Thanks, Soundy! Very good illustration of how the availability of in-coming light changes as the lens gets longer...and explains why at maximum zoom my light levels in winter just aren't sufficient for what I was trying to do.
I never did get my hundred-finch take-off pic. But it gives me something to shoot for in future.
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."