Female and male Rufous Horneros eject Shiny Cowbird eggs using a mental image of the size of their own eggs

Brood parasites reduce the reproductive success of their hosts, which selects for the evolution of antiparasitic defenses, like recognition and ejection of parasite eggs. Most hosts use differences in color between their own and parasite eggs to discriminate the foreign eggs, but there are a few hosts that use differences in egg size as a cue. Generally, females are the sex responsible for ejecting the parasite eggs, but in hosts where incubation is shared, males also can eject them. Hosts may require the simultaneous presence of their own and parasite eggs to discriminate between them (i.e. they use a discordancy rule), or they may use a mental template of their own eggs. The Rufus Hornero, (Furnarius rufus) is a common host of the Shiny Cowbird (Molothrus bonariensis) that ejects parasite eggs using egg size as a cue. We studied the cognitive mechanism underlying this discrimination by parasitizing hornero nests with artificial eggs of different size, with and without the presence of host eggs. We also studied which sex was responsible for ejecting the parasite egg. We found that horneros do not need to compare the parasite egg with their own eggs, which is consistent with the hypothesis of a mental template. Both sexes ejected parasite eggs with similar frequencies and latencies. Our results provide new knowledge on a remarkable parasite-host system, as the hornero is the first host in which both sexes eject parasite eggs using egg size as a cue paired with a template based cognitive mechanism.