Wildlife photography and P-TTL (pre-flash)

I know this has been raised before under slave and studio flash problems, but I now have a new problem associated with P-TTL.

I'm using a *ist and MZ6, both use the P-TTL with the 360FGZ flash. I wish to use the cameras in auto exposure on a remote lead of 100m which I've made up, to take photos of birds on a feeder. If I try to use the flash with fill in, it emits a pre-flash, sufficiently far ahead of the main flash and shutter release, to disturb the birds. Alternatively, I can use other dedicated flashes, but can't set the exposure compensation on the flashes to give me a decent fill in.

There must be a way of defeating the P-TTL pre-flash, but still be able to set the flash compensation? Do I need to get yet another flash, like the 500?

I need to use the auto modes, as the light levels are constantly changing, but I can't attend the camera, only watch through a telescope.

Regards

Taliety