Experimentally induced innovations lead to persistent culture via conformity in wild birds

Abstract

In human societies, cultural norms arise when behaviours are transmitted through social networks via high-fidelity social learning. However, a paucity of experimental studies has meant that there is no comparable understanding of the process by which socially transmitted behaviours might spread and persist in animal populations. Here we show experimental evidence of the establishment of foraging traditions in a wild bird population. We introduced alternative novel foraging techniques into replicated wild sub-populations of great tits (Parus major) and used automated tracking to map the diffusion, establishment and long-term persistence of the seeded innovations. Furthermore, we used social network analysis to examine the social factors that influenced diffusion dynamics. From only two trained birds in each sub-population, the information spread rapidly through social network ties, to reach an average of 75% of individuals, with a total of 414 knowledgeable individuals performing 57,909 solutions over all replicates. The sub-populations were heavily biased towards using the technique that was originally introduced, resulting in established local traditions that were stable over two generations, despite a high population turnover. Finally, we demonstrate a strong effect of social conformity, with individuals disproportionately adopting the most frequent local variant when first acquiring an innovation, and continuing to favour social information over personal information. Cultural conformity is thought to be a key factor in the evolution of complex culture in humans. In providing the first experimental demonstration of conformity in a wild non-primate, and of cultural norms in foraging techniques in any wild animal, our results suggest a much broader taxonomic occurrence of such an apparently complex cultural behaviour.

Extended Data Fig. 1. Wytham Woods (51°46’N, 01°20’W), showing the location of replicates and puzzle-boxes

Total area of Wytham Woods is 385ha; location and size of the separate woodland areas within this are labeled on the map. Green points indicate puzzle-box locations for three ‘control’ replicates C1-3: Broad Oak, Bean, Singing Way. Blue points indicate location of puzzle-boxes for two ‘option A’ replicates T1-2: Common Piece, Brogdens Belt. Red points indicate location of puzzle-boxes for three ‘option B’ replicates T3-5: Great Wood, Pasticks, Marley Plantation. (d) indicates locations where trained demonstrators were caught from and released to.

Extended Data Fig. 2. Social network data collection

a, Feeding station (shut), with sunflower-feeder, RFID antennae, and data-logging hardware. Cage is to restrict access to small passerines only. b, Map of study area showing placement of 65 feeding stations. Stations are approximately 250m apart and open simultaneously dawn-dusk on Saturday and Sunday over winter. c, Grouping events are inferred from the temporal data stream gained from feeding stations, with individuals assigned to grouping events in a bipartite network. d, Repeated co-occurrences are used to create social networks (adapted from Psorakis et al. (2012)).

Extended Data Fig. 3. Social Networks showing diffusion of innovation

Red nodes are individuals that acquired the novel behaviour after 20 days of exposure, black nodes are naïve individuals and yellow nodes are trained demonstrators. Networks are heavily thresholded to only show links above the average edge strength for each replicate (T1-5: 0.09, 0.05, 0.08 0.07, 0.07). a, Social network for T1 replicate (n=123). b, Network for T2 replicate (n=137). c, Network for T3 replicate (n=154). d, Network for T4 replicate (n=95). e, Network for T5 replicate (n=110).

Extended Data Fig. 4. Individual trajectories (option A/B) for each replicate

Only individuals that performed both options are included, and Individuals that moved between replicates are excluded. Lines are running proportions of seeded variant for each individual over its last 10 visits. a, T1 (option A), n=30; b, T2 (option A), n=10; c, T3 (option B), n=19; d, T4 (option B), n=4; e, T5 (option B), n=15.

Extended Data Fig. 5. Food preferences trials

Birds were presented with a freely available mix of 40 mealworms, peanut granules and sunflower seeds for 1 hr on 2 days over 1 week at 6 sites (3 sites in T4 and T2). Trials were conducted 2 weeks after the end of the main experiment, in March 2014. Food choice was identified from video camera footage, and the trial was halted when all of one prey item was taken. Only great tits were included, but birds could not be individually identified. Birds clearly preferred the live mealworms to either peanut granules or sunflower seeds.

Fig 1. Cultural diffusion experiment

a, Puzzle-box where birds can slide the door either way (from left, option A; or right, option B) to access a reward. Puzzle-box records identity, visit duration and solution choice, and resets after each visit. b, Diffusion curves for treatment sub-populations with demonstrators (T1-5; n=91, 130, 132, 90, 50) and control sub-populations without demonstrators (C1-3; n=56, 87, 61). c, Total number of solutions of each option in each replicate; x-axis indicates demonstrated option. Points show mean proportion of option A performed by individuals with 95% CI; y-axis on right. No. solvers=5, 46, 19 (control); 76, 89 (A); 96, 69, 37 (B).

Fig. 2. Evidence for social conformity

a, Proportion of solutions as seeded technique in each replicate significantly increases over time. Points are proportion as seeded technique on each day; lines are GEE model fit. b, Comparison of frequency of option A in previous group with an individual’s first learnt option. Node size represents number of individuals (n=1-147). Black line shows expectation under unbiased copying, red lines show model fit with 95% CI. c, Solution trajectories from individuals that used both possible options in T2 replicate (n=10). Lines are running proportions of seeded technique for each individual over last 10 visits.

Fig. 3. Local traditions persist between years

a, Diffusion curves for initial (T1 2013-I/T3 2013-I; 1-20dy) and second exposure (T1 2013-II/T3 2013-II; 1-5dy). Uptake rate in second exposure is much higher for prior solvers (T1 2013-II/T3 2013-II; pop. sizes=23, 26), but also higher for naïve birds (T1 2013-II/T3 2013-II; pop. sizes=28, 27). b, Number of solutions as option A/B. In T1 one circuit board failed, so data are from 2/3 devices. Bars are split into prior solvers (ps) and naïve birds (nb). c, Proportion option A/B in initial and second exposure; x-axis indicates initially demonstrated option. Points show mean proportion option A performed by individuals with 95% CI.