Monarch tags are tiny, round stickers made of polypropylene. They are 9mm in diameter, a little larger than a hole-punch. A tag weighs 10 mg. A typical monarch weighs 500 mgs, so the tag is about 2 percent of a butterfly's weight.

The tag is encoded with unique numbers and letters. Tags are supplied by Monarch Watch, a University of Kansas program that documents the monarch migration and promotes conservation of butterfly habitats.

Tagging information helps answer questions about the geographic origins of monarchs, the timing and pace of the migration, mortality during migration, and changes in geographic distribution.