I found several problems in trying to focus for such head-on shots:
- very little time
- very small focussing target, so hard to keep the focus spot on the head
- worst possible direction for grabbing focus (a target moving perpendicular to you stays at the same distance, more or less)
Add to that that the actual spot is often not quite centred (and smaller) than the spot indicated in the viewfinder, and it really becomes a gamble.
If the birds follow a predicable path, you could try pre-focussing on manual and wait untill one is kind enough to pass through your focus zone...
For the multiple birds: several targets at different distances => DoF problems...
If you can, smaller aperture would of course help here (F16-22, if shutter speed stays acceptable), perhaps at higher ISO (you don't mention what you use now)?
No magic recipes, I'm afraid