To understand the exception of the Native Americans and the various laws in the US, one must take into consideration on exactly what, legally, the tribes (and thus their members) are in relation to the United States.

The Tribes are, formally, independent nations which are under the protection of the US government. The members of the tribes all hold dual citizenship as part of the US and part of the Tribal nation. The relationship of the Tribe and the US government is defined by Treaty. Tribes have the right to have their own money, passports, et al. where racial or socio-economic groups do not have the same rights.

The aforementioned treaties actually define and stipulate the United States can not outlaw the use of anything if it has a traditional value to it from the history of the tribe. Thus, why registered Native Americans can have eagle feathers, use Peyote (a Schedule I controlled substance, same schedule as Heroin), and the previously mentioned use of land for hunting, or the hunting of specific animals, in ways otherwise banned for other US citizens. They also designate specific territory (the Res) to each tribal nation as independent, extraterritorial property, much in the same vein as an Embassy is actually, by law, part of another country.

Now, to consider the use of Eagle feathers by the hierarchy of the Clans inside the US, one must look not only to US Code, but to international treaty. The prevailing treaty is the Geneva Convention on International Relations. This document states visiting citizens to a host country is required to follow the local law, unless the visiting citizen is officially recognized by both the home and host country as a diplomat. In the case of a diplomat, the home country's law apply, although the host country has the authority to demand the diplomat remove themselves within a specifically stated time frame.

So, the long and short of this ramble....

If a Scottish Chief, Chieftan, or Armiger is traveling into the US, unless they are designated as a Diplomat of Scotland or the UK, they would be wise to not bring their real eagle feathers with them. US Customs would be required, if they identified said feathers as eagle, to confiscate said items, unless said person shows legal permits for the ownership thereof (and unless said chief is a part of a a traveling conservation education group, the likelihood of getting such is almost nil). If a Chief, Chieftan, or Armiger is from the US (yes, I know...), then, by law, they are NOT allowed to have eagle feathers.