Prior to traveling to a location to photograph birds, you may want to gather the following information which could increase your chance of success.  This information can be gathered from the internet or communication with a local park ranger etc.  Another method is to rely on the experience of a professional guide or photographic tour which specializes in the type of photography you want to experience.  

          -  Time of year

          -  Bird food supply

          -  Sun up or sun down time and direction

          -  Wind direction and velocity in morning / afternoon

          -  Water levels - bay, ocean, streams, rivers

So now you're in the right place at the right time to photograph flying birds.  Whether you use a 100-400 mm zoom or 500 mm telephoto will depend on how close the birds will let you get to them.  While professional photographers can produce commercial grade images without auto focus and motor drives, these features will greatly increase the number of acceptable images taken by an amateur.  Image stabilization and dynamic auto focus will also greatly increase the percentage of acceptable images.  

The terminology below refers to the Nikon F100 but the concepts also apply to the other camera manufacturer's equipment.

• Use a 200+ ISO film.

• Set metering to spot. (may have to adjust over/under compensation)

• Switch Nikkor lens to M/A mode. 

• Set FOCUS MODE selector to "C" for Continuous Servo AF.

• Set AF AREA MODE selector to "Dynamic AF".    [+]

• While waiting for flying bird, set lens focus to INFINITY.

• When bird in sight, place CENTER AF INDICATOR on BIRD before depressing shutter release button.

• Once slightly depressed, bird should stay in focus regardless of birds location in view finder.

• Rapidly moving wings and all white or black colors may confuse auto focus.

• Fill flash may be required for birds up close in the shade.

Every bird has flight zone.  Birds that are use to humans have a much closer flight zone.  While approaching bird, stand upright and stop at intervals. Flapping straps on equipment may scare birds away. Take "insurance" photographs as you get closer to the bird.