(c) 2011 Jerry Grady

(c) 2011 Jerry Grady

(c) 2011 Jerry Grady

The following is from the Wisconsin DNR website
Birds that are mature enough to reproduce will develop "nuptial tubercles," or large ridges on top of their bills, and ornamental plumes atop their head. Once chick feeding begins, the ridges fall off and the birds develop black feathers on the back of the head. Their eye color also changes from dark to light as their breeding status changes. Nests are built in depressions on the ground and lined with dried plants.