Winter survival strategies for small birds: Managing energy expenditure through hypothermia

Theoretical studies on energy management in wintering small birds have focused on energy reserves. Here we extend these analyses to include energy expenditure controlled by nocturnal hypothermia. Our models indicate that even the minimal use of hypothermia, when dusk reserves are insufficient, can significantly increase the probability of surviving the winter. Optimal hypothermia, which allows employment of hypothermia even when dusk reserves are sufficient for surviving the night, outperforms minimal hypothermia only when the food supply in bad weather spells is very low. Our models illustrate the use of the dynamic state variable approach in a broader theoretical analysis involving multiple trade-offs among alternative strategies.