Scrutinizing assortative mating in birds

It is often claimed that pair bonds preferentially form between individuals that resemble one another. Such assortative mating appears to be widespread throughout the animal kingdom. Yet it is unclear whether the apparent ubiquity of assortative mating arises primarily from mate choice (‘like attracts like’) which can be constrained by same-sex competition for mates, from spatial or temporal separation, or from observer, reporting, publication or search bias. Here, based on a conventional literature search, we find compelling meta-analytical evidence for size-assortative mating in birds (r = 0.178, 95% CI: 0.142 – 0.215, 83 species, 35,591 pairs). However, our analyses reveal that this effect vanishes gradually with increased control of confounding factors. Specifically, the effect size decreased by 42% when we used unpublished data from nine long-term field studies, i.e. data free of reporting and publication bias (r = 0.103, 95% CI: 0.074 – 0.132, eight species, 16,611 pairs). Moreo...