A good field bag helps you stay outside comfortably. It does not need to contain a solution for every possible photograph.
Start with what you know.
Bring a camera and lens you are comfortable using. A telephoto lens is useful for giving birds space, but familiarity with your controls often matters more than bringing your longest lens. Test any new setup before you travel.
Pack for the hours between photographs.
Water, a layer for the early start, sun protection, and something to sit on can make a long session more comfortable. Add spare batteries, memory cards, a lens cloth, and simple weather protection. Keep the items you use most within easy reach.
Ask about the day ahead.
Check the walking distance, terrain, weather, and transport arrangements with your guide. A tripod can be useful for a stationary session and cumbersome on a longer walk. Match the bag to the actual day, rather than the day you imagine.