If you want to read some more there is an active thread on contractortalk.com which discusses the absence of birdsmouth notches in rafters with tails. This alternative method allows the rafter rest on bearing blocks which are sistered to the ceiling joists. Interesting concept, I guess. The seat cut is therefore not level, but matches the roof slope.
As always, don't expect to participate in the discussion unless you can demonstrate your existence as a professional contractor.
Some full time framers do this but they use a saw that has a swing table on it from Big Foot tools that cuts up to 75°....the saw is actually called Big Foot.
There was also a specialized tool for cutting birds mouths. Worm drive circular saw with a wide table and about a 4" wide set of stacked dado blades. You lined up your rafters, set tha angle on the table and hogged out the entire birdsmouth all at once.