Used Australian Reed-Warbler (Acrocephalus australis) as guineapig for lens tests in Tyto today. Birds weren't too co-operative but over a few pleasant hours of chatting in hide to visiting birders a few came to the edge of scleria.
First up, 600 lens plus 1.4x.
Then, 600 plus 2x.
Bonus, bird snatches long-legged insect or spider.
Verdict: steadier camera mount needed.
Second bonus, back to 600+1.4: Brush Cuckoo (Cacomantis variolosus). Full frame uncropped, portrait, tripod used as shaky monopod.