Mating patterns in avian hybrid zones - a meta-analysis and review

Hybrid zones provide a unique opportunity to study the dynamics of isolation barriers. Pre-zygotic isolation through assortative or conspecific mating is thought to be an important aspect of reinforcement and speciation. Although assortative mating has received much attention and nearly a hundred theoretical models have been published, there is a large gap between theory on the one hand and empirical data on the other. To fill this gap, I carried out a meta-analysis on 58 studies which were identified as suitable for the analysis. Most studies had been carried out in the field (n = 52), and six were based on mate choice experiments. Fifty-three studies used plumage scores and five used genetic evidence to assess parental types. I found no correlation between the magnitude of effect sizes and date of publishing (r = 0.181, P = 0.174, n = 58). A fixed effects model without any underlying model structure showed a heterogeneity of Q(total) = 5454.6 (df = 57; P < 0.001) and a significant mean effect size of 0.47 (95% Cl 0.46-0.48). The results of the meta-analysis indicate that there is a significant effect of medium strength of assortative mating in avian hybrid zones. By partitioning the data, I found that effect sizes were very large in mate choice trials, and medium in observations in nature. Based on the inspections of the CIs of the mean effect sizes, assortative mating appeared strongest in Passeriformes and Charadriiformes hybrid zones. Further, assortative mating was stronger in narrow hybrid zones compared to wider ones but I found no difference between stable and moving zones.