Despite, only this year they will finally make the bald eagle the national bird. Which, I also learned that they are not official symbols until a President signs a document for them to be. It is just a recognized symbol but not official. (Just my opinion but why do they really need a bill for this? What changes would making anything like this official really do?)
Here is a list of other US symbols (article includes links to other countries of the world national symbols, some official, some not made official) if you are interested:
https://en.m.wikipedia.org/wiki/National_symbols_of_the_United_States
How else would it be official though?
Presidential decree could be overturned every time the president changes. If there was an agency that declared things to be official, then they would have to pass a law to grant them the power to do that.
Being official always starts with laws, might as well do the whole thing at once.
Official by tradition or custom https://en.wikipedia.org/wiki/Customary_law