BOAC, the bird is a visual representation of your flight path vector (FPV), laterally showing your track, and vertically showing your flight path angle (FPA).
We can either select Heading/VS mode OR Track/FPA. With Track/FPA we have a little visual representation of the aircraft showing your FPV that appears.
Generally we use Track/FPA mode if flying a NPA or flying visually. We also have a flight director associated with the bird, but it isn't very good to fly with manually. So, generally we fly an NPA with the bird on, the FD's on (known in this case as the flight path director) AND the autopilot on; or autopilot off, FDs off. e.g. you get on your inbound QDM, you are in track mode, so you are laughing all the way down. Regarding FPA, 0.3nm from the FAF select 3 degrees down, and it will fly down beautifully. So varying speed or wind doesn't really affect you with AP on, without AP on you'll have to turn the aircraft to keep the bird on the selected track. Don't get onto managed NPAs for now!
Quite handy also, if you put the top of the tail of the bird on the 0 degree line on the PFD, you will be going down at a flight path angle of around 3 degrees.