The only explanation I have in my mind would be the mimicry of the moving antennae, meaning birds are rather attacking the "unimportant" tail thinking that's the head and eyes rather than going directly to the head. This prevent the butterfly from being killed by the first attack. If you look at my link you see how effective this works and note also the perfect symmetry within the butterfly:
http://www.projectnoah.org/spottings/8183368
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