The countries were once human, able to blend in with their people seamlessly. However, thanks to a "gift" from an unknown deity (there's so many worshipped, it's hard to tell which one exists and/or did this) caused them to grow large feathery wings. Some countries were thrilled, able to fly with the birds they had admired in their lives. Others, however, hadn't gotten the hang of any of it or just didn't enjoy having large feathered structures on their backs, but there was no known way to get rid of it.
Now living on an uncharted island together, they all communicate with their bosses through technology (thank the age of WiFi!) to organise things, and treaties can be signed immediately between the actual nations as they're all in an enclosed space together.
On the island, there was an abandoned settlement that is now thriving with winged nations, even consisting of various research facilities for the countries that want to reverse it. With no outside interruption or aggression, the nations' secret is safe.
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