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

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