One Eye Open: Mallard Sleep
"Fieldnotes," September 8th & 9th, 2013: "Mallard Sleep," written by Kassy Holzheimer, read by Nicole Schegg. http://www.montananaturalist.org/
"Although technically, the mallard is sleeping, one side of its brain remains active throughout the night. The open eye usually points towards potentially dangerous directions, and when it recognizes a danger, the mallard becomes fully awake quickly and can usually escape."