I don't do lots of bird photography and hence have not invested in any of the longer lenses. For this shot I used my Canon 70-200 f/2.8 L IS MKII with a Canon 2x MKII extender. The combination is surprisingly sharp (in my opinion the MKI version of this lens was terrible with the 2x extender) and is perfect for my occasional wildlife shots.
I liked the late afternoon winter sunlight in this shot.
Canon EOS-1D Mark III, f/6.3 @ 400 mm, 1/1600, ISO 800