A THEORETICAL INVESTIGATION OF THE FAT RESERVES AND MORTALITY LEVELS OF SMALL BIRDS IN WINTER
A THEORETICAL INVESTIGATION OF THE FAT RESERVES AND MORTALITY LEVELS OF SMALL BIRDS IN WINTER
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DOI：
10.2307/3676736
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发表时间：
1993-07-01
期刊：
影响因子：
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通讯作者：
MCNAMARA, JM
中科院分区：
文献类型：
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作者：
HOUSTON, AI;MCNAMARA, JM
我们假设，一只小鸟在冬天的脂肪水平反映了饥饿和捕食之间的权衡。这种权衡是通过找到在给定环境中使总死亡率（饥饿加捕食）最小化的脂肪水平来形式化的。一只鸟的特征在于它的能量储备水平。如果这些储备降到零，鸟儿就会挨饿。一般来说，鸟被捕食者杀死的概率随着储备的增加而增加。我们考虑两个模型。在一天中的每一个时间单位，鸟类都可以根据其储备的功能选择觅食或休息。在觅食与休息模型中，只有一种觅食选择。在觅食强度模型中，鸟类可以从一系列选项中进行选择，其中具有高能量增益的选项也会导致高捕食风险。我们找到了各种环境的最佳储备水平，以及由此产生的饥饿和捕食水平。除非食物的供应是非常高的，在黄昏时的平均储备（MRD）的整体可用性的改善的结果减少，在觅食强度模型也有一个趋势，选择更安全的觅食选择较低的平均收益。变异性的增加，无论是由于寻找食物的概率降低，还是由于觅食中断的增加，都会增加MRD。储备的增加不足以防止饥饿的增加。因此，储备和饥饿可以在各种环境条件下呈正相关。饥饿的水平往往低于捕食的水平，但饥饿与捕食的比例往往随着条件的恶化而增加。隆冬时节，昼短夜长且寒冷。对日照减少的最佳反应包括增加黎明和黄昏的储备水平。当过夜支出增加但白昼长度不变时，也会出现这种情况。在这种情况下，我们表明，当温度很低，一个小的节能可以导致生存概率大幅增加。这一结果的相关性进行了讨论，以评估低温的重要性。
We assume that the fat levels of a small bird in winter reflect a tradeoff between starvation and predation. This tradeoff is formalized by finding the level of fat that minimizes total mortality (starvation plus predation) in a given environment. A bird is characterised in terms of its level of energy reserves. The bird starves if these reserves fall to zero. In general, the probability that the bird is killed by a predator increases with increasing reserves. We consider two models. In both at each time unit during a day a bird can choose, as a function of its reserves, to forage or to rest. In the forage vs rest model there is only one foraging option. In the foraging intensity model the bird can choose from a range of options, where options with a high energetic gain also incur a high predation risk. We find the optimal level of reserves for various environments, together with the resulting levels of starvation and predation. Unless food availability is very high, an improvement in overall availability results in decrease in the mean reserves at dusk (MRD); in the foraging intensity model there is also a trend towards choosing safer foraging options with lower mean gains. An increase in variability, either as a result of a decreased probability of finding food or an increase in interruptions of foraging, increases MRD. The increase in reserves is not sufficient to prevent an increase in starvation. As a result, reserves and starvation can be positively correlated across environmental conditions. The level of starvation tends to be lower than the level of predation, but the ratio of starvation to predation tends to increase as conditions become worse. In the middle of winter, the days arc short and the nights are long and cold. The optimal response to a decrease in daylength involves an increase in both dawn and dusk levels of reserves. This pattern is also found when overnight expenditure increases but daylength is constant. In this context we show that when temperatures are very low, a small saving in energy can result in a substantial increase in survival probability. The relevance of this result for evaluating the importance of hypothermia is discussed.