>> This is the biggest part of the problem that is left ambiguous, namely because you have to make assumptions about the liar in order to succeed.

I don't understand why the author then proceeds to outline three separate types of liars. This part of the problem seems very simple to me. If you present any question to the lying bird, the lying bird will first evauate the question in the exact same way as the truth-telling bird, to arrive at its answer. Then, instead of telling that answer truthfully, the lying bird will lie.

My answers:

For the first form of the riddle, I thought "Would the other bird say "Yes" if I asked it whether this door goes to paradise?" would work well enough.

For the second form of the riddle, I tried to reduce it to the first answer and side step this 'cannot reference the other bird' problem by referencing that type of bird, maybe something like:

"Given that you are one of two types of birds (Truthful and Liar), if you were the other of the two kinds (the opposite of what you actually are), would you say "Yes" if I asked it whether this door goes to paradise?" (This could be made more concise and clear.)

For the third form of the riddle I had to sketch out logic charts. I arrived at:

Is it true that ( you are a truth telling bird, and that door goes to heaven) or that (you are not a truth telling bird, and that door does not go to heaven)? (Pointing to a particular door throughout).

What about a simple "Does the left door go to heaven and am I asking you this question?"

Just combine a known truth with what you really want to know.

Damn, that was complex. Would a simpler "What would be the wrong answer to 'which door goes to hell?'" solve it?