After shooting birds in flight all last week I thought it would be a good idea to share with you some tips on setting your camera's focusing up to be better able to capture these fast little critters.
First thing you want to do is make sure you are in AF Continuous Mode. If you are using a Canon camera this is called AI Servo. This will allow your autofocus system to focus on a subject and track it as it moves across the frame.
Next you will want to set which focus "area" your camera is in. Typically you will have better luck in a Zone focus area. This will allow your camera to use more then one of the autofocus points to focus on your subject. Typically the "zones" are configured as you would think...one "zone" on the left of the frame, one in the center and one on the right. This will improve your tracking ability over trying to use a single autofocus point. Depending on your camera model you may have several "zone" areas, not just three. If I use this focus mode, I have much better luck with the center most "zone". The center is usually where all of your cross type focus points are located.
Another option is to use a Multi Point Focusing mode. My Sony A77ii is a bit different in how it names it's focusing areas. I prefer to use what Sony calls Expanded Flexible Spot. Using this mode the camera allows me to select one of the many main focus points in the viewfinder. It then searches the main point I've selected. If that main point loses focus it scans the surrounding eight focusing points. This allows me to use the center most focus point, which is actually the nine center points. Again, using focusing points in the center is typically where all of your camera's cross type focus points are located, too. Cross type focus points will track for contrast changes both horizontally and vertically, where as a non cross type will only search vertically.
Since I have become more and more familiar with my Sony system, I've become less familiar with the others. In my Sony system I have a couple of other settings that can help me to capture birds in flight. One of those is called "AF Track Duration". This setting basically allows me to fine tune my tracking speed. It has selections from 1 to 5 with 1 being the lowest and 5 being the highest. A low settings is better for predictable, slow moving subjects or focusing in lower light situations. The higher setting allows for a more responsive focusing for different subjects at different distances...I keep this on 5 for birds in flight.
Another setting I have on my Sony camera is called "AF Drive Speed". This has two options...fast or slow. Slow is a better choice for lower light or more critical focusing. The main advantaged of using Slow Mode is to cut down on the camera having to "hunt" on subjects or in lower lit situations. Then there is Fast Mode. Fast Mode works better for sports or action, such as birds in flight, so that's what I use when I am shooting birds.
Those are the focus settings I use on my camera. You should have similar settings on your camera, they will just most likely have different nomenclature.
Here's an image I was able to capture of a bird I had never seen before, a Northern Gannet. Two of the days we were at The Space Coast Birding Festival we got the opportunity to go offshore on a large boat to photograph birds. This Northern Gannet was one of those birds. It spends its life feeding at sea and only nests on offshore islands, so the only real chance of seeing it is by boat.