I'm not sure what Wolfgang is talking about the e-m1 being huge. I have both the e-m1 and the e-m5 and the e-m1 is marginally larger than the e-m5. Perhaps he is talking about the Nauticam housings for both. I have the housing for the e-m5, but not for the e-m1. However, I've seen the Nauticam housing for the e-m1 at DEMA and I didn't notice that much of a difference. The e-m1 housing comes with a tray that has two "arms" vertically attached so maybe that is what he means. However, if you mount two flash units, then those "arms" are integrated into your overall unit.
As for shutter lag, I am a professional photojournalist and previously was using Nikon and Canon systems. Although the e-m5 has a short lag, I find the e-m1 to be nearly as fast as my DSLRs. S-AF is actually faster than the DSLRs and I have not had any problems with the 2x crop factor.
With Nikon, I was using the D300 and D700 with the 14-24, 24-70, 70-200, all 2.8 lenses. With the Canon, I was using a 1Dmk3 and 1Dmk4 with the same lenses. I originally bought the e-m5 for strictly underwater work since the housings were so much smaller. Little by little, I began to incorporate the e-m5 into my working equipment until I was using the e-m5 about 30% of the time. As of the release of the 40-150 PRO lens, I switched over to Olympus and sold all my digital DSLR gear. I now own the e-m5 and e-m1 with the 8mm Panasonic fisheye, 9-18, 45 1.8, 12-50 (range and macro are nice for uw) , 12-40 PRO, 40-150 PRO and the 1.4 converter. While the c-af with the e-m5 is pretty much useless, I find that I can use the e-m1 well. There is a new firmware version for the e-m1 (V.3.0) which promises to increase the c-af accuracy at up to 9fps. I have downloaded the firmware but have not been able to check out the c-af yet. If it works well, I see no problem with you using it for your birding photos.
Not wanting to get too detailed about my experiences here, message me if you have further questions about the system.