Osprey in Silhouette
When photographing birds in flight the most important consideration is the direction of the wind. Due to the need for the maximum amount of lift, birds nearly always land and take off into the wind. For the shot above, the situation was as shown below, with the wind blowing into the sun.
Back Lit, Wind blowing into the direction of the sun
For this shot the wind was blowing as below to achieve a front lit shot.
Front Lit, wind blowing from the direction of the sun
Of course, most of the time the wind is not blowing either directly into or away from the sun. in this case to get a good shot you position yourself so that the bird is not directly flying into or away from the camera as below.
Osprey Landing on the Nest
Finally not only the direction but the speed of the wind plays a factor On very windy days the speed of taking off and landing is much slower, and predictable than on calmer days. Therefore I get a much higher keeper rate on windy days.
On my photo tours when planning the morning's or afternoon shoot when planning on photographing birds in flight, I always check the weather report and check the direction and speed of the wind and plan accordingly.