It took me some time to work out what needed to be done to photograph birds in flight. Searching that phrase on the web pulled up some useful information which sank into my brain after a while. When I first tried to photograph flying birds with a 70-300mm zoom on a Nikon D7000 I was not very successful. Many shots were blurred, empty or half-filled frames. I would like to blame the camera or the lens, but really it was my poorly practiced skills. 300mm on a crop sensor is equivalent to 450mm, so not a bad reach.
There are some easy parts of the equation to change eg start with slower-flying big birds, rather than fast flying small ones. The technical part is getting your camera set up properly - I now have my settings memorised in a preset, so I can easily switch to bird in flight mode! The part which requires most patience is trying to point your camera and lens at the bird you are trying to photograph. Again there are many opinions out there on the net, but most often I hold my camera and lens in my hands (with my strap round my neck) and shoot. Based on observation of the bird I have some idea which way it is going to go and I move with it. It is not unusual to have several hundred shots or more on my card after 15 minutes when trying birds in flight, only a small number of these are worth more than a glance! There can also be longish periods of time just watching and waiting (and waiting …) for an exciting event. Another important step is that you have to be in a place where the bird you want to photograph likes to be!
Some settings: I am assuming that you are working with a DSLR camera.
f5 - f6.3, autofocus continuous (AF-C) on a Nikon or AI servo on a Canon, shutter speed needs to be up at 1000th of a second or more to freeze motion and your ISO needs to be high to get the correct exposure. My camera allows an auto-ISO where you can specify maximum sensitivity and minimum shutter speed. I usually have the metering centre-weighted because that’s where I hope the bird will be in the frame!! Using this group of settings I then look through my view finder keeping my other eye open for the birds. I let the auto ISO deal with the exposure, while I use back button focusing and continuous high speed shutter release. I tend to shoot at no more than 90 degrees to the Sun. Choose a good safe spot to stand, because it does get very engaging at times. Swans can weigh over 10kg and fly at over 20 mph and I have had one coming straight at me.