are well-proven natural material with good compressibility and very good shape stability. Goose feathers are the most frequently used ones. But... there are not feathers like feathers.
Thermal insulating quality of feathers is dependent on the ratio of down and small feathers. These are the numbers: 90/10, 80/20 etc. The first means that there is 90 % of down feathers and 10 % of small feathers. Another quality feature is so called "fullness". It expresses total elasticity and expansibility – ability of feathers to occupy certain capacity. This quantity is measured in laboratories by internationally acknowledged method and it is denoted in cubic inches (cuin). Plain downy feather (100/0) has about 950 to 1,000 cuin, mixture 90/10 approximately 650 cuin and 70/30 has 500 cuin.