The yearling population submodel is fairly simple. The number of squirrels born each year (births) depends on the number of squirrels available to give birth and the amount of food available; when little food is available, the squirrels mate less frequently and produce smaller litters. Each year, some yearlings die due to various natural causes (lack of food, death of parents, illness, etc.), although food is, at least in this model, the primary determining factor for the number of surviving yearlings. Those who survive become adult squirrels in the next year; those who do not are removed from the stock via the "deaths1" flow. (The name "deaths" is used for adult squirrel deaths.)