Down Throws Feathers, the principal covering of birds, are of flat, almost two-dimensional construction. They have a hard, tubular quill shaft from one end to the other. As a 2-D structure, feathers are unable to trap air effectively and therefore have little insulation value.
Feathers are strong as a result of a vane center, however, feathers have little softness value.
Down is the light, fluffy undercoating of waterfowl, consisting of clusters of filaments growing from central quill points. A Down cluster has a quill point but no quill shaft, so it is far more resilient than a feather. Down is lighter than a feather, it is three-dimensional rather than two and therefore Down has more loft or filling power. When Down is compressed, a good shake is all it needs to make it bounce right back to its original fluffy loft. Because the big trick to insulation is trapping air.