Seasonality of the Daily Weight Cycle in Wintering Passerines and Its Consequences

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Abstract
Nocturnal weight loss in birds varies seasonally due to variation of photoperiod and temperature. Birds can maintain weight balance (1) by compensating for the loss of the previous night or (2) by anticipating the loss. In case (1) minimum (morning) weights will decrease the longer and the colder nights are whereas in case (2) the morning weights will remain constant. The concept of winter fattening should be restricted to mean only the cases in which (3) the minimum (morning) weight increases before winter starts. The morning weight is the only momentary weight that is globally comparable in day-active animals. Consequently, the seasonal ''adaptive weight variation'' must be divided into two components and causative factors behind each of them considered separately. First, by adjusting the daily weight amplitude the bird can cope with photoperiodic and temperature challenges. Secondly, variation in the daily minimum weight reflects the safety margin enhancing survival under poor feeding conditions. Constant morning weights are sufficient only in highly predictable environments. A greater unpredictability of the opportunities to replenish daily fat reserves should lead to a seasonal increase in the daily minimum weight. 1 measured the seasonality of overnight weight losses and diurnal gains in three passerines. In the Greenfinch and the Yellowhammer diurnal gains closely equalled nocturnal losses throughout the winter and showed seasonally, which could be explained by night length and temperature. In the Great Tit daily gain remained low in midwinter, but overnight losses varied seasonally as predicted. In all the species morning weights increased before winter started, indicating varying degrees of true winter fattening.