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Group formation and the evolutionary pathway to complex sociality in birds


Group-living species show a diversity of social organization, from simple mated pairs to complex communities of interdependent individuals performing specialized tasks. The advantages of living in cooperative groups are well understood, but why some species breed in small aggregations while others evolve large, complex groups with clearly divided roles is unclear. We address this problem by reconstructing the evolutionary pathways to cooperative breeding across 4,730 bird species. We show that differences in the way groups form at the origin of cooperative breeding predicts the level of group complexity that emerges. Groups that originate through the retention of offspring have a clear reproductive divide with distinct breeder and helper roles. This is associated with reproductive specialization, where breeders invest more in fecundity and less in care. In contrast, groups formed through the aggregation of unrelated adults are smaller and lack specialization. These results help explain why some species have not transitioned beyond simple groups while others have taken the pathway to increased group complexity.

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Fig. 1: The diversity of social group complexity across cooperative birds.

Patrick Kavanagh (a), Bob Brewer (b), Graham Lee (c) and Andy Reago and Chrissy McClarren (d)

Fig. 2: The evolutionary origins of group formation.
Fig. 3: Evolution of divided reproduction.
Fig. 4: Evolution of task specialization.

Data availability

All data generated and analysed during in this study are available via the Dryad Digital Repository

Code availability

The R code detailed all analyses in this study is available via the Dryad Digital Repository


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We thank S. West and G. Wild for providing comments on the manuscript. We also thank the Swedish research council (grant no. 2017-03880), the Knut and Alice Wallenberg Foundation (grant no. 2018.0138), a National Environment Research Council studentship and a Royal Society University Research Fellowship for funding.

Author information




A.S.G., C.K.C. and P.A.D. conceived the study. P.A.D. designed the study and collected the data. P.A.D. and C.K.C. analysed the data. All authors contributed to writing the manuscript.

Corresponding author

Correspondence to Philip A. Downing.

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The authors declare no competing interests.

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Extended data

Extended Data Fig. 1 The evolution of group size.

Differences in the maximum (a) and mean (b) group sizes of family and non-family groups. Dots represent raw data for individual species with the back transformed mode and 95% CI estimated using a BPMM.

Extended Data Fig. 2 Patterns of survival in groups.

(a) The change in survival in relation to group size (Zr) in family and non-family groups. (b) The relationship between the change in survival in relation to group size (Zr) and how maternal care changes with group size (Zr). In larger family groups (blue dots) survival increases and maternal care decreases. Dots represent raw data for individual species and parameter estimates are the back transformed posterior mode and 95% CI estimated using BPMMs.

Extended Data Fig. 3

Publication bias tests and heterogeneity for each of the Fisher Z transformed correlation coefficients used as effect sizes in our study.

Supplementary information

Supplementary Information

Supplementary methods and Tables 6–8.

Reporting Summary

Supplementary Tables

Supplementary Tables 1–5, 9 and 10.

Supplementary Data 1

Details of how each effect size used in the analyses was calculated.

Supplementary Data 2

R code needed to recreate the analyses.

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Downing, P.A., Griffin, A.S. & Cornwallis, C.K. Group formation and the evolutionary pathway to complex sociality in birds. Nat Ecol Evol 4, 479–486 (2020).

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