Down is the natural plumage that forms the undercoating of waterfowl. It forms in tufts, or plumules, and consists of fluffy, wispy filaments. It is an exceptional insulator, prized for being light, easy to compress, durable and breathable. It excels in cold, dry conditions. Down is more expensive than synthetic fill, but it maintains its loft (which provides its heat-trapping ability) at a near-original state longer than synthetics. That makes down a good value over the long haul.
The 'most' ethically sourced down is that gathered from nests, however this is a time consuming process which is not feasible for production processes. Down is also recovered 'ethically' as a bi-product of the meat industry when birds are killed for food, this is the most common source of ethical down.
Down plucked from live birds is considered cruel and unethical and should be avoided. We only sell down products from companies who source the down ethically.
The effectiveness of any batch of down can be measured and this measurement is called Fill Power, it concerns the volume a given ounce of down can fill in a standard measuring device. There are two different 'standard measuring devices' one used in Europe and the other in the US. For simplicity we try to stick to one standard, that's the US one.
Quality down starts with a Fill Power of 650, however good quality garments and sleeping bags will use 750-850 Fill Power down. Some very high quality bags and garments will use 900+ Fill Power, such high quality down is in relatively short supply and is expensive.
This is simply how much down is put into a garment or sleeping bag and can be used as a rough guide to effectiveness - e.g. a sleeping bag with 350grams of 850 Fill Power Down will be warmer than one with 350 grams of 750 Fill Power Down.
The comparison assumes that both sleeping bags are well constructed and the down has enough volume to expand into (called Lofting). Simply stuffing more down into a compartment could be counter productive if the down can't expand to its full potential, sleeping bag designers focus on how to make the compartments work best to let the down loft while keeping it distributed over the body without creating cold spots.
This is a relatively new development and is a treatment applied to down that makes it water resistant, there are a number of different treatments and names, Nikwax treated Hydrophobic Down is the one used by RAB, Hyperdry Eco is used by Montane, Hydro Down by Berghaus, Dry Q Elite by Mountain Hardwear and there are many more out there too.
Although it is not waterproof, Hydrophobic down absorbs a lot less water, dries more quickly and retains loft more effectively in damp conditions. If Hydrophobic down gets soaked it will still 'clump' like regular down and lose its insulating value.