Cowbird Eggs Don't Look Much Like Kirkland's Warbler Eggs, but the Warblers

Cowbird eggs don't look much like Kirkland's warbler eggs, but the warblers don't discriminate, so don't kick the foreign eggs out. Why not?

A) Warblers aren't that smart.
B) Warblers, like most birds, can't see too well.
C) Cowbirds only recently invaded the warbler territory and the warblers probably haven't had time to adapt yet.
D) Cowbirds are so rare that this doesn't happen enough for the warblers to have to respond.
E) The competitive exclusion principle doesn't apply to these species.