Hej,
Well sorry I was kinda too happy so I didn't put on any explanation...
I just put some old bread with the camera in auto mode with flash disabled and macro mode enabled, cam being around 20cm from the bread.
So I started the motion detection script :
threshold : 10
delay : 5
Didn't change any other setting from the script.
So this is mostly the script which did the trick, I hadn't to set much things, tested the script with my hand.
Of course I set the power saving from the camera off, and the LCD off with CHDK set to "script".
So I don't think anything I did is usable with a DSLR.
The only trick is not to be there for the bird gets close to the cam without flying away, so the cam must work by itself. ^^