In experienced hands it manages well
The Southern Ocean I recently spent 3 weeks on board a ship exploring the Southern Ocean including the New Zealand and Australian sub-antarctic islands. There were plenty of opportunities for photographing birds in flight.
Main Bird Types We had on board many highly experienced bird watchers who could and did distinguish between species and sub species by minute variations in morphology. For my rather simplistic purposes there appeared to be three main types of ocean birds.
* Big Ones: These were mostly albatrosses, with some giant petrels for variation. These birds glide along on outstretched wings, rarely flapping. They are relatively easy to photograph, especially if some helpful person chucks fish pieces off the back of the boat to bring them in.
* Mid Size: The commonest at this size were the Cape Petrels. These fly very fast, frequently changing direction and elevation. They are very difficult to photograph.
* Little Ones: These were mostly Prions. These birds are only about 25cm long. They flap their wings a lot, dive, soar and change direction several times per second. I have no idea how they survive in the southern ocean which can be a very wild place. They were completely beyond my ability to photograph. I couldn't even hold one in the frame let alone get a focus on it.
Camera Gear I elected to go simple for this trip. I mounted a Lumix 14-140mm lens on a Panasonic GH3 and left it there for the whole trip. The only other camera which I took was a Canon G16 compact as backup in case the GH3 got trashed by some misadventure. The GH3 and lens were subjected to plenty of salt and fresh water splashing plus extremes of cold and humidity. There were no equipment problems at all.
Limitations of M43 system for B.I.F I found that in fact the camera's autofocus system could keep up with the birds quite well. The main functional limitation is the EVF blackout time after each exposure. This makes it difficult to keep the birds in frame. I have plenty of practice working with this issue so got a reasonable percentage of in frame/ in focus shots but a newcomer to M43 might struggle.
Camera Settings
* Focus Mode: AFC
* Drive Mode: Burst (=Continuous) With a 95Mb/sec card this gives about 4.5 frames per second using the 14-140mm lens.
* Burst Rate: M with live view
* Autofocus Mode: 1 Area (Do not use Tracking) I set the AF area to center position, size 1/4 or 2/4.
Others: OIS on, RAW capture, Multi Metering.
The GH3 is very pleasant to use as the settings for Drive Mode and Focus Mode are on hard control modules with the setting in use easily visible and quickly changed.
Improvements Needed The main limitation of M43 cameras for photographing moving subjects is the EVF blackout time. This needs to be greatly reduced before M43 can be widely accepted as a desirable camera system for sport/action type photography.
These are just guesstimates but I put the viewfinder blackout time of a good DSLR at about 20% of the total time during continuous shooting. For the GH3 I guesstimate blackout time to be more like 60%.
The capability of the autofocus system is actually quite good. On board our ship in the southern ocean there were many high powered Cano Nikons with very large lenses. Most photographers trying for BIF photos appeared to be running off about 500-1000 frames per day and counting themselves lucky to get 5-10 really good shots out of that number. My keeper rate, by which I mean clear, sharp printable frames, was about 5% with the GH3. This was similar to that of the DSLR's. I know 5% doesn't sound very good but conditions were very challenging most of the time.
Recommendation At this time I would not recommend any Panasonic M43 camera for B.I.F. If Panasonic can substantially reduce the EVF blackout time and maybe increase the sensor readout rate for AFC from 256x to 512x per second, I think M43 (even with contrast detect AF) could be very competitive with the best DSLR's for follow focus on moving subjects.