I'm most familiar with Micro Four Thirds, a system I bought to replace my unwieldy APS-C kit for motorcycle and backpack travel. I'm so impressed with MFT IQ, lightness and lack of bulk that I'm selling the APS-C kit for travel and landscape, and planning on buying a used 5D (original) mostly for studio use as backup for MF and LF film.
I have Olympus E-PL1, which I chose over Panny's cameras mainly due to IBIS. I can fit the entire kit (body, 4 lenses, Gitzo 1541T tripod with ballhead, misc. stuff) in a standard carry-on and have room for 5 days of clothes, gym gear, personal hygiene, etc.!
Olympus is rumored to announce two new cameras in the coming months: an E-PL1 replacement at CES, and a "pro" level body later in the Spring.
Lens selection is ok with zooms, but (good) dedicated primes are very few. In general, Panny's glass has better IQ (go figure), while Oly's lenses are cheaper, smaller and lighter. One can use all kinds of adapters with varying success to mount anything from Four Thirds to Canon to Leica glass, although some of the lenses make the weight and bulk savings of MFT moot, of course. Voigtländer has started making highly regarded manual focus lenses in MFT mount as well.
There are brand new zoom lenses
from both Panasonic and Olympus which reach up to 600mm (FF equivalent). You'll have to do your own research on the lens selection's suitability for birding as I don't use long teles at all.