In reviewing the experience with the Nikon D3X with 24.5 MB sensor back in 2009, several observations were drawn from the conclusions of the In-depth Review by DP review below:
http://www.dpreview.com/reviews/nikond3x/page33.aspIn specifics, quote: "The class-leading image detail comes at a price though. A 14-bit RAW+JPEG image occupies 30MB + on your memory card and this inevitably has an impact on in-camera processing, buffering and continuous shooting. Having said that the D3X's five frames per second (12-bit RAW and JPEG) in continuous shooting will still suffice for most applications, and if they don't you can always switch to DX mode to speed things up and shoot 10.5 megapixel images at seven frames per second. It's not quite D3 speed but still impressively fast.
The other area in which the D3X, mainly because of its pixel-denser sensor, can't quite keep up with the D3 is the high ISO performance (and that's not really a surprise). The D3X is designed for 'high resolution' applications such as studio or landscape photography' and low light is not really its natural environment; nevertheless it still offers a maximum sensitivity of ISO 6400 and, despite its pixel-packed sensor, delivers surprisingly good results at higher sensitivities......"
Another consideration observed was that the high density sensor began to test the resolution of the quality of the lens optics as this quote from Nikon D800 Brochure: " To best draw out the full potential that a 36.3 megapixel camera can offer, lens quality is of vital importance. Even subtle differences in optical performance make a difference when utilizing such a large pixel count..." One may infer that an investment of top quality optics may be prudent to go hand in hand with the all new D800 36.3 MB sensor, or otherwise it may become a counter-productive exercise. As a result, for Bird Photography application, one must take into consideration the parameters of continuous frame rate, high ISO performance and lens quality while looking forward to the new D400 APS-C format DSLR body.
Good birding and photography.