Newbie's response - For quickness, I'll take my 100-400 with a 1.4x. Trouble is, with lenses, long is never long enough; the tree is just too high, there's too much tick-ridden brush/grass in the way, or they've shut down beach access.
The last happened to me a year ago. I didn't know 'til the trip was on, but every May, New Jersey closes off nearly all its southwestern beaches so the Red Knots can feed on the over-farmed horseshoe crab eggs. There was only one spot where you could reliably see them up "close", and they never got to where I was able to get a good frame and focus on them. Without a digiscope set-up, I wasted precious time driving up and down the shoreline on pot-holed roads, and came away empty except for a busted engine mount I still need to get fixed!
I also got to see how scopers see the world; an SLR viewfinder is no comparison. 'Scope to study, scope or lens to capture, depending on the situtation - and so, I'm hoping to enter a new world this year. Catch you elsewhere on here!