An aigrette is a hair ornament designed to hold or depict feathers and is usually jewel encrusted and sometimes mounted en tremblant. The name comes from the egret - the feathers of this particular bird were often used as adornments in hair and on hats. This fashion reached it's peak at the end of the 19th century when hats were often made entirely of feathers and occasionally even whole birds.
Aigrettes were a logical accompaniment to the prodigious hairstyles and wigs favored during the Georgian period. Hair continued as a focal point and aigrettes were again favored during the latter part of the Victorian period. Simpler, shorter Art Deco hair styles were often adorned with bandeaus and other hair ornaments, including aigrettes. Our fascination with birds has maintained the feather as a popular jewelry motif.
As an interesting aside, The National Audubon Society was founded to combat the extinction of birds sacrificed for the purposes of ornamentation.