ensure you have the latest firmware
reduce viewfinder blackout by:
get the exposure correct and shutter to stop the action:
consider Manual exposure mode or Shutter Priority mode
set shutter speed to 1/1000th - 1/2000th sec to reduce motion blur of the bird
set ISO to auto ISO if not using Manual exposure, and have upper setting set to around ISO 6400
aperture will generally default to wide open if shooting Shutter Priority, but if you need more depth of field, then use Manual Mode and set everything individually
if not using Manual mode you may need to set exposure compensation, especially shooting into a bright sky
you could consider using spot metering, but then your bird MUST be in the centre of the image and depending upon whether that part of the bird is white or dark will affect your metering, but this won't be such an issue with manual mode as you can determine this before the shot
consider disabling image stabilisation
Olympus OM-D cameras have the best image stabilisation available so you may as well be using it if you are panning with slower shutter speeds
how general BIF, use a shutter speed faster than 1/2000th sec in which case you can turn IBIS and OIS off although many still have it on for better EVF experience and perhaps faster AF acquisition in which case you may wish to use S-IS 2 mode to correct only for vertical shake
decide upon Release Priority options
set Noise Filter to OFF or LOW
set the focus range on the lens if it has one
set AF points
consider choosing a group of 9 points
in some situations just a single central point may work better, especially for stationary birds
avoid using teleconverters
do NOT use C-AF + Tracking mode unless you have the E-M1 Mark II
this does not work well enough on other cameras, on the E-M1 Mark II you can consider using C-AF+tracking, but all other cameras, use S-AF+MF or S-AF
set Fulltime AF to OFF
set Release Lag-Time to SHORT
set EVF Frame Rate to HIGH
consider setting EVF to OVF mode