You are skilled in using a shield, allowing you to gain greater defensive benefits from it.
Prerequisite: Shield Proficiency.
Benefit: Choose one type of shield from the following list: buckler, heavy, or light. When using a shield of the appropriate type, you increase its shield bonus to AC by 1.
Special: You can take this feat more than once. Each time you select it, choose a different type of shield.
A fighter can select Shield Specialization as one of his fighter bonus feats.
Source: Player’s Handbook II, page 82.