Bird Photography - Advise needed

Hi, I am an amateur photographer with interest in travel, landscape and people. Lately, taken an interest in bird photography. Without forking out more money, I'd like to use my existing equipment and would welcome any advise on the following questions:

1) Which combination of my existing equipment will be best suited for bird photography? And why?

2) If I get serious about birds and wanted to take my hobby to the next level, which lens would you recommend I buy? And why?

Existing Canon equipment
Cameras - 1DMk3 & 5DMk2
Lenses - 70-200mm f/2.8L IS USM, 100-400mm f/4.5-5.6L IS USM
Extender - 1.4x

Thank you