Energetics of Winter and Migratory Fattening

Abstract

Many birds live in environments that undergo marked variation in the course of the day and the year. From day to night the light level can change 1,000,000-fold, and under extreme conditions the air temperature can drop from +30°C to 5°C, or the relative humidity can rise from 20% to 80%. The same kind of variation can apply to the alternation between summer and winter, with months of permanent light followed by months of permanent darkness at the poles, with a temperature range of +30°C to -40°C in a continental climate and a several-meter snow cover that can persist for several winter months in mountain areas or high latitude with high precipitation. Superimposed on these predictable changes in the abiotic environment are unpredictable weather events. The climatic fluctuations are paralleled by changes in the biotic environment of a bird; certain prey animals or food plants disappear temporarily or are periodically not accessible. Birds have a relatively high metabolic rate, which implies continual energy consumption and a constant supply of food. To ensure that this supply is uninterrupted despite the extreme variation in availability of food, birds have developed many strategies, without which they would not have been able to survive in a diversity of habitats.