"The birds are cleaning the hippo with their beaks."
Translation:Les oiseaux nettoient l'hippopotame avec leur bec.
"Leurs becs" could be interpreted as each bird having more than one beak. In French, leur bec means one beak for each bird, leurs becs means multiple beaks for each bird.
EDITED: Mea culpa. It is more common in French to use the singular when each party possesses one of them "leur bec," but it is still understood to mean "one of each" if the plural "leurs becs" is used, because of common sense. Context will tell.
I read your edited comment stating "leurs becs" should be acceptable but my answer as such was marked incorrect. I am getting the impression that the singular, "leur Bec" applies to one beak per bird. Much of what I am learning on Duo has to do with how the French speak, think or write, which is more important,IMO, than my being right. I really appreciate Duo and the great French moderators!