I have no idea, but probably not as much as you'd imagine. The Q is virtually impossible to answer with any degree of accuracy as it very much depends on how spread out they are. One tonne spread out in a 1ft thick layer would contain a far greater volume than if stacked in a 1ft square area because then the bottom ones would be very squashed and so occupy only a fraction of the space as what the top ones do.
To demonstrate the difference in volume imagine one large cup in volume of feathers in a neat heap on the table. About 4" high??? Now put a piece of cardboard on top of the feathers and put the cup on top of the cardboard. See how the volume of the feathers is now less than 5%.
If you knew the density of a feather, you can then calculate the minimum volume of a tonne of feathers. To fit the feathers into this minimum volume, you would have to compress them so that no air was present in between feathers. However, because of the nature of feathers and the way they pack together in a pile, a non-compressed volume of a tonne of feathers would be much larger.