The answer depends a bit on your budget. Full frame or APS sensor body. Smaller sensor bodies can get by with less length, of course. But the 1DMkll makes a very nice bird camera on the Canon side of the fence.
The 500 f4 is very useful - but large, heavy, and sometimes not long enough. I use a 1.4 TC with it fairly often. I don't think you will use it with a monopod, but a tripod and a Sidekick or a Wimberly style head. I find this much weight to be very unstable on a monopod. 8.7 pound + camera weight == about 12 pounds or so - doesn't sound heavy sitting here at a desk, but in the wild, it gets heavy quick. Not much handholding with this lens unless you are superman.
400mm is easier to handhold, and can be long enough in Florida where the birds are more tolerant of tourists with cameras in hand. But generally just a little short of the mark for birding. The 400mm f4 DO is very light, less than 5 pounds.
A 300mmf2.8 is too short by itself, but pairs nicely with a 1.4 or a 2x TC. And is easily handholdable.
I find all are useful, and helpful. I would find it very hard to limit myself to just one lens for birds. Song birds are even more demanding, than larger birds.
Longer always sem to be better, but a LOT more difficult to manage with regard to weight, tripod, blurring due to camera shake, and just finding the subject through very long glass. Less length is always easier, if long enough.
20D 300mm - only in FLa can I get this close with 300mm
1DMkll 500mm +1.4 TC
1DMkll 400mm DO