One consideration might be to look at some compact body mirrorless cameras. They're not perfect for all photography, but for the purposes you mention could be a very useful thing indeed. I picked up a Sony NEX-5N for the purposes you mention - when I go birding, I often like to have a second camera body on me to shoot shorter focal lengths, either if a bird is too close, or there's some particular landscape shot or detail closeup I want to shoot - it's more convenient to not have to change lenses, so I go with the second body. I weighed the idea of a second DSLR body, but in the end, WEIGHT was most of the issue...it's simply more convenient going with the much lighter and smaller NEX, even with a 'superzoom' type lens onboard, then even the smallest DSLR (as Popo mentioned, moving to any other brand's mount means you'd have to get a new superzoom lens to go with that mount). I can have the NEX hanging around my neck, or on a belt holster, and barely notice it. Yet it still has an excellent 16MP APS-C sensor with full low light capabilities and excellent dynamic range, and via cheap adapters I can use any lens from any mount and any brand on the NEX, if I feel like playing around or traveling light with just the NEX and leave the DSLR home. There are at least 2 current superzoom lenses available for the NEX bodies - the Sony 18-200mm and the Tamron 18-200mm.
Just something to consider. The M4:3 cameras from Oly or Panasonic might similarly work for such a purpose, with the smaller sensor their lenses can be a little smaller too - whether it would meet your IQ needs depends on whether you would be doing a lot of high ISO shooting with it.
If weight and size are absolutely of no consequence to you at all, then the second DSLR body idea would make sense, especially if you stuck to the same mount you have, that way you could share lenses between both bodies as needed.
Sony DSLR-A68 / Sony 18-250mm / Minolta 50mm F1.7 / Tamron 150-600mm / Minolta 300mm F4 APO
Sony A6300 / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / 10-18mm F4 / 35mm F1.8 / 16mm F2.8 / FE70-200mm F4 G OSS / FE70-300mm F4.5-5.6 G OSS / via manual adapter, lots of Pentax K mount, Konica K/AR mount, and Leica M mount manual lenses