Birds without own brood help other birds with parenting, but not selflessly
(PHYSorg.com Biology)

Birds will sometimes care for the offspring of other birds of their own species if they anticipate future benefits. Being tolerated in another bird`s territory and the chance to inherit that territory later are considered rewards for which some birds are willing to postpone their own chance of reproduction. On 23 October 2017 veni researcher Sjouke Kingma from the University of Groningen has published an article on this subject in Nature Communications.