Just feathers pinned to it. Was common to have overly decorated hats if you could afford it in ways we wouldn't consider fashionable today (Mrs. True's hat looks like a modern flower cake also lol). This tradition is not dead, for some modern examples check out the royal ascot races, kind of a fun/funny tradition.
And because they ran out of birds. Passenger pigeons, for example, became extinct largely (Edit: partly. Also lots of other reasons for the slaughter, plus habitat loss. Thanks Wiki!) because they were turned into hats.
Now I'm off to Wikipedia to check whether this fact I learned in childhood is true or not. And that's why I have a donation to them on Autopay