I don't even think you have to go that far. Most of the recent rural to urban migration in poor countries is really family members, that will, without a doubt, move back home to the family farm in the event of a crisis. It's not really that they are investing in social capital. They have transiently moved to where they can make better money. In the event of a famine, this tide will reverse, and the urban poor will quickly migrate back to the countryside.

Once back on the farm, they will quickly reintegrate with the village because they never really left in the first place.

Also, in the real event of a famine, the idea of mono cropping cereals for cash will be ditched quickly. These people may be poor, but they aren't so stupid they will accept death. The cereal crops they raise will be abandoned for integrated gardening just as quick as they can manage it. This will put further pressure on grain prices, but will allow much of the poor to live out a crisis. Most rural families who import food do so because it is relatively easier to grow a single crop, sell it and buy what you need than managing a complete garden, especially when several family members are earning money in the city and sending it home.

No, the 3rd world poor will quickly adapt during food scarcity. They are only a decade or two removed from it, and the knowledge hasn't been lost.

It's the first world non immigrant poor who will starve. That's where the crisis will be.