I will guess 400 mm, 1.7X converter, with 1.5X DX crop factor ... But like others would be interested to know which lens, whether hand-held or tripod-mounted, etc. I occasionally photograph a flock of herons who live nearby, but lately have noticed that I've been coming home with more and more photos of the gulls that fish, scavenge, and perch in the same area. They're graceful in flight, and often quite expressive just standing around. These are great photos!