Why Did Some Dinosaurs Have a Crest on Their Head?

Lambeosaurs had two kinds of crest on their heads. The young had small ones that developed as they matured. No one knows for sure why they were crested, but maybe the crests served the same purpose as a peacock's tail feathers do today, distinguishing males from females. Lambeosaurs was one kind of Hadrosaurs or duckbilled dinosaur. Not all duckbills had crests but those that did had crests of different shapes. Probably the shape of the crest was a sign of the particular breed or type of duckbilled dinosaur. Hadrosaurs, or duckbilled dinosaurs, are named for their facial features rather than their eating habits. Growth of the crests throughout a duckbill's life suggests that their most likely function was as a secondary sex trait.