Down sleeping bag:

Down is natures way of keeping ducks and geese warm. It’s the plumage between the birds body and thick feathers. It captures air molecules in small pockets creating a thermal barrier. When it comes to maximum warmth with minimal weight, nothing is more efficient than down. The 3D structure of down clusters create loft that traps air. The greater the loft, the warmer the insulation. This is accomplished by measuring how many cubic inches an ounce of down displaces.

Down fill

Often when discussing lightweight sleepingbags, it mainly comes down to the down filling, or fill power (loft). For example down 650 will fill 650 cubic inches per ounce, while down 900 will fill 900 cubic inches per ounce. Or, in laymans terms: one ounce of higher quality down will loft more and hold more heat than a lower quality down for the same amount of ounces. The loftier down 900 will, because of it’s holding more heat per ounce, be warmer and lighter than down 650.

Often the higher the down rating, the higher the price. Down 650 bags tend to be more on par with synthetic bags as far as price and warmth. While down 900 tends to be the premium priced bags.

Durability

Down bags are known to hold for upwards of 20 years or more if properly taken care of. So while the initial investment of a down bag is usually much higher than a synthetic equivelent, the lifespan of a down bag is exponentially longer.

Build of baffles

With down, sleeping bag construction tends to be far more important than on a synthetic bag. This is because down has a tendency to shift around within a bag. The larger and longer the baffles, the easier it is for down to get stuck at the far ends of the baffle, and the harder it can be to stay warm within the bag.

Compressability

Because of downs lofting capabilities, this also means that down compresses much tighter than synthetic. This is of course also dependent on the down fill. A down 650 bag, while being more compressible than synthetic, will be large and clumsy when compared to a much more compressable down 850 bag.

Climate - Wet vs Dry

Down’s main weakness is moisture. As down gets wet the feathers clump together cancelling all loft and warming properties. While new processes such as Downtek is making down more resillient to water, even this chemical will wear off after a few washes. On top of that, treated down has been shown to not loft properly or as much as it’s non-treated counter part. If you often take long backpacking trips in wet climates, than it’s important to keep you bag as dry as possible. Both in a dry, watertight stuff sack and let your sleeping bag dry out whenever possible.

In the winter you can dry freeze your sleeping bag as well - this is the process of setting out your bag in the freezing cold and then scraping off the frozen moisture.

Ethical issues

Down is bird feathers, or rather bird plumage and is among the worlds finest insulator - plain and simple. However, a bird either needs to be skinned alive, or killed inorder to retrieve the down. If animal rights are a big issue for you, than perhaps down in not the best choice. There are more ethical down practices, and if possible purchasing a down sleeping bag from a company that follows proper ethical standards is to be preffered. Most companies that have good down practices will usually market that factor and will even source their down from such services as Track my down. https://trackmydown.com/ which is a varied, traceable source.