I was shooting Common Moorhens some 25 meters away at Candaba the other day and 700 mm was a bit shy of the length I need. Having enough light at that time, I stacked a Tamron 1.4x behind a Canon 1.4x on the 500 f4 IS + 40D to make 1000 mm.
Out of nowhere, this Barred Rail appeared suddenly in front of me, not more than 10 meters away. I didn't have time to remove the excess TC, so I just fired off some bursts, intending to do some close crops later. ;D
40D + 500 f4 L IS + stacked Canon/Tamron 1.4x TCs, 1000 mm, f/11, ISO 320, manual exposure, Ix! bean bag: