Michael C. Smith wrote:

Aperture Pro's (Miami) largest lens, a 400/2.8 L, rents for $125
daily, $500 weekly, and a $7,500 deposit. Adorama is $100 daily, $300
weekly, and $6,500.00 new. Adorama rents a 600mm/f4 (call for price).
Insurance isn't listed; thanks for the reminder.

Hmmm. That's better than I expected. That's a big deposit though!

Is 400mm too short for birding? I can afford (monetarily and
politically) only one long lens.

It depends on location. In many places yes, others no.
For example, look on my site and see the focal lengths used.
http://www.clarkvision.com/galleries/gallery.bird
I also shoot with a second body and either a 300 f/4 or
100-400 f/5.6. But have few bird photos on the site
are with the shorter focal lengths (maybe one or two).
The images are about 40% 500mm + 1.4x TC, 55% 500mm straight,
and 5% other (lower FL and 500mm+1,4x+2x). So I use the 500
most of the time. While I've taken many images with
other lenses, the 500mm ones always seem better.

You might consider also the 300 f/2.8
as you can add a 2x and get 600 f/5.6. Or, as an intermediate
the 300 f/4 and you can add a 1.4x and get 420mm f/5.6 (cheapest
solution to get started).
Also check out used lenses.

Keep in mind that hand holding a 400mm f/4 lens may be possible,
but only for a short time, and you'll quickly become unsteady.
Try adding a weight to your longest lens to make it weigh
what it would with a 400mm f/4 and see how long you can hold
it up. I often wait many minutes pointing at one bird hoping
for some particular action. I can't imagine hand holding that
long.

Also, with any big lens be prepared to add the cost of tripod
and head to hold the lens. If you go 500mm you need something
like a good carbon fiber (or much heavier aluminum tripod).
I found the qitzo 1325/1328 the minimum for the 500 f/4.
I had a gitzo 3021, but it wasn't quite up to the task.
Then a good head able to carry the weight. A wimberly
really helps follow action with a big lens (if you rent,
you will still need all this too). The gitzo +
wimberly plus plates will run somewhere around $1200-1500.
(I know, its discouraging...I just went through this the last
couple of years). Large format lkandscape is cheap in comparison!

Thanks for you suggestions, Roger. Great images & info on your site!

Thanks,
Roger