I wear a hat with feathers stuck in the band. Can I bring/wear them through customs into UK & Rep of Ireland? And then re-enter Australia? (possibly with a couple more I find when over there.. :-)
From the Restricted items page on HMRC:
The following are also banned but in certain cases may be brought into the UK if they are not for commercial use:
fur skin of harp and hooded seals pups and products made from such fur cat and dog fur and products made from such fur finished goods of certain fur skins covered for personal and private use
Note that this is not a full list, but it includes the most important examples.
So given that they need you to declare fur items, it's almost certain they'll want to know about feathers. As they say, if in doubt, declare - they may say it's ok, or fumigate it to make sure (a friend brought an ostrich egg into NZ, they fumigated it and then were fine).
For Australia, from their biosecurity page:
feathers, bones, horns, tusks, wool and animal hair
are definitely in their list of items to declare. Again, it doesn't mean it will be removed from your possession, just that they want to check it - often to make sure it doesn't have bugs or something.