To follow up, when a famine occurs in a population, is it due to an actual scarcity of calorie-containing food in a geographic area?
To what extent do cultural norms affect the ratio of available calories that the biosphere of an area can provide to the amount of calories actually used for consumption by a population?
Why couldn’t (or don’t) starving people convert to a diet of calorie rich insect protein for example? There are plenty of stories about people eating leather and cats during famines - why not grubs and ant eggs?
This question as a whole may be leaning more science-ily than history-ily - but do we have examples of people (individually or as groups) choosing to starve due to cultural preferences regarding diet when plentiful culturally discouraged options abound?
What defines/delimits a “famine food”? Are there recognizable patterns for the stages of reintegration of a former famine food into the standard diet of a population (lobsters in new england for example)?
To provide a specific-context answer to your third question, during the Cambodian famine in the 1970s people did indeed eat insects. It's not uncommon to find people eating tarantulas and other things (like large centipedes) in Cambodia and some Cambodians even consider this as part of their national cuisine (although I'm not sure to what extent they did before the famine.)
While some insects are high in protein, that's true, they are not amazing calorific sources. Too much protein and little to no fat and carbohydrate leads to a phenomenon called rabbit starvation where the population slowly just rots away. If you eat only super-lean meat you will die. In terms of just insects, which have been used to historically supplement, not constitute, diets, there just isn't enough important nutrients in the biosphere. For that you need agriculture, a lack of which constitutes... famine. In the middle of an already existing famine, reliance on protein-only sources would just exacerbate the conditions. Which is exactly what it did in Cambodia.
A last comment: yes, people can ignore taboos if they are hungry. In the siege of Paris in the 1870 German war, the population slaughtered and ate every animal in the Paris zoo. Now the French have ... different, shall we say, culinary habits to the English, or Americans, but during this siege they broke all taboos of eating certain animals (cats, dogs, rats). A good summary article can be found here for the Paris 1870 siege.