Lots of opinions here. PEEDYC remarked that the Royal Company of Archers take the feather business to the extreme, while Jock Scot remarked that it was no more extreme than the bearskin or the Royal Marines’ topi.
Personally my take on the Archers is that their bonnets are stranger than their feathers (which admittedly are very long in proportion to the hats). For my liking the bonnets stand too straight – anything that wide at the top ought to tilt over.
And Jock, the busby is not a bearskin. It is a much smaller type of fur hat. But blame the drummie brigade for misnaming their fluffy bits of fake fur “busbies”.
The band of the Cape Field Artillery (a Citizen Force unit, equivalent to Territorial Army or National Guard) adopted a colonial-type uniform in the 1960s which included karakul busbies. They looked quite spiffing, I thought.
Karakul was very highly regarded at the time, since it was a product of South West Africa (then regarded as the fifth province of South Africa) and was also produced in the Northern Cape.

Dave Dove (with tongue firmly in cheek, I am sure) suggested a coloured ostrich plume. Well, coloured plumes are generally associated with showgirls’ outfits, although I have seen men wearing them in blue, stuck into their broad-brimmed hats.
And when I worked on an ostrich farm I also had a broad-brimmed hat with a white ostrich plume and a smaller black (body) feather. It certainly caught people’s attention, but it was tricky getting it to work even with the broad-brimmed hat. Much to showy for a bonnet, to my mind.

I rather like Dale Seago’s choice of an African grey parrot’s red tail feather. My wife intends to become an African grey owner, so I will probably have a source of supply for those!

I have a coat of arms, but I would not class myself with a Scottish armiger because my arms are not currently registered.
But even with a registration certificate on my wall, I am not sure that I would wear an eagle feather. A smaller feather would not attract as much attention.
On the other hand, African fish eagles inhabit the estuary where my best friend has a holiday home, and I might pick a feather up there sometime . . .
Regards,
Mike