Dominant nestlings displaying female-like melanin coloration behave altruistically in the barn owl

► When competing over parental resources, young animals are typically selfish. ► In striking contrast, we show that nestling barn owls frequently feed their siblings. ► This altruistic behaviour predominates in dominant nestlings. ► It also predominates in individuals displaying a female-like plumage trait. ► In vertebrates altruism can evolve in offspring still dependent on their parents.