In birds that breed cooperatively in family groups, adult offspring often delay dispersal to assist the breeding pair in raising their young. Kin selection is thought to play an important role in the evolution of this breeding system. However, evidence supporting the underlying assumption that helpers increase the reproductive success of breeders is inconsistent. In 10/19 species where the effect of helpers on breeder reproductive success has been estimated while controlling for the effects of breeder and territory quality, no benefits of help were detected. Here we use phylogenetic meta-analysis to show that the inconsistent evidence for helper benefits across species is explained by study design. After accounting for low sample sizes and the different study designs used to control for breeder and territory quality, we found that helpers consistently enhanced the reproductive success of breeders. Therefore, the assumption that helpers increase breeder reproductive success is supported by evidence across cooperatively breeding birds.

Data were collected following standard meta-analytic techniques.

The three supplementary files are

- R script
    + name: Rcode.R
    + details: code for reproducing the analyses using the metafor MCMCglmm R packages 

- Effect size calculations
    + name: effectSizes.txt
    + details: describes how each effect size reported in Table S2 and used in the analysis was calculated

- Supplementary Tables
    + name: TableS1-S4.xlsx
    + details: all data used in the analysis including and model results