How fast are these birds when in flight?
"Hawk owl speed in predatory flight dive, for example, is about 60-70kph. The flight is often undulating and the bird sways from one side to another unpredictably, making it really hard to keep the owl in the viewfinder. The great grey owl is not especially fast in flight, but it normally comes out to hunt in the twilight, in low light conditions."
What was the weather like in the locations you chose, and how many hours of daylight did you have?
"It was February. I spent some time in Kuhmo, where the daylight lasted just short of eight hours, and some in Utsjoki, Finnish Lapland, where the day was about five hours long. This winter has been unusually mild even in the north, and the temperatures ranged from 0 to -25°C during this shoot.
"One evening, I got to shoot a great grey owl in flight about 40 minutes after sunset. ISO51200 gave me 1/500 sec shutter speed, just enough to freeze the owl. I was amazed to find that the focusing system could follow the owl in what was indeed very low light."