Birds - Nature Study Using the Outdoor Hour Challenge

Red Birds - Robins, Cardinals, House Finches
Blue Birds - Jays and Bluebirds
Yellow Birds - Goldfinches and Meadowlarks
Hummingbirds - Flight
Brown Birds - House Sparrows, House Wrens, and mourning Doves
Black Birds - Crow, Red-Winged Blackbird, Starling, and Cowbird
Black and White Birds - Woodpecker, Chickadee, Nuthatch, Towhee
Gray Birds - Pigeon and Mockingbird

Owls (Summer Series)
Geese (Autumn 2010)
Winter Bird Study (Migration)
Winter Wednesday Birds
Spring Bird Study (songs)
Woodpeckers

Here are specific challenges from the Handbook of Nature Study for different bird topics if you are interested in learning more:

Feet

Flight

Eyes and Ears

Beaks

Feathers

Wing and Tail Shape