The Cuckoo and The Warbler
These species are an example of Commensalism. The Cuckoo bird lays its eggs in the Warblers nest. The eggs look so similar, that the Warbler bird assumes its her own. When the Cuckoo bird eggs hatch, they destroy the Warbler eggs. Being the only birds left, the Warbler bird cares for the Cuckoo birds as if they were her own.
This is classified as Commensalism instead of Mutualism because even though the Warbler bird is not harmed, it is also not benefited from the relationship. The Warbler bird is unaware of this relationship and that eggs are not her own.
Cuckoo is top picture, Warbler is bottom.