Animals usually show distinct periods of diel activity and non-activity. Circulating baseline levels of glucocorticoid hormones (corticosterone and cortisol) often peak just before or at the transition from the non-active to the active period of the day. This upregulation of glucocorticoids may function to mobilize stored energy and prepare an animal for increased activity. Usually, the alternation of active and non-active periods is highly predictable; however, there is one group of animals for which this is not always the case. Many otherwise diurnal birds show nocturnal activity during the migration seasons. Nocturnal migratory flights are alternated with stopover periods during which the birds refuel and rest. Stopovers vary in length, meaning that nocturnal migrants are inactive in some nights (when they continue their stopover) but extremely active in other nights (when they depart and fly throughout the night). This provides an ideal natural situation for testing whether glucocorticoids are upregulated in preparation for an increase in activity, which we used in this study. We found that in northern wheatears (Oenanthe oenanthe), corticosterone levels peaked in the few hours before sunset in birds departing from stopover that night, and, importantly, that this peak was absent in birds continuing stopover. This indicates that corticosterone is upregulated in the face of an increase in energy demands, underlining corticosterone's preparative metabolic function (energy mobilization). The timing of upregulation of corticosterone also gives a first insight in when during the day nocturnally migrating birds decide whether or not to resume migration.

中文翻译：

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皮质酮的狄尔变异和迁徙鸟类的离境决策。

动物通常表现出不同时期的diel活动和非活动。糖皮质激素（肾上腺皮质激素和皮质醇）的循环基线水平经常在一天中从非活动期过渡到活动期之前或之时达到峰值。糖皮质激素的这种上调可以起到调动储存的能量并使动物准备增加活动的作用。通常，活动期和非活动期的交替是高度可预测的；但是，有一组动物并非总是如此。许多其他昼夜鸟类在迁徙季节表现出夜间活动。夜间候鸟飞行与中途停留的时间交替进行，在此期间，鸟类会加油和休息。中途停留时间长短不一，这意味着夜间移民在某些夜晚（当他们继续中途停留时）不活跃，而在其他夜晚（当他们离开并整夜飞行时）则非常活跃。这为测试糖皮质激素是否被上调以准备增加活性提供了理想的自然环境，这是我们在本研究中使用的。我们发现，在北部麦穗（Oenanthe oenanthe）中，当晚从中途停留的鸟类在日落前几个小时内皮质酮水平达到峰值，而且重要的是，在持续中途停留的鸟类中没有此峰值。这表明在能量需求增加的情况下皮质酮被上调，突显了皮质酮的制备性代谢功能（能量动员）。