Abstract
Cultural traditions have been observed in a wide variety of animal species. It remains unclear, however, what is required for social learning to give rise to stable traditions: what level of precision and what learning strategies are required. We address these questions by fitting models of cultural evolution to learned bird song. We recorded 615 swamp sparrow (Melospiza georgiana) song repertoires, and compared syllable frequency distributions to the output of individual-based simulations. We find that syllables are learned with an estimated error rate of 1.85% and with a conformist bias in learning. This bias is consistent with a simple mechanism of overproduction and selective attrition. Finally, we estimate that syllable types could frequently persist for more than 500 years. Our results demonstrate conformist bias in natural animal behaviour and show that this, along with moderately precise learning, may support traditions whose stability rivals those of humans.
Highlights
Where pi,j is the probability of types i and j occurring in one bird’s repertoire, and pi is the probability of type i in the sample
Inference in ABC is subject to the summary statistics used, and so we used an exhaustive approach, employing as many statistics as possible. These were as follows: (1) Singletons: proportion of syllable types that were sung by only one individual in the sample; (2) Rare syllable types: proportion of syllable types that were sung by 4 individuals or less; (3) Intermediate-frequency syllable types: proportion of syllable types that were sung by more than one but less than or equal to c individuals, where the threshold c was 5% of the total number of syllable types in the sample; (4) Common-frequency syllable types: proportion of syllable types that were sung by more than c individuals; (5) ns, The number of syllable types in the sample; (6) The number of individuals singing the most common syllable type in the sample; (7) H, an index of diversity, calculated as:
Statistics (12) and (13) both capture information relating to the probability that individuals share more than just one syllable type in their repertoires, and are intended to be related to a tendency to learn songs from one particular demonstrator
Summary
We recorded the song repertoires of 615 adult male swamp sparrows between 05 May 2008 and 12 July 2009 from six populations across north-eastern USA (Supplementary Table 1). We recorded the location of each individual’s territory using a hand-held GPS device. Recordings were made with a Sony PCM D50 digital recorder at a sampling rate of 44.1kHZ and at 16bits, using a Shure SM57 microphone mounted in a Sony PBR 330 parabola. To maximize the chances of recording each of the syllable types in a male’s repertoire, we recorded each male for either at least one hour or until it cycled through its supposed repertoire at least 1.5 times[57,58]. We selected one exemplar for each syllable type in each individual’s repertoire, based on a subjective assessment of recording quality
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