Subjects and housing

Subjects were ten subadult ravens (6 males and 4 females), all hand-raised and socially housed at the Haidlhof Research Station, Austria (see Supplementary Table 1 and Supplementary Fig. 1a for details). Birds were taken as nestlings at the age of ∼4 weeks, individually marked with coloured leg bands and hand-reared to fledging by human caretakers for the purpose of behavioural studies. As fledging, they have been subjected to daily observations and cognitive tests; accordingly, they have been trained to come by name and individually participate in physical and visual isolation from their group members. All birds were fully habituated to the experimental setup and testing procedure before the onset of the study but none of them had any experience with caching experiments.

The experiment was conducted from May to October 2013. At the time of testing, nine birds were in the second year of life and one bird (female Astrid) was in its fourth year. All birds but this older female were kept in one social group in the non-breeder area (Supplementary Fig. 1a); the older female had already formed a pair bond with another male and thus was kept in a separate compartment adjacent to the non-breeder area. Keeping compartments were 5 m high and equipped with roofed areas for rain and sun-protection, several perches, live and artificial trees, bathing tubs and different types of ground substrate (sand, gravel and wood chips).

The ravens were fed twice a day with a mixture of fruits, milk products and meat. During experimental days, meat was provided only in the afternoon feeding (after the experiments). Water was provided ad libidum.

Ethical note

The ravens originated from captive breeding pairs in zoos (Zoo Wels, Austria; Wildpark Haag, Austria; Nationalpark Bayrischer Wald, Germany; Gymanisium Spanga, Stockholm, Sweden). The study complied with Austrian law and local government guidelines. The study subjects remained in captivity at the Haidlhof Research Station after the completion of this study for further research.

Experimental design and set-up

Experiments were conducted in an experimental compound that was directly connected to the birds’ keeping compartments (Supplementary Fig. 1a). We used two adjacent rooms of identical size (each 3 × 4 × 4 m3) and equipment (sand floor; three perches; one wooden platform of 2.5 × 1 m2 at 3 m height; one wooden box of 1 × 1 × 1 m3 on the ground; one metal bowl filled with fresh water). The rooms were connected by two functional windows (35 × 25 cm2, 85 × 185 cm2), both of which were covered with wire-mesh (transparent barrier) but could also be visually closed with wooden panels (opaque barrier; Supplementary Fig. 1b). For the experiment, the left room was designated as caching room and the right room for holding potential competitors. Accordingly, individual ravens were presented with food (four pieces of cheese and dried dog food) that could be cached in the left room, whereas they did not receive any food in the right room. In the observed condition, the wooden covers on the windows were open and ravens in the right and left room could see each other. In the non-observed condition, the wooden covers were closed and the ravens in the right and left room could not see each other. Note that in both conditions, auditory contact was possible. Before the onset of the experiment, we drilled two peepholes (1 cm radius) into the wooden covers of the windows but made those non-functional by placing a small wooden board on top.

The experiment consisted of baseline trials (n=8) and interspersed playback trials (n=2). Baseline trials featured either an observed or a non-observed condition, that is, the focal subject was given food in the left compartment, whereas potential competitors were visibly present (observers) or just audibly present (non-observers) in the right compartment. Playback trials simulated the presence of conspecifics in the right compartment by playing raven sounds recorded from non-observers in the non-observed condition of the baseline phase. Note that during playback trials the covers of the door and window were closed (resembling a non-observed condition) but one of the two peepholes was open (offering possibility of being observed).

To make sure that focal subjects were knowledgeable about the open peephole, each bird got individually introduced to it from the caching room’s perspective by a human experimenter before the playback trial. The human took position on the other side of peephole and got the bird’s attention by calling its name and showing a piece of cheese through the peephole; when the focal bird looked through the hole, the human cached the piece of cheese in the sand and opened the door to the right room, allowing the raven to come over and search for the hidden food. Each bird had to pass the criterion of finding the cached food within 30 s two times in a row on two consecutive days before it proceeded to the playback trial. All but one bird passed the criterion in two or three sessions; one male (Rufus) appeared to be scared of the peepholes and consequently was excluded from the tests. After each individual session, the peephole was closed to prevent ravens from obtaining experience with it from the competitor’s perspective.

All baseline and playback trials were administered to each ravens in a given sequence: four baseline trials (n=2 trials observed and n=2 non-observed, presented in random order) were followed by the first playback trial; another four baseline trials (n=2 trials observed and n=2 non-observed, presented in random order) were followed by the second playback trial. Irrespective of condition, a trial lasted 4 min and was followed by a 1-min feedback period, in which the large window to the competitors’ room was opened and observers and non-observers, respectively, were allowed to enter the caching room and search for the caches in the presence of the focal subject. In playback trials, those birds whose sounds were played back were allowed to enter the experimental compound before the door to the caching room was opened.

Data collection and analysis

Even though the ravens were naïve to the experimental setup, they readily cached (parts) of the food within the given time frame of 4 min. Hence, no warm-up or training phase was necessary and data collection started with the very first trials of the baseline phase (observed and non-observed, in randomized order). In case a bird ate all four food pieces within the first minute, it was given another four pieces by the experimenter and the total time increased to 5 min.

During the first baseline phase of the experiment (per bird, n=2 trials observed and n=2 non-observed, respectively), we recorded the sounds made by the competitor (non-subject) raven in the non-observed condition using a Sennheiser directional microphone (ME 66) connected to a digital sound recorder (Zoom H4n Handy Mobile 4-Track Recorder). Loud calls (‘rab’) of two male and two female group members were recoded. For both sexes, the call providers were non-siblings and medium-to-high-ranking animals (second and third rank in males, top and second rank in females). Seven loud calls with high signal-to-noise ratio (0.288 s average duration) were chosen from each individual. Using Adobe Audition (Version 4.0 × 1815, 1992-2011 Adobe Systems Incorporated) peak amplitude of all calls was equalized to an average level of −9 dB Sound Pressure Level (SPL) (in Adobe Audition wave display) to prevent abrupt changes in loudness. For each focal subject, an individual acoustic stimulus set was created consisting of calls from one single non-sibling, same sex call provider. A set contained two tracks with one bout of three calls each. Within a bout, there was a call onset every 0.7 s. Both bouts in a set were composed of six different call recordings randomly assembled by a custom-made Python script (programmed by Jinook Oh). In addition, each set was assigned one of two sound recordings of a raven’s wing flapping when flying up or down from the platform. Both of these recordings were peak amplitude equalized to an average level of −15 dB SPL using again Adobe Audition (same version).

During all experimental trials, the behaviour of the focal subject and its potential competitors was recorded on video, with one camera covering the right and a second camera the left experimental room. In playback trials, a loudspeaker (LD Systems Roadboy 65, frequency response: 80–15,000 Hz) was hidden behind the wooden box (1 × 1 × 1 m3) in the right compartment (Supplementary Fig. 1b). Sounds of a potential competitor (of the same sex as but non-affiliated to the focal bird) were played back via an iPod-touch device (fourth generation, MC540LL/A) and a radio transmitter-receiver system (Sennheiser EW 112-p G3-A Band, 516-558 MHz) three times per trial: (i) a ‘rab’ call right before food was given to the focal subject for caching should alert it to the presence of a particular bird in the adjacent compartment; (ii) a wing-flapping sound presented ∼30 s after the food has been given to the focal subject should indicate that the competitor is moving and potentially interested in the focal bird’s behaviour; (iii) another ‘rab’ call during minute 2 should remind the focal subject that the competitor is still there. All sounds were played at natural loudness in accordance with the current wind conditions. Playback amplitude, measured at the proximate wall of the neighbouring room, reached on average 50 dBC (SPL, measured with a Voltcraft SL-100 Digital Sound Level Meter 5 Hz to 8 kHz).

Supplementary Table 2 summarizes the behavioural parameters, which were scored from the videos by Thomas Bugnyar. Only those parameters that produced significant results between the observed and non-observed condition in the initial baseline step (before the introduction of the peepholes) were used as predictors for our hypothesis (Supplementary Table 3). Note that in the peephole condition, we additionally estimated the visibility of a cache. For this purpose, a picture was taken through the peephole at the end of the experiment; all caches that were located in the area that was visible in this picture were scored as ‘inside view’, whereas those that were placed in the area not visible at the picture were considered ‘outside view’ (Supplementary Fig. 2). A randomly selected subsample of the videos (10%) was also coded by Stephan Reber. Inter-rater reliability calculations showed a high level of agreement for both behavioural frequencies (Cohen’s Kappa: κ=0.785, exact P≤0.001) and durations (Spearman’s rho correlations: ρ-correlation coefficients≥0.723, P≤0.018).

For the initial analyses, we employed generalized linear mixed models. The order of the experimental trials (first four baseline trials and playback 1, second four baseline trials and playback 2), the conditions (observed, non-observed, peephole) and the interaction between these two were used as the fixed effects (repeated measures within subjects, subjects as random effects with intercept). The target distribution was kept as a linear model, but testing was performed with robust covariances to account for potential violations of model assumptions. Initial models were stepwise reduced and the best fitting model selected with the Akaike Information Criteria. In the absence of a significant effect of trial order, we calculated the mean values per condition (non-observed, observed, peephole) for each subject and used these values for the subsequent analyses. We applied Friedman tests to compare ravens’ behaviour between the observed, non-observed and peephole condition and exact Wilcoxon signed-rank tests for post hoc pairwise comparisons. Owing to multiple testing, we conducted Bonferroni corrections. All analyses were performed in SPSS (v. 20).