skulling

skulling
 n.—Gloss: Wiktionary defines this as “the act of aging a young bird by measuring the degree of ossification of its skull.” «The age of the birds can be determined by examining the feathers (color and shape), eye color, soft parts around the beak, and what’s known as “skulling” the bird. Young birds, like infants, go through a process like the soft spot on a baby’s head. These openings can be seen by adding water and gently parting the feathers on the top of the bird’s head and viewing the skull through the bird’s skin. This bony structure can be used to determine the bird’s age.» —“Bird banding a labor of love for Candor man” by Anne O. Stout Ithaca Journal (New York) Oct. 5, 2007. (source: Double-Tongued Dictionary)