For those lenses I don't see why you would NEED the tripod down here in the sunny Florida Keys. I use one with my 500mm due to weight, but never put my 400 5.6 on one. I don't know what your expectations are, the birding in the Keys isn't as abundant as other parts of Florida, but if you know where to go you can get some great stuff; just have to work harder to find it. You might want to reconsider not going fishing, since fishing might bring some of your best birding opportunities here in the Keys. Also, jumping fish such as Tarpon and Sailfish can be a blast (and challege) to try and capture in the air. Flats fishing in the backcountry you are bound to see Egrets/Herons/other waders on the shallows, far enough out back you might see an American White Pelican or Roseatte Spoonbill flying about. Offshore the seabirds are just about trained to follow the fishing boats, and while most captains hate them, they are easy opportunity for some great BiF action. Seeing the huge wingspan frigate birds effortlessly flying through the air offshore and is something to see.
On land that time of year you should be able to head out anywhere with some clean skys, sit around long enough and some hawks/osprey's/eagles/ect should fly by; birds of prey are very active here in the fall/winter. Also along the shallow shorelines at low tide the wading birds and Herons/Egrets should be out working the exposed bottom for a meal. If you can time low tide around sunset/rise you can get some great light too. If you don't like holding onto the camera for long periods of time those land situations I guess the tripod would be nice for rest, but shouldn't be needed for light. I am not a big small bird shooter but we do get some smaller songbirds and cardinals/jays/ect.
Certainly don't forget the wide angle, we have some great landscapes and sunsets here!
Whereabouts in the Keys are you staying, if you don't mind my asking? Upper/Middle/Lower, Key West? For birding I would certainly recommend taking an early morning trip into the Everglades National Park if you are far enough up the Keys to make it there (especially at or near sunrise, but hey, no need to get up early, you are on a vacation), you will see a lot of birds in the park that time of year from freshwater dwellers and gators up north at the Anhinga trail (just a couple miles past the main entrance), to saltwater dwellers down by Flamingo (45min down the road from the entrance), and who knows what you might happen past in between. Also, one or two cold fronts should have come through by then to blow the worst of the bugs away.
Anyway, hope you have a fabulous trip!
*edit*
It would help if I read all of the posts, I see you are staying in Marathon! Should be able to make it up to the Everglades for the day if you leave early. I am a bit north of there, so I don't shoot in Marathon that much, but have gotten a few good birds shots at low tide at Curry Hammock State Park (http://www.floridastateparks.org/curryhammock/default.cfm
). Bahia Honda State Park is about 30 minutes south of Marathon, and is a great stop with the old railroad bridge (http://www.floridastateparks.org/bahiahonda/
), you might see birds there, but it is mainly known for its great beach.