Data from: Male zebra finches have limited ability to identify high-fecundity females

In species with biparental care and lifetime monogamy, the fecundity of a male’s partner can be a major component of his fitness but it is unclear whether males can assess female fecundity before breeding. We carried out an experiment in which we measured variation in female fecundity (repeatability 39%, 213 females) in a captive zebra finch population and tested whether males preferred unfamiliar females of high fecundity (approximately top 10% of the population; 30 eggs laid on average) over those of low fecundity (bottom 10%; 6 eggs). We first tested whether naïve human observers could identify the high-fecundity female when confronted with duos of high and low fecundity. Humans guessed correctly in 58% of the cases (95% confidence interval [CI] 50–66%) indicating that differences in female condition were not highly obvious to humans. Zebra finch males preferred the high-fecundity female in 59% of choice tests that lasted 20 min (CI 52–66%). When extending such choice tests over several days, male “success” in associating with the high-fecundity female was still modest (61% correct choices, CI 44–76%). Overall, male zebra finches seem to have only limited abilities to identify the better mate when faced with a choice between extremes in terms of female fecundity. We found no male preference for heavier females. We speculate that such a preference may not have evolved because, in contrast to many ectothermic species, predicting fecundity from female weight is not sufficiently accurate (r2 = 0.04) for the benefits to outweigh the costs of increased male–male competition for heavy females