Source of test subjects
Adult noisy miners (n = 32) were caught using mist nets from four locations near Armidale, New South Wales, Australia: Roumalla Creek TSR (30°25'28.56"S 151°11'37.14"E, n = 6), the University of New England Armidale campus (30°29'04.1"S 151°38'28.6"E, n = 8), Burying Ground Creek TSR (30°32'47.5"S 151°46'07.7"E, n = 7) and a privately owned property near Argyle (30°33'17.5"S 151°51'18.3"E, n = 10) from July 2020 – May 2021. These sites were selected to ensure naivety of test subjects as locations were a minimum of 14 km apart in linear distance (more than 100 times further than average home range of this species, Higgins et al. 2001). All sites consisted of dry, open woodland vegetation with a large eucalypt overstory and sparse understory vegetation, except for the privately-owned property where regeneration efforts have led to relatively dense understory vegetation (LF personal observations).
After capture, birds were transported back to the University of New England Armidale campus and housed individually in a 76- x 45- x 45-cm cage with 30 mL of Wombaroo Nectivore wet-mix (Wombaroo Food Products, Glen Osmond SA, Australia) and supplied twice daily in conjunction with ad libitum madeira cake and mealworms (Tenebrio molitor). All individuals were left overnight under natural light conditions to habituate to holding conditions prior to the commencement of trials. Experimental trials were conducted between 0800h – 1600h, with individuals housed for no longer than three consecutive nights prior to release. Individuals housed in the same room were always from the same colony and were therefore already familiar with each other. Birds housed together from the same colony were released as a group at their point of capture during periods of benign weather with at least two hours of daylight remaining.
Ethics statement
Ethics approval was granted by the University of New England’s Animal Ethics Committee (AEC20-032), with no injuries or deaths recorded during the experiment. Banding of caught individuals was conducted under license SL100314 provided by the National Parks and Wildlife Service, using bands supplied by and in accordance with protocols approved by the Australian Bird and Bat Banding Scheme (authority A2259).
Playback procedure
Calls used during the playback procedure were obtained by Farrow et. (2020) with all call exemplars used being unfamiliar (temporally separated) to subjects in order to avoid past experience impacting responses. Vocalisations from signallers used in experiments were recorded from colonies located at Dumaresq Dam (30°30’S, 151°40’E) and the University of New England Newholme Field Research Station (30°25’24’’S; 151°38’38’’E). Only signallers that had been fitted with unique leg colour band to allow for identification were used, ensuring that recordings could be assigned to a specific signaller without ambiguity. Additionally, only those signallers that had a minimum of 25 different calls from each call type were used in the playback experiment to avoid pseudo-replication impacting results. Consequently, this allowed for 11 signallers at Dumaresq Dam and 10 signallers from Newholme Field Research Station.
Subjects were placed within a 76- x 45- x 45-cm cage located within a sound booth (Acoustical Design PL, Blackburn South, Australia) to ensure subjects were reacting to broadcast sounds only. The sound booth was regulated at temperatures between 18–25℃, with each cage having a single wooden perch to control the orientation of perched birds. Wet food (Nectivore wet mix, Wombaroo, Glen Osmond, South Australia) and madeira cake were supplied ad libitum during the experimental trials, with feeders washed and replenished between test subjects. Vocalisations were played via one of two speakers (Advent AV570, Advent, Cambridge, MA) connected to an iMac (2015 model, EL Capitan, Apple Inc, Cupertino, CA) and placed at either end of the holding cage perpendicular to the single perch. This ensured that all individuals were a similar distance from either speaker, with playback randomly assigned to each side to account for any hearing biases (McDonald 2012).
Each subject heard the same number of stimuli, with calls presented at 30 second intervals. Subjects were monitored via an overhead camera (Panasonic HC-V270) placed on a Manfrotto tripod (MKCOMPACTLT – BK Compact Light 4 Section Tripod) that allowed an observer in the adjacent room to monitor real-time behaviour without disturbing subjects. Any birds deemed to be exhibiting behaviours linked to high stress levels, such as excessive movement or gular fluttering, were planned to be excluded from the experiment and treated accordingly, however this was not required. Subject responses during the experimental period were recorded and this footage used to score responses as outlined below.
Playback method consisted of a given subject being exposed to a new call exemplar broadcast at natural volume (89dB, Holt et al. 2017) every 30s following the procedure depicted in Fig. 2. Rarely, if a subject was distracted when a call was due to be broadcast, for example moving in the cage, playback of that specific exemplar was delayed slightly until the subject was back in a stationary position on the central perch. This was assessed by viewing video footage in real time, and only involved a delay or several seconds at most.
Broadly, playback involved a “Habituation Phase” where the subject was exposed to 23 different exemplars of the same call type (either aerial or terrestrial contexts) from the same signaller (Signaller A). A 24th “Test” call of the same call type as in the Habituation Phase was then played, however this time the exemplar broadcast was from a new signaller (Signaller B). Call 25 in the series was the “Rebound” call, which was used to control for any false-positive responses potentially observed in the Test call exemplar (McDonald 2012) and, as such, was a new exemplar of the same context as the Habituation Phase from Signaller A. The 26th call in the series was the “Transference” test and was of the alternate context to the call used in the habituation phase (i.e. if terrestrial calls were played during habituation phase, call 26 would be of aerial alarm context). Importantly, to confirm that responses seen to these transferred calls were true indications of transferred status of the signaller (and thus TIR), half the subjects received the transference call from the signaller to which they had habituated (Signaller A; Groups A & B) and half received the transference call from the signaller to which they had not habituated (Signaller B; Groups C & D). Doing so, we reasoned that if subjects in Groups A and B maintained habituation when hearing the transference call from Signaller A in a new context, and subjects in Groups C and D dishabituated if hearing Signaller B in a new context, this would be empirical evidence of TIR. Finally, the 27th call was the “Attending” test call, which was of a new context (fledgling begging “chip” call) also from a new signaller (Signaller C). The Attending test call was used to ensure subjects were still attending to calls and any lowered responses observed in prior signals were part of the experimental procedure and not a change in the subject’s attention. As such, any subjects not dishabituating when hearing this call would have been dropped from the analysis, however again this was not necessary as all subjects responded to the Attending test call exemplar.
Response scoring
Final Cut Pro version 10.5.2 (Apple Inc., Cupertino, CA) was used to strip audio from the footage and cut samples into smaller videos of a single playback call and its response (using the audio waveform to identify when each exemplar was broadcast). These edited sequences were renamed randomly using the inbuilt Mac OS Finder tool and saved without audio to ensure that observers were blind to the exemplar context when scoring sequences.
Due to the difficulty of determining gaze direction, we used the method deployed in past experiments that investigated differentiation of signallers (McDonald 2012; Farrow et al. 2017; Farrow et al. 2020) to measure the most informative responses in playbacks to this species, which were: 1) initial time to first respond (a head turn > 5 ° without the body moving) to the signal (measured in 24 frames per second), 2) degree to which the head moved in response to the signal and 3) duration of this initial head turn (fps). Degree of head turn was measured by Pixel Stick version 2.16.2 (Plum Amazing Software LLC).
Statistical Analyses
All statistical analyses were conducted in R (Version 4.0.3 for Mac OS X GUI, R Core Team 2021), with the measured responses log-transformed to generate normal distributions before they were reduced with a principal component analysis (PCA). Only one component had an eigenvalue above 1 (PC1 = 1.61), and this component accounted for 79% of the variance in the data set. PC1 loaded most heavily and positively on the measures “response time” (0.591) and “duration of the initial head-turn” (0.351). Consequently, PC1 was treated as the dependent variable in a generalised linear mixed model (GLMM), with a Gaussian link function using the package “lme4” to assay the fixed effect of “Stimulus type”, a seven-level factor that corresponded to the playback stages of interest; the first (call 1), middle (call 12) and last call within the Habituation Phase (call 23), the Test call (call 24), the Rebound test (call 25), the Transference test (call 26) and the Attending test (call 27). The fixed factors of “context” (aerial or terrestrial alarm call) and treatment (whether call 26 was from the Habituation Phase Signaller A or the Test call Signaller B) were also tested. All two-way and the three-way interactions between stimulus type, context and treatment were also included. Additionally, two random terms were included as potentially confounding covariates: 1) source colony (3 levels) of the subject and 2) subject identity to account for repeated measures of a subject.
Likelihood Ratio Tests (LRT; package “lme4”, Bates et al. 2015) were used to determine the significance of dropping terms from the model, starting with the three-way interaction. Planned post-hoc comparisons comparing individual levels of interest in main effect of stimulus type were then carried out using Estimated Marginal Means (emmeans and emmip; using package “emmeans”, Lenth 2021). No formal adjustments were made to alpha levels to account for multiple testing, with p values presented unaltered instead for the reader to assess.