Back LightingBack-light is the bane of the bird photographer. The best lighting comes early and late in the day when the sun is behind the photographer and not behind the subject--or behind you and off to the side, left or right.
The worst and hardest to adjust back lighting happens when the sun is directly behind the bird you are trying to photograph.
Even when the sun isn't directly behind the subject back lighting can still be a problem, especially so when birds are perched on a branch with a bright blue sky background. When birds are posed against a blue sky background you have to be careful. The camera's light meter will likely underexpose slightly because it gets confused by the bright background. Using the camera in a "Center Weighted Metering Mode" helps get the right exposure. For brightly back-lighted bird subjects you might want to over expose a full F-stop.
Otherwise you might end up with a properly exposed sky and a dimly-lighted bird. This sort of of exposure compensation is more important for Jpeg shooters than RAW image shooters. When shooting RAW you can let the camera expose properly for the sky and then fix the dimly-lighted bird with post-processing software.
The Golden Eagle below was badly back lighted on a gravel road North of Bozeman MT. The sun was directly behind the bird when I took this photo. I also accidentally under-exposed.
The first image below is the Jpeg I the camera made. The second is my best effort editing the RAW image iwht Darktable and Gimp. It's not a great photograph because of severe backlighting combined with a badly under-exposed image. But it does illustrate the power of editing when working with a RAW image.
How to Over ExposeI often shoot (with the exposure mode set to a large matrix area rather than a smaller "spot metering" area) II in aperture priority mode at ISO 800 with the iris set to the smallest F-stop I can hand hold and still get sharp results. Depending on light conditions that might be anything from F-8 to F-11. For really narrow iris openings I like to use a tripod. When in either Shutter Priority or Aperture Priority Mode my camera has an "Exposure Compensation Dial" that tells the camera to over or under expose by any increment I choose, from 1/3 F-stop to 3 full stops. I seldom over or under expose by more than on stop. All good cameras have an exposure compensation mechanism. Exactly how it's done will vary slightly from camera to camera.
Fill FlashIf you have an optional/removable speedlight flash you can set it to its lowest power (usually 1/128th power) and experiment with exposure. If you are shooting in a backlighted situation you might want to make a test shot or two of a fence post perhaps, before any interesting birds show up. In order to be ready. I've used fill flash a lot with shorebirds. At 1/128th power they hardly seem to notice. The flash is hardly brighter than a sharp reflection off the water. At full power the exposures--on closeup birds--are not good and the birds do take notice. Set the flash to its lowest setting and then go a stop or two brighter if you have to. But not more than that.
I All RAW images contain an embedded jpeg for use by the camera's LCD image review. This jpeg is saved at a medium quality rather than high. But it is useful. I extracted this Jpeg from the RAW with software. A high resolution Jpeg would have been a bit sharper perhaps. But the colors would have been the same. Editing a Jpeg this badly under-exposed would not have accomplished much. The edited RAW isn't a prize winner. But it is an acceptable image.
II I set the focus mode to a small area rather than a larger matrix area, and then set the exposure mode to just the opposite. I make the light meter look at the whole view port image area. And I make the focus area look at a small portion of the center of your subject.