Black-tailed Godwit. 5DIII 300mm f/2.8 II + 2XTC III at f/5.6. This is at the limits of resolution for a decently sharp image for me. It is 100% crop, and the image spans only 800x300 pixels.
5DS R, 7D II, EOS-M, Powershot G3 X, 300/2.8 II, 1.4xTC III, 2xTC III, EF 35 f/2 IS, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, EOS-M 18-55, f/2 22, 11-22