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 Specifically, the effect size decreased by 42% when we used unpublished data from nine long-term field studies, i.e. data free of reporting and publication bias (r = 0.103, 95% CI: 0.074 – 0.132, eight species, 16,611 pairs). Moreover, in those data assortative mating effectively disappeared when both partners were measured by independent observers or separately in space and time (mean r = 0.018, 95% CI: -0.016 – 0.057). Likewise, we also found no evidence for assortative mating in a direct experimental test for mutual mate choice in captive populations of zebra finches (r = -0.020, 95% CI: -0.148 – 0.107, 1,414 pairs). These results highlight the importance of unpublished data in generating unbiased meta-analytical conclusions, and suggest that the apparent ubiquity of assortative mating reported in the literature is overestimated and may not be driven by mate choice or mating competition for preferred mates.
Unpublished data (nine long-term field studies): All the pairs that have been identified across the nine studies where both pair members have at least one morphological record, including repeated records from different years. This dataset also includes latitude and longitude of the nest site (Lambert azimuthal equal-area projection, units = meters) and year and, if available, the putative date of the first egg.
Unpublished data (nine long-term field studies): All available records of morphological traits which covered more than 95% of individuals included in S1_Data.This dataset also includes the location where the individual was caught, the date of catching, and the observer who measured the individual
Unpublished data (nine long-term field studies): Data where we combined the information from data S1_Data and S2_Data
Published data (Fig 1A): Excel spreadsheet containing two separate sheets with data from literature: 1) data extracted from the publications and 2) the original references.
Data for Fig 1B: Strength of assortative mating for size as a function of data source.