That sounds very cute, but the robin parent wasn't wasting energy on the Ovenbird chick since it was just tagging along, though I would really love to have that picture on my wall! I hope it got some appropriately sized food eventually.

Cowbirds aren't the same since the adoptive parents "think" it's their own baby because it's in their own nest to begin with. 

I guess in the case of the  Chickadee voluntarily feeding Robin chicks, or of the Black Headed Cardinal feeding KOI,  the evolutionary advantages of having such a high drive to care for nestlings must outweigh the disadvantages of occasionally feeding someone else's offspring.  Maybe it's not so different from humans wanting to raise and nurture pets, at least the kind that have no purpose other than to be nurtured, like me with my 4 dogs!