From what I recall of my vast absorption of PBS Nature specials, this isn't too uncommon... there's some large bird that adds its egg to the nest of a small, unsuspecting bird with a trusting nature ~ she incubates them all, and when they hatch, the interloping egg has produced a big, ugly, squawky monstrosity that strongwings its way to the front of the line every time the mother brings back any food. Her own brood perishes, and adding insult to injury, I think I recall the big mean one eating its nestmates. No recollection whatsoever, of course, as to which bird this is. It's called *selective memory.

EDIT: This may be it... google criteria were "bird egg nest surrogate" (it was the best I could come up with in free association): http://members.aol.com/WHobbs7491/web/cbird.htm It's called the Brown-Headed Cowbird, and apparently I made up the part about eating the other young. It's evidently a major threat to songbirds.