I'm spending the summer in London and recently took a trip to Kew to visit the botanical gardens. While there I found a beautiful peacock feather that a friend of mine back in the US would love. However, I'm concerned that it would cause an issue at Gatwick when I return home. Are bird feathers prohibited?
It's like any animal product - you'd have to declare it - better safe than sorry.
However, if they're anything like New Zealand - who are VERY strict, they can fumigate / sterilise / wash any suspect item. For example, a friend brought an Ostrich egg back from South Africa. They took it for a couple of weeks, processed it, and mailed it back to him, all customs-approved.
Similarly, bugs that are dead and pinned can be fumigated to kill any eggs, and wood carvings can be dipped/sprayed as well.
Gatwick won't care - you're exiting with it, but you'll need to declare it at the US point. Odds are they'll wave you through, or possibly just inspect it.
As long as you declare it, you won't get fined. It's if you're caught trying to sneak something through without telling them that you'll get into trouble.
EDIT - Update for US
I've come across the US Customs regulations on Prohibited and Restricted Items - Fish and Wildlife - and indeed, you need to declare feathers.
As it turns out, no one said anything to me. I packed my clothes and then put 2 notebooks on top of them, placed the feather on the notebooks, and then closed the whole thing down with the compression straps. I'm not sure if it didn't show up in the Xray or what, but it went through checked baggage with no issues. Not sure if that's the norm for US flights.
I just walked right by two customs agents in Newark International with a hand fan made from Peacock feathers in my hand that I purchased at a floating market in Bangkok. It's a big fan (approximately 12" by 24") and it was right in front of them, I didn't need to declare it, just as I thought. It was too delicate to pack so I had to carry it by hand, well worth the money.