrete areas of open eucalypt and urban habitat,
 preferring areas with a high edge-to-interior ratio (Taylor et al. 2008).
 Colonies are made up of highly-social cooperatively breeding subgroups,
 known as ‘coteries’, containing up to 20 individuals in over-lapping
 ‘activity spaces’ (Dow 1978; Dow & Whitmore 1990). Coterie members
 regularly interact with one another within these activity spaces,
 including foraging, roosting, and regular mobbing behaviour. Cross-coterie
 groups also form during larger mobbing events as birds leave these centres
 of activity to chase other noisy miners or mob heterospecifics (Arnold
 2000). Noisy miners are highly aggressive to a range of species, actively
 expelling both avian predators and other small to medium-size birds (i.e.
 potential food competitors) from their colonies (Dow & Whitmore
 1990, Clarke & Oldland 2007). Like much of noisy miner behaviour,
 mobbing is coordinated vocally; miners use two functionally referential
 alarm calls (Cunningham and Magrath 2017; Farrow et al. 2017; Holt et al.
 2017), a high-pitched ‘aerial’ alarm call given to raptors in flight and a
 broad-frequency mobbing (‘chur’) call given to ground and perched
 predators, as well as competing heterospecifics (Kennedy et al. 2009;
 Magrath and Bennett 2012). Past work has established that chur calls
 provide information about individual identity (Kennedy et al. 2009), but
 whilst noisy miners can differentiate between callers in a captive setting
 (McDonald 2012), field-based playback experiments suggest that it may not
 always be beneficial to do so (Kennedy et al. 2009). Individuals can
 adjust their response to vocalisations in relation to the relative costs
 and benefits of doing so, but the potentially fatal cost of not responding
 to alarm calls may outweigh the costs of discriminating between callers,
 even when information about caller identity is vocally encoded (Schibler
 and Manser 2007).<strong> </strong> This
 study was conducted on a wild population of noisy miners at Newholme Field
 Research Station (30° 25’ S, 151° 38’ E), a rural property approximately 7
 km north-west of Armidale managed by the University of New England, chosen
 as it supports a high density of noisy miners. This colony of ~100 adults
 inhabits a patch of remnant eucalypt woodland (~ 75 ha) in a
 heavily-cleared agricultural landscape, and has been studied since 2011
 (Barati et al. 2016, 2018; Barati and McDonald 2017, Farrow et al. 2020).
 We elected to run playback experiments on a single colony, as whether
 there is geographic variation in the mobbing behaviour of the species is
 currently unknown. While only one colony was tested, the sampling decision
 as to mob or not is taken at the individual level in this species and the
 experimental protocol was such that multiple coteries were exposed to
 playbacks (akin to testing multiple neighbouring social groups within a
 non-colonial species). To increase the likelihood that different playback
 sets (see ‘Playback experiments’ below) were targeting different colony
 members, the six playback pairs within the colony took place at least 145
 m apart (mean±SE: 304±34 m, range 145–595 m), a distance greater than the
 average activity space of individual noisy miners (mean diameter 129 m;
 Dow 1979). Moreover, different individuals from within the large colony
 respond to different mobbing events, so by moving the stimulus to
 different locations within and external to the colony, different birds
 were challenged in each playback. All research was authorised and
 conducted under NSW National Parks and Wildlife Service Scientific License
 (SL100314) and UNE Animal Ethics Committee (AEC20-099).
 <strong>Call collection and preparation of playback
 stimuli</strong> Playback stimuli were
 constructed with chur-call recordings (Fig 1) from noisy miner mobbing
 events collected during February and March 2023 at the Newholme colony and
 at a colony of noisy miners at Baker’s Creek, 25 km from Newholme.
 Recordings were made using a Marantz PMD661 professional solid-state
 recorder (sampling rate 44.1 kHz, 16-bit resolution; Marantz America,
 Mahwah, NJ) with a Sennheiser ME67 directional microphone (Sennheiser UK,
 High Wycombe, Buckinghamshire, UK) and Rycote softie windshield (Rycote
 Microphone Windshields, Stroud, Gloucestershire, UK). Mobbing events were
 provoked in calm conditions using a taxidermy model of a boobook
 (<em>Ninox boobook</em>) – a local owl species known to be
 aggressively mobbed by noisy miners – secured on a tree branch, with
 recording equipment mounted on a tripod 5–10 m away.
 For use in the ‘familiar’ treatments, six mobbing events were
 recorded at different locations within the Newholme colony, while for use
 in the ‘unfamiliar’ treatments, six mobbing events were recorded at
 different locations within the Baker’s Creek colony. Baker’s Creek is far
 enough for the colonies to be considered discrete and callers to be
 unfamiliar; recent analysis found little evidence of marked gene flow
 between colonies over 8 km apart (Barati unpublished data 2023; Dow
 1979). Twenty-four (i.e. 12 familiar and 12 unfamiliar)
 10-min playback tracks were constructed by extracting chur calls from
 original recordings and inserting them into ambient sound (recorded at the
 start/end of mobbing events) using Raven Pro (version 1.5, The Cornell
 Laboratory of Ornithology, NY, USA). To avoid pseudoreplication, two
 different tracks were constructed per mobbing event. Mobbing recordings
 contained chur calls from 8–12 individuals, with the number of individuals
 calling matched within each set of playback tracks (see below). To ensure
 consistent between playbacks in the amount of time a signal was present,
 all tracks were constructed as 30 s of chur calls followed by 30 s of
 silence for the 10 min duration. As Baker’s Creek is adjacent to a major
 road, a low-pass filter at 500 Hz was applied to all recordings from both
 sites to remove low-frequency disturbance. Tracks did not include any
 other vocalizations from noisy miners or other species.
 <strong>Playback experiments</strong>
 To assess whether the response to chur calls is influenced by
 call location in relation to colony borders and/or caller familiarity, a
 playback experiment was performed. Six playback sets were conducted, where
 each playback set comprised four treatments: familiar chur calls broadcast
 inside the colony (‘familiar inside’), unfamiliar chur calls broadcast
 inside the colony (‘unfamiliar inside’), familiar chur calls broadcast
 outside the colony (‘familiar outside’), and unfamiliar chur calls
 broadcast outside the colony (‘unfamiliar outside’). Colony borders were
 based on long-term observations of noisy miner occurrence and the
 distribution of recorded nests (Barati et al. 2016; Barati 2017). The
 order of the four treatments was counterbalanced between the six sets of
 trials. Unique tracks were used for each of the six playback sets, with
 the two familiar tracks in a given set constructed from recordings of the
 same Newholme mobbing event and the two unfamiliar tracks from the same
 Baker’s Creek mobbing event. To avoid habituation, models were placed in a
 different tree for each pair of inside/outside treatments within a given
 playback set. Paired trees were &lt;100 m of each other (mean±SE:
 52±11 m, range 20–96 m) – this distance is less than the mean activity
 space of individual noisy miners (Dow 1979), thus in each pair, the
 nearest miners were likely members of the same coterie (see Study species
 above). Playbacks experiments were conducted on calm,
 dry days from 07:00 – 12:00 h in February and March 2023. Experiments took
 place outside of the breeding season to avoid any changes in mobbing
 behaviour that might be associated with nest protection and brood
 provisioning (Arnold et al. 2005). When weather allowed, a maximum of two
 playbacks were conducted per day, at least 3.5 hours apart. Playback
 locations were alternated such that if a day’s first playback took place
 inside the colony the second took place outside and vice versa. To ensure
 that consecutive playbacks were not targeting the same colony members (and
 so minimising risk of habituation to the stimuli), consecutive playbacks
 took place at least 455 m apart (mean±SE: 545±28 m, range 455–646 m), over
 3 times the average activity space of individual noisy miners (Dow
 1979). Before each playback, one of four 3D-printed
 boobook models was placed in a tree within arm’s reach when no noisy
 miners were visible in the immediate vicinity and covered by a pillowcase
 (as in Mesken 2021). All four models were used in a given set and their
 order of presentation counterbalanced. A portable Bluetooth speaker (JBL
 Xtreme 2, Harman International, Connecticut, US) was placed in the tree
 below the model. A Digitech QM-1589 sound meter was used to determine
 playback amplitude prior to experiments and all calls were broadcast at
 natural levels (~80 dBC at 1 m; Holt et al. 2017). After setting up the
 equipment, the observer removed the pillowcase and then entered a hide
 with it positioned &gt;25 m away, before starting the relevant
 playback. While it was impossible to be blind to location (inside/outside
 the colony), the observer was blind to caller type (familiar/unfamiliar).
 If a natural disturbance occurred (e.g. aerial alarm call or natural
 mobbing event), the trial was abandoned and repeated on another day
 (N=2). During trials, the following data were recorded
 to a Dictaphone (Sony Europe Limited, Surrey, UK): response latency (time
 between start of playback and the first noisy miner to approach &lt;10
 m of the model); latency to give the first chur call (time between start
 of playback and the first noisy miner &lt;10 m of the model to produce
 a chur call); maximum number of birds that responded; duration of the
 response (total time where noisy miner/s were &lt;10 m radius of the
 model and responding to the model). Responding to the model was
 categorised as either looking toward the speaker, movement toward the
 speaker, and/or chur calling (Cunningham and Magrath 2017). As playbacks
 only commenced when no birds were within the immediate vicinity, we were
 able to count individual birds as they flew within the 10 m radius, aided
 by the patchy nature of the habitat. During noisy miner mobbing events,
 the number of birds generally peaks early on, with individuals flying in
 singly or in small clusters to the nearest trees where they perch until
 gradually leaving the mob as it subsides. Data collection ended at the end
 of the 10-min track. Thus, where no birds had responded or called by the
 end of the 10 min trial, latency was assigned a cut-off value of 600 s; if
 birds were still responding response duration was recorded as 10
 minutes.
description: # Location and caller familiarity influence mobbing behaviour and the
 likely ecological impact of noisy miners around colony edges
 [https://doi.org/10.5061/dryad.xsj3tx9pf](https://doi.org/10.5061/dryad.xsj3tx9pf) ## Description of the data and file structure #### **Mobbing_playbacks_responses_data.csv** Responses recorded during playback experiments broadcasting chur-calls. Trial: 1-6, denotes playback set. Six playback sets were conducted, where each playback set comprised four treatments: familiar chur calls broadcast inside the colony (‘familiar inside’), unfamiliar chur calls broadcast inside the colony (‘unfamiliar inside’), familiar chur calls broadcast outside the colony (‘familiar outside’), and unfamiliar chur calls broadcast outside the colony (‘unfamiliar outside’). Treatment: Familiar inside, Unfamiliar inside, Familiar outside, Unfamiliar outside. CallerType: Familiar/Unfamiliar. Chur calls used in the ‘familiar’ treatments were recorded at different locations within the Newholme colony, while calls used in the ‘unfamiliar’ treatments were recorded at different locations within the Baker’s Creek colony, 25 km from Newholme. Location: Inside/Outside the Newholme colony boundaries. PB_Order: Order of playback of the four treatments within each set. Date: date of playback Time_slot: A/B, 07:00 – 8:30 h (time slot A) and 10:30 – 12:00 h (time slot B). Response_binomial: 0/1, denotes whether noisy miners did (1) or did not (0) respond to cur-call playback. Response_latency_log: log transformed response latency (time between start of playback and the first noisy miner to approach &lt;10 m of the model). Call_latency_log: log transformed latency to give the first chur call (time between start of playback and the first noisy miner &lt;10 m of the model to produce a chur call). Response_duration_sqrt: square-root transformed duration of the response (total time where noisy miner/s were &lt;10 m radius of the model and responding to the model). Total_no_responders: maximum number of birds that responded. #### **Mobbing playbacks_mixed models** R script used to analyse data in Mobbing_playbacks_responses_data.csv with linear/generalized linear mixed models