Subjects, their housing and the period of data collection

Subjects were 9 sub-adult common ravens (6 males, 3 females) of our captive colony at the Haidlhof research station of the University of Vienna and of the Veterinary University of Vienna, in Bad Vöslau, Austria. At the time of testing, the birds were housed in one social group made up of 10 individuals that were derived from four nests in captivity (Stockholm, Bayrischer Wald, Wels and Haag). All birds arrived at Haidlhof research station in 2012 as nestlings at the age of 3 to 5 weeks, were hand-raised to fledging and kept in this social group ever since. They were housed in a large outdoor aviary complex (15 × 15 × 5 m) composed of several compartments. The compartments could be closed off by sliding doors and used for separating of one or more individuals from the group for testing. All birds were familiarized with the separation procedure shortly after fledging using positive reinforcement. They were never food deprived and fed a diet consisting of meat, milk products, bread, vegetables and fruits twice daily; water was available ad libitum. Participation in experiments was voluntary, with birds coming in after being called by their name and in expectation of small rewards not included in their daily diet. In the current studies, the lowest ranked male did not participate in any of the tests and therefore was excluded from all analyses. Two other subordinate males (Paul, Rufus) participated only in Study 2 (see Supplementary Table 1), since continuous and consistent conflicts between them and other group members led us to temporarily separate them from the rest of the group, together with the other subordinate bird. Consequently, they could not participate in the group experiment. Fortunately, the relationships between these birds improved over the course of this study, which allowed us to include these two birds in study 2. Study 1 was conducted between September and December 2013 and study 2 between January and March 2014.

Ethical note

Participation of the birds in our experiments was voluntarily. Since all experiments were non-invasive, the study complied with Austrian law. Moreover, the study received oversight from- and was authorized by the ethical board of the behavioural research group at the faculty of Life sciences, University of Vienna (case number: 2015-003). The birds remained in captivity at the Haidlhof Research Station after the study.

Procedure and design

Familiarization

Even though the set-up was positioned outside of the aviary, all individuals were habituated stepwise to the apparatus, taking possible effects of neophobia towards novel objects into account. The ravens were first introduced to the big panel (200 cm × 60 cm × 1,5 cm) and then to the small panel (78 cm × 10 cm × 1,0 cm) 2.5 weeks before the experiment started. During this habituation phase the whole group had permanent access to the set-up and we regularly placed small treats on the set-up that were in reach for the ravens. We only started testing after we had established that all individuals regularly came to the set-up to take some of the treats.

Cooperative string-pulling test (group)

The whole group participated in one round of cooperative group tests in order to asses whether the ravens would solve the cooperative problem spontaneously and to subsequently asses which factors influence cooperative success when individuals are able to choose their cooperation partner among all group members. Per day we conducted 3 to 9 sessions with at least a 1 hour break after every third session took place. Each session consisted of 20 trials with a 20 s inter-trial interval and 5 min inter-session interval. In total we ran 30 sessions.

Cooperative string-pulling test (dyads)

We ran two rounds of dyadic cooperation tests, which also included control trials. With these tests, we aimed to investigate which factors might influence cooperative success when individuals are forced in certain dyads and whether the ravens understood the need for a partner within this paradigm. Every individual was tested with every other individual of the group (36 dyads) in one round and this was then repeated in the second round. The order of dyads was randomized in both rounds. Each session consisted of 16 cooperation trials with a 20 s inter-trial interval, plus, per individual in the dyad, 4 control trials; i.e., 2 solitary trials and 2 delay trials. In the former, the focal bird was solitary and we were interested to see whether it would not pull the string, since successful cooperation was impossible. In the latter, the other bird in the dyad was granted access to the testing compartment as soon as both ends of the string were placed in the aviary and we were interested to see whether the focal bird would wait for its partner to arrive at the apparatus before commencing to pull.

Coding

All trials were videotaped with 2 cameras. CR coded all trials live and afterwards from the videotapes. In study 1 she coded who pulled on which side of the string and whether it eventually led to successful cooperation; i.e., two individuals had to pull each on one end of the string simultaneously in order to move the panel as far to the fence that the food was in reach. A trial was considered unsuccessful when the string got loose, which means that the other end of the string was pulled back out of the aviary and therefore out of reach for the partner. In study 2 she additionally coded whether individuals pulled or not in the solitary trial and whether they waited in the delay trials. Moreover, in study 2 she coded whether after successful cooperation the two birds would divide the rewards equally or not; i.e., would they both take the one piece of cheese on their side of the feeding platform, or would one of the birds chase away the other and take both rewards. A research assistant independently recoded 10% of the videos of study 1 and 11.11% of the videos of study 2 and inter-rater agreement was good (Cohen’s kappa = 0.70) and excellent (Cohen’s kappa = 0.93) for both studies respectively.

Additional data

Tolerance data

We tested all individuals’ ability to tolerate other individuals in front of the apparatus at the same time. For this, pieces of Frolic® dog food were attached to two separate strings. Birds thus had to pull on one of the strings to reach the reward, without the need of a cooperation partner. Both ends were simultaneously moved into the aviary and were placed in a way that the ends were either 30 cm, 60 cm or 120 cm apart from each other. Providing these different distances could potentially reveal specific distances in which specific birds tolerate each other’s presence in a food-context. Per day, we ran 3 sessions, each with a different distance. One session consisted of 20 trials with a 20 s inter-trial interval and 5 min inter-session interval. Order of distances was counterbalanced over a total of 6 days. Consequently, we conducted 18 sessions, 6 for each distance, per round. We ran 3 of these rounds: one before study 1, one in between study 1 and 2 and one after study 2. Tolerance data were very consistent over the different distances (ICC: 0.855, p < 0.001) and over the different testing rounds (ICC: 0.574, p < 0.001) and were therefore lumped. However, since there were additional animals involved in study 2 vs. study 1, for the analyses of study 1, we used combined tolerance data based on the tolerance tests before and after study 1, whereas for the analyses of study 2, we used combined tolerance data based on the tolerance tests before and after study 2.

Dominance hierarchy data

To determine the dominance hierarchy in the group, during the study period, we conducted 3 different monopolization experiments in which we placed two highly preferred, but monopolizable food-items in the group for 30 minutes and videotaped the whole session. Afterwards, all unidirectional displacements75 were scored and data were arranged in a matrix with actors in rows and recipients in columns. A dominance order most consistent with a linear hierarchy was determined, calculating Landau’s linearity indices (h’) using MatMan 1.176 and the matrix was reordered to best fit a linear hierarchy77,78. We found a significantly linear hierarchy in the group (h’ = 0.858, n = 9, p < 0.001, based on 524 interactions and 5.56% unknown relationships).

Statistical analysis

Study 1

To test what possible factors might explain the difference in success between the different dyads, we ran a generalized linear mixed model (GLMM) on the number of successful cooperation trials, in which we added inter-individual tolerance levels, dominance rank, rank distance, sex, sex-combination and kinship (yes/no) as fixed effects and the identity of the dyad and the subject of analyses as random effects to control for repeated measurements. We ran a full model and several reduced models using a backward step-wise approach. The best fitting model was chosen based on comparisons of corrected Akaike Information Criteria (cAIC).

Study 2

To test what possible factors might explain the difference in success between the different dyads, we ran a GLMM on the number of successful cooperation trials, including the same factors as in study 1. In addition, here we also included session number/individual (ranging between 1–16) to see whether there were any learning effects. In addition, to test whether birds reacted to the reward distribution after a successful cooperation trial, we analyzed for all trials after the first successful cooperation trial within a session, what would be the probability that both birds would cooperate successfully again. Therefore, we ran a binomial GLMM with a logit link function on whether a dyad would cooperate (Y/N) and entered the same fixed factors as previous analyses, although now we treated session number as a random variable. In addition, we now entered whether the reward distribution in the previous successful trial was equal (Y/N) as a fixed factor. And finally, we tested whether the amount of rewards an individual received in the previous successful trial would influence whether it would pull at all (be it cooperative or unsuccessful solitary pulls) in the next trial or not. Therefore, we ran a binomial GLMM with a logit link function on whether a bird would pull or not and only entered the amount of rewards that birds had received in the previous trial as a fixed variable, whereas we included the identity of the dyad and the subject of analyses as random effects to control for repeated measurements and session number as a random effect to control for learning/motivation effects. We ran full models and several reduced models using a backward step-wise approach. Best fitting models were chosen based on comparisons of corrected Akaike Information Criteria (cAIC).

Overall

We used Wilcoxon signed ranks tests and Mann-Whitney U tests for post-hoc comparisons and used a Holm Bonferroni79 correction for calculating corresponding p-values when multiple comparisons were made on one data-set. All tests were two-tailed and we set alpha to 0.05.