Male American Robin (Turdus migratorius) — note the dark feathers on the head, and compare them to the female's paler head feathers (below):
Cedar Waxwing (Bombycilla cedrorum) — I liked how the yellow band at the tip of the tail blended in with the leaves.
Female Northern Flicker (Colaptes auratus) — females lack a red stripe on the face (while males have a red stripe).