First off, I would point out that limiting yourself to a single brand is likely to greatly limit your options, and force you to choose something extremely expensive when there are viable alternatives for much cheaper with other brands.
Second, I think people are greatly diminishing the weather sealing on modern "professional grade" cameras. One need not explicitly buy the top of the line Nikon or Canon cameras, or specifically a Pentax camera, to get good weather sealing. That is "the best" weather sealing, and if you suspect you'll be crawling through the mud in a rainforest during the wet season with the camera drenched 90% of the time on long days for weeks on end...then you probably want the best of the best. Good weather sealing can be found for much cheaper than the $6000 you would have to spend on the likes of the Nikon D4, or the Canon 1D X.
Given your budget, I would actually recommend the Canon 7D. It rarely crops up in discussions of good weather-sealed cameras, but owning it myself, I can attest to this cameras resilience in an extremely broad range of rough weather, including extreme cold and blowing snow. I've used my 7D in temperatures ranging from 106°F to -19°F, in blistering sun, rain, sleet, snow, hail, even once up in the middle of a frozen lake at 11,000 feet in a blizzard driven by 60mph winds.
The 7D, as of yet, hasn't skipped a beat. It is a low-end professional-grade camera with good weather sealing, a great AF system, very rugged, durable body, highly customizable, and capable of working with a battery grip for extra power and battery life. The battery grip is probably a must-have for temperatures below 0°C, as battery life will be shortened by the cold. The cropped sensor and high end AF system will support your desire to photograph birds as it will extend the reach with long lenses. For a good beginner birding lens, you could pick up the 100-400mm L. For landscapes, you'll want to pick up the EF-S 10-22mm lens (equivalent to a 16-35mm on FF, a great range for landscapes).
The only real drawback of the 7D is the noise at very high ISO. IQ is great up through 1600. By 3200, it gets pretty noisy, and by 6400 it is kind of useless...at least for artistic usage where the images could be printed or published online. The 7D is not really a true "low light" camera, and at times this can limit your ability to photograph fast moving subjects (i.e. birds). At the moment, the 7D is kind of a unique camera in that it combines an excellent AF system and a high frame rate (8fps, essential for bird photography) with a high resolution crop factor in a high quality body. The only real alternative that can offer the same kind of AF performance would be the Canon 5D III or a top-tier body from either Canon or Nikon. The problem with full-frame cameras is you lose the reach, and need longer lenses. Not only would you need to spend $3500 on the camera body (for the 5D III), you would need to spend $8000-$14000 on even JUST ONE LENS! Given the cost chasm, I again feel the need to recommend the 7D. As a bird, wildlife, and landscape photographer myself, I have found it to be a superb camera.
Currently you can pick up the 7D for around $900-1000 on sales. The EF 10-22mm f/3.5-4.5 lens can be found for $600-$700. That puts a starter landscape package right around your budget of $1500. If you want to pick up a longer lens for birds, you could grab the EF 100-400mm f/4.5-5.6 L IS for about $1500. It is a good lens, although a bit soft. For a better lens, you could pick up the 300mm f/4 L IS for about $1400, and with the EF 1.4x TC III ($450) you would have a great birders combo. The longer lenses just plain and simply aren't cheap, and for the full package, you are probably looking at about $3500.
You could pick up an off-brand super-zoom lens that goes from the wide or normal lengths to supertelephoto lengths. You might get such a lens for fairly cheap, however I wouldn't recommend it. For one, cheaper lenses usually don't have any kind of weather sealing, so they render the weather sealing of your body useless (every time I have used my 7D in extreme weather, I've used a sealed lens.) Second, cheaper lenses are also slower lenses. Winter tends to be a darker time of year, wrought with storms, so available light is limited. Faster lenses are better, which is why the EF 300mm f/4 L IS lens is probably the best option for birds. On the 7D it is the FOV equivalent of a 480mm lens on FF, and combined with the higher pixel density of the 7D's sensor, you get considerably greater reach.
Canon is rumored to be releasing the Canon EOS 7D Mark II this summer, around August. Its initial price is expected to fall around $2100, and when it hits, the original 7D is expected to drop a fair bit in price. If you are able to hold out that long, you could probably pick up the 7D for a steal, and save a little more money for lenses. If you can't really wait, I'd still say go for it. It is a great camera, and the high ISO noise issues become less and less of a problem as better and better NR tools are released.
