Down Sleeping Bags
Down sleeping bags are much lighter than their synthetic filled counterparts due to the construction of the down clusters. These are not true feathers, but the fluffy plumage that waterfowl grow next to their skin. Under the microscope they look like chaotic pipe cleaners growing in lots of different directions, but with finer detail. It is the large surface area that these clusters create that traps a lot of warm air for relatively little weight, keeping you warm. These down clusters can also compress a lot further than synthetic insulation, making them ideal for people who need a warm bag that has a low weight and pack size. Compressing a down sleeping bag is just a matter of either stuffing it in a compression sack or any empty space in your backpack. Doing this will not damage the down insulation, you just need to give at a good waft before use to get the loft back - as you would a duvet.
The only major thing you need to be wary of with a down sleeping bag is dampness. The down clusters stick together when damp, thus reducing the insulation properties. If totally wetted out, these clusters collapse and the amount of heat retained will be negligible. Unlike synthetic sleeping bags, drying down is a major task with the best results achieved by using a tumble dryer.