Photographing Birds in Flight

Planning

As with most endeavours, some time spent planning before you set out rarely goes amiss. Here are a few things to consider.

Timing
It makes sense to choose the time of day when you will have the best light; this almost always means the morning or the evening – the golden hours when the sun is lower in the sky.
Get there early and stay late. Not only will your birds have a nice glow but the low sun will produce a nice reflection in the birds eye, which could otherwise just be a black space.

Location
Locations near rivers or lakes are great because of the abundant food sources for the birds, but other feeding, nesting or roosting sites can be equally rewarding. 
With sites of this type you will give yourself opportunities to capture birds arriving and departing; not just in flight but also at take off and landing.

Positioning
If possible, pick a spot that will put you at eye level with your subjects. BIF photos generally look best when the photographer is at the same height as the subject. Failing that, try and find an elevated position; better to be a little above your subject than below it.
Next, check the position of the sun. Ideally you want the sun behind you or slightly to the left or right of your position. This will ensure that you get good lighting on your subject. 
Except for very specific reasons, avoid shooting into the sun; if there is too much light behind the subject it will appear as a dark shape in your photograph and you’ll struggle to recover any detail.
Take account of the wind; birds, especially large birds, will usually take off and land into the wind. Being aware of the wind direction will help you predict the direction they’ll be flying in. (This rule is often over ruled by Murphy’s Law; the one that dictates that birds will always fly away from you when they take off.)

Know your subject
This is something worth doing no matter what you are photographing. 
Study their habits; if you are photographing birds that are nesting/roosting learn which direction they arrive at and depart from the nest/roost. Some birds fly straight, others will drop down or dive when leaving the nest. Some birds will have a perch they regularly use on their approach to the nest. 
Songbirds often have a favourite perch from which to sing; take a little time to try and find it.

If your subject is one that has a particular characteristic, like swooping down or skimming, then be ready for it. Try to put yourself in a position where you can keep photographing until the bird has completed its manoeuvre. Much better to photograph it coming towards you and finishing up level with your position than to start with it level and then only being able to photograph it flying away from you.

Not all birds fly in the same way. Small birds tend to flap their wings very rapidly and are more erratic in their flight whereas large birds like buzzards and eagles flap their wings much more slowly and will use thermals to hover and glide; making their flight path much easier to follow. Some birds, like Bee Eaters and Swallows will return to the same perch after each sortie; you just need to be patient.

Background
It is not always possible to achieve, but a clean background is much better than a messy one. Your BIF will look much better against clear sky than it will against a tangle of bushes or branches. It is also much easier to achieve focus if there is less to distract your camera’s sensor.