I've done a fair amount of photography in the tropics, and I have now been to Brazil twice. This year (2006), I finally did things right. If you'd like to judge how I did to decide whether my advice is worth listening to, here is a page of the best bird photos from this trip. The images on the web are, of course, tiny and at low resolution: mostly about 800 pixels wide. The camera took photos that are 4000 pixels wide, so the images I would print from are of much higher quality.
I've got the following, older, general page on tropical photography that might also be of interest.
The key thing, of course, is to be there with your equipment. If you leave the camera at home or in the lodge, it never seems to take any decent photos on its own. This year I made it a point always to have the camera along: on all the trips, even the night ones, and while traveling in the car.
For bird photography, you want a big telephoto lens. I took a variety of lenses, but used one of them 98% of the time: my largest telephoto, a 200-400mm f/4 zoom lens with Nikon's VR (vibration-reduction) system. (I happen to own Nikon stuff, but there's nothing magic about it; Canon and other manufacturers have equivalent equipment; it just has model numbers that I don't know...)
With the 200-400mm lens's f/4, I could add a 1.4x tele-extender (making it effectively a 280-560 f/5.6) and still preserve auto-focus, and I usually did this.
The VR technology allows you to get some shots even when hand-holding the lens, since (Nikon says) the VR gives you about three more stops. In other words, you can take the same quality photo with 8 times less light, or alternatively, with a shutter speed eight times as fast. Since the little birdies hardly ever cooperate and they're always moving, the VR is a wonderful addition.
I also had the 70-200 f/2.8 VR lens, but I hardly ever used that since the larger lens was, well, larger. In retrospect, I probably should have used the smaller, faster lens for the flight shots. At f/2.8 you can shoot with twice the shutter speed than you can at f/4, and the f/2.8 allows faster auto-focus which is also critical for flight. Usually I didn't need the magnification for flight shots since with a moving bird it's hard to keep it in the frame anyway, and I usually had the 200-400 lens at the 200 mark. And of course the 70-200 is a lot easier to move around. The problem is that it's hard to guess when you're going to be going for a flight shot, and lugging another body plus lens would have been a lot of trouble. And I couldn't talk my wife into being my sherpa...
If you go to the top of the main Brazil 2006 page there's a panorama that I made from four successive shots of a flying Black-collared hawk that were pasted together with photoshop. The 200-400 lens was set at 200mm for all of the photos. You can click on the image on that page to see a larger version, and I've also made a full-resolution version (Warning -- it's large: 5.3 megabytes) of that image avaliable to show just what the camera/lens combination is capable of. For this image, the camera was in auto-focus mode, set to focus on the nearest object, and taking 5 frames per second.
Whenever possible, of course, I carried the lens mounted on a sturdy tripod (a Gitzo carbon-fiber model) with a ball-head (the Arca Swiss monoball). When we were driving, of course, this was impossible, and when I needed to take a photo from the car window, I would brace the lens against the window frame and that worked pretty well. Have the driver turn off the engine if you're trying to shoot from the car; the VR will remove some vibration, but you might as well have as little as possible to begin with.
I also always had the big external flash mounted and turned on. If you're too far away, the flash won't have any effect, but it's really rare that the flash will hurt. I probably took 95% of the photos with a flash, even in broad daylight.
On the left is a photo of me and our guide (Giuliano) where you can see how big the lens/tripod/flash combination is. You can click on the image to obtain a larger version.
When you carry the lens plus tripod combination, it'll be over your shoulder, and for the first couple of days, you'll have a sore shoulder at the end of the day. Also, you have to realize that with the tripod, you'll miss some birds, since you have to set down the tripod and pick up your binoculars, and that'll take some time which is unfortunately, enough time for you to completely miss some birds. I personally am so interested in the photography that I'm willing to pay that price. A hard-core birder might not.
I also took a high-end body (the Nikon D2x) with a 12 megapixel sensor and that allowed me to crop photos and still get reasonable quality even if I couldn't get as close to the bird as I wanted. These days I would only shoot digital. It is wonderful to get a quick review of your shot immediately, and if the exposure or something is way off and you've got more time with the subject, you can fix it on the spot. Also, you can delete bad photos right away: often when you're approaching a live subject, it's just a series of "insurance" photos since if you can get two steps closer, that'll be a better shot. So take two steps and a photo, then two more and another, and repeat. The last "insurance" shot is probably the one you want since the bird flew on your next two steps.
I took a lot of other stuff, too: a backup body, about 6 gigabytes of compact flash cards, a 17-55mm and a 12-24mm lens, plus a 105mm macro. I used the other lenses only a tiny bit, although my wife took some photos with them using the backup body. I also took a small fisheye lens (10.5mm) thinking that I'd take some "art shots", but never got around to it.
Of course you need battery rechargers for both the camera battery (which uses a Nikon-specific device) and for the AA batteries in the flash. I always recharged everything every evening, and always carried spare sets of batteries, but never exhausted them during the day. Last year I used non-rechargeable AA batteries, and that was a bad idea. You hate to throw them out before they're exhausted, but you miss some shots because at the end, it can take 10 to 20 seconds to reload the flash's capacitor to be ready for the next shot.
Make sure your rechargers (in Brazil) can take either 110 volts or 220 volts. You find different voltages in different places.
Finally, I carried a small laptop with me and at the end of each day, dumped all the photos onto it and burned them onto a CD. Next time maybe I'll burn the backups onto DVDs; it took about 25 CDs for about 12 days of birding. It's also nice to have the laptop to review the photos (so that you can delete immediately any that are total crap, and so that you can show the ones you can't identify to your guide that very evening and record it).
My camera allows me to make a short audio recording to associate with any picture, so when possible, I would mutter the name of the bird into the camera as soon as I had the photo if I thought there was any chance I'd forget.