So can you reference a Buddhist text that clearly describes a cause for ignorance?
The entirety of the Buddhist canon couldn't be clearer on the point of dependent origination, which is one of the few non-negotiable doctrines that define orthodox Buddhadharma. But the most comprehensive treatment is probably Nagarjuna's
Mulamadhyamakakarika, which is also relevant to the discussion at large. I don't need to point to a cause for ignorance; the burden would be on you to explain how ignorance could be the only phenomenon in existence that somehow stands outside Buddhist phenomenology by being unconditioned (and therefore eternal and not subject to cessation). All scriptures are written with this assumption in mind. You do see some references to "beginningless ignorance" (I just saw one in the
Sutra of Queen Shrimala of the Lion's Roar), but that's not literally saying that ignorance is unconditioned, which would constitute a heresy, but that it's a problem that goes back to the very beginnings of human existence—i.e. not something that just popped up one day.
So can you give some specific examples from Buddhist texts where the straightforward meaning of birth and death doesn't make sense?
I'd say none of them do, given that there is nothing that experiences birth or death. But if I have the chance to do some extra-curricular research on it and dig up some of the specific things I'm halfway remembering, I'll get back to you.
By the way, putting an end to birth simply means that a Buddha will not be subject to future births.
Doesn't work in the context, which is about what the Buddha has come to do for the world, not for himself. Though I suppose it depends on what one understands by "not subject to future births." That's not something that makes a great deal of sense if taken literally either.
I assume you're familiar with the way that birth, ageing and death are described in SN12.2? I don't see how those descriptions could be viewed as metaphorical:
http://www.accesstoinsight.org/tipitaka/sn/sn12/sn12.002.than.html
I still don't see a coherent basis for your assumption that birth and death are intended metaphorically in the Buddhist texts, it seems more like wishful thinking. There
are plenty of similes in the suttas, but they are clearly labelled as such.
Apart from the fact that everything is functionally a metaphor in Buddhist thought, since concepts are merely mental models and not reflective of reality-as-such? I'll grant that "metaphor" may not be the sharpest term—perhaps "conceptual constructs." Anyway, that sutra is good and clear, for whatever value of "clear" can rightly be applied to Buddhist sutras. However, it seems to be clear to me in a different way than it is to you.
So, birth, aging, and death arise because of ignorance. What does that mean? Do cells literally divide, deteriorate, and cease functioning because people don't understand the nature of things? No, that's a stubbornly literalist interpretation that just doesn't make sense. Cells divide, deteriorate, and cease functioning because they are conditioned phenomena, and that's what conditioned phenomena do. What is
not inevitable is the arising of a concept of self-identity, whose self-referentiality labels those things as "birth," "aging," and "death"—i.e. as things that occur to the imagined self. What the sutra is trying to communicate is not literal physical realities but
conceptual ones, which is what Buddhist scriptures do. Birth, aging, and death are concepts that arise because of ignorance, namely the failure to understand that there is no persistent entity to which those things happen. Why say "birth" and not "the concept of birth"? Because in Buddhist thought the first has no reality, so the second would be redundant.
In other words, the sutra is functioning on a level that is considerably more profound than what you seem to be getting from it.
And this is the "simple" version of the birth-and-death business, not even bringing in its development at the hands of the Yogacarins, who probably worked on it more thoroughly than anyone. I guess we could bring Vasubandhu in here and see what he has to say.