Bird Colonel
A full colonel, designated by the eagle emblem on the insignia.
Full Bird, as opposed to the lower rank Lieutenant Colonel.

This slang originated in the U.S. Marines.

  In the U.S. Army, Air Force, and Marines, a Lieutenant Colonel has a silver oak leaf as his rank insignia.  A full Colonel has, as his insignia, an eagle with a shield.  The term “full-bird Colonel” refers to the eagle, which distinguishes the Colonel from the lesser Lieutenant Colonel, whose insignia does not include an eagle nor any other avian creature.

  Below are the insignia for a “full-bird Colonel” and a Lieutenant Colonel, respectively.

I always find myself just adding to Perryman's answers, but....

Full Bird is a Colonel. The emblem Colonels wear is an eagle.

A Lt Colonel (one rank below Colonel) is referred to as a Light Bird.

However, (at least in the Marine Corps), it is acceptable to address a LtCol as "Colonel" when addressing him verbally. Therefore necessitating a differentiation between a Light Bird and a Full Bird.

There are two colonels, a LT. Colonel and Colonel or "full bird" colonel. A Full bird colonel is an silver eagle, opposed to an Oak Leaf.