Falling snow and birds in flight can be difficult. Captured this image on one of my Alaska eagle tours a few years back. Keeping my single focus point on the head where there is lots of contrast allowed the camera to focus correctly. I had a rock wall in my BG fairly close behind the bird so I opened up my 300mm lens to blur out the BG to keep it from competing with my subject.
Image cropped from horizontal capture.
Camera Model: Canon EOS-1D Mark IV
Copyright: D. Robert & Lorri Franz
Shutter speed: 1/2500 sec
Aperture: 3.2
Exposure mode: Manual
Flash: Off
Metering mode: Multi-segment
ISO: 400
Focal length: 300mm