A butterfly should never call itself a bird because, even it can fly with the birds, people would be very quick to remind it that it doesn’t belong with them.
On the other hand, if a bird calls itself a butterfly, people will remind it of how far it has travelled, of how strong its wings are and how much it has achieved- how very “birdy” it is.
The former is done more often than the latter, but whenever people choose to do the latter, they go wild with it.