A barn owl composite that unexpectedly hunted over a wetland (while I was looking for stonechats) - very far off, but that helped with the autofocus, which is usually too slow and unreliable for birds in flight with my setup (500L II + 2x II). Considerable cropping required.
Second, lots of siskins on a garden feeder gave me the opportunity for some passerines in flight shots. Actually mostly wing-flapping displays, but it's nice to get shots of small birds doing something other than perching
Poor light meant pretty high ISO required for this - flight-freezing shots needed 1/1600-1/3200 sec, so it was ISO 2500-4000 with the bare 500 and with the 1.4x III.
Current equipment: 5Ds, 5D mark III, 50D, 24-105L, MP-E, 70-200 f/2.8L II, 100L macro, 500L IS II; 1.4xIII + 2x III extenders; 600EX-RT.
Former equipment includes: 300D; EOS-M, EF-M 18-55, Samyang 14mm f/2.8, EF 35 f/2 IS, 70-200L f/4 non-IS, 85L II, Sigma 180 macro, 200L 2.8, 400L 5.6