We would like to know more about this. There is evidence that the effects of tannins can be felt when there is not enough to cause a migraine. This might allow for experiments to be done without actually causing migraines. It would not be pleasant to ask someone to suffer migraines for the sake of research. Tannins in tea can be reduced by making the tea weaker.
Caffeine is a vaso-restrictor.
Tannin must be causing vaso-dilation somehow to cause a migraine. They do extensive work on dietary matters. They are doing a major project for the European Union.