Brood-parasitic female cowbirds have better numerical abilities than males on a task resembling nest prospecting behaviour.

Selection can act in a sex-specific manner on cognitive abilities, including numerosity, especially when ecological roles differ between sexes. However, few systems exist in which numerical abilities would be expected to differ between the sexes, and even fewer focus on systems in which females are predicted to outperform males. In obligate brood-parasitic brown-headed cowbirds (Molothrus ater), only females select and parasitize host nests, and would benefit from enhanced numerical abilities to distinguish suitable host nests in the process of egg laying from unsuitable nests that have begun incubation. To test this hypothesis, we trained cowbirds to use touchscreens and discriminate between sets of images differing in number. Cowbirds distinguished images based on number alone (i.e. without using non-numerical cues), and females outperformed males across combinations of objects ranging from one to six (range in host egg numbers), but this difference disappeared across higher numbered combinations. In addition, males spent less time deciding on the correct stimulus than females, but made less accurate decisions overall, suggesting they 'guessed' correct answers more than females. We add to the growing evidence for complex numerical abilities in diverse taxa, and show these abilities can be shaped by ecology in a sex-specific way.