Changes

In most birds, the body is divided into areas that contain feather tracts, '''pterylae''' and areas that do not, '''apteriae'''.  The location and the number of tracts varies between species.  By originating from tracts rather than being randomly distributed, feathers can smoothly overlap and conform to the natural contours of the body.  By laying feathers over apteriae, air can be trapped beneath, providing insulation.

In most birds, the body is divided into areas that contain feather tracts, '''pterylae''' and areas that do not, '''apteriae'''.  The location and the number of tracts varies between species.  By originating from tracts rather than being randomly distributed, feathers can smoothly overlap and conform to the natural contours of the body.  By laying feathers over apteriae, air can be trapped beneath, providing insulation.

The portions of the feather can be decribed as follows:

The portions of the feather can be described as follows:

*'''Calamus''': The short, hollow, tubular, unpigmented end of the mature feather inserted into the feather follicle and thus present below the skin level.

*'''Calamus''': The short, hollow, tubular, unpigmented end of the mature feather inserted into the feather follicle and thus present below the skin level.

*'''Rachis''': The long solid, tubular extension of the calamus above the skin.  The rachis contains pith, which is composed of air-filled keratinised epithelial cells surrounded by a solid keratinised outer cortex.

*'''Rachis''': The long solid, tubular extension of the calamus above the skin.  The rachis contains pith, which is composed of air-filled keratinised epithelial cells surrounded by a solid keratinised outer cortex.