Big lens with a fast shutter. Also as @kerristephens said move up to them very slowly and take plenty of shots. More you take the greater tue chance of a good shot.
Spend time watching the birds & get to learn their behaviour. That way you will begin to know where they go, and will be able to either predict where you need to be to get the shot, or at least which direction to pan to follow them through the lens. Oh yes, and a nice long lens helps too!
Is it any particular species you are trying to catch, or just birds in general?
bird feeders,watch them and find out what time they are around so you can have your camera and stuff all set up waiting. Mine come around in the morning and mid afternoon so I am always ready. Be patient and coordinated. I tripped once trying to get near a bird and well I never got the shot lol
I've got a bird feeder shot earlier in my project, and I made a comment about the birds being skittish. A friend of mine commented that what they've found works is to not fill the feeders full, and add to them every day at the same time. The birds get used to your pattern, and recognize that you're the one that brings the food. He even stated that the birds would follow his wife as she headed out on the golf cart to fill the feeders.
I agree with the bird feeders. . we set-up a bunch of them outside the back doors and I stalk them through a window opened just enough to fit the long lens through. .. .. . and snap quick!
Is it any particular species you are trying to catch, or just birds in general?
he is absolutely correct...stole the suggestion out of my mouth