In reply to Removed UserAvaunt: As others have said, it's PROBABLY all about the fill power. As far as I know there has not been a study that proves that FP 800 is significantly (in a statistical way) warmer than 700, 600, 500... In theory it makes perfect sense, and there's a lot of evidence from products, but it's something I am looking into experimentally soon. If anyone knows of any existing scientific studies that relate fill power to thermal resistance I would love to hear about it! It's worth noting that the differences between duck and goose down are not limited to their thermal resistance, but that's the thing most people are bothered about.
Where does eiderdown fit into all this? It's got a fill power of about 700 (it's a myth that it's any higher) but is - again - reputedly warmer than goose or duck down. Why's that then? OOh, big questions...
The other massive influence on warmth is fit and design, which so many people forget. We wear clothes not fabrics and I'd rather wear a well-fitting jacket filled with marshmallows than an eiderdown one sized for a hippo.
I recently wrote some stuff about down on my blog (link is above (thanks Charlie!)) but didn't cover quite a lot of the above as it's currently a bit of an unknown factor and so little research has been done on it.