Communication about threats including those posed by the presence of predators occurs mainly through acoustic signals called alarm calls. The comprehension of these calls by receivers and their rapid antipredator response are crucial in terms of survival. However, to avoid overreaction, individuals should evaluate whether or not an antipredator response is needed by paying attention to who is calling. For instance, we could expect adults to be more experienced with predator encounters than juveniles and thus elicit stronger antipredator responses in others when alarming. Similarly, we could expect a stronger response to alarm calls when more than one individual is calling. To test these assumptions, we applied a playback experiment to wild ravens, in which we manipulated the age class (adult or juvenile) and the number (one or two) of the callers. Our results revealed a seasonal effect of age class but no effect of number of callers. Specifically, the ravens responded with stronger antipredator behaviour (vigilance posture) towards alarm calls from adults as compared to juveniles in summer and autumn, but not in spring. We discuss alternative interpretations for this unexpected seasonal pattern and argue for more studies on call-based communication in birds to understand what type of information is relevant under which conditions.

有关威胁（包括捕食者的存在所造成的威胁）的沟通主要通过称为警报呼叫的声音信号进行。接收者对这些呼叫的理解及其快速的反捕食者反应对于生存至关重要。然而，为了避免反应过度，个人应该通过注意谁在打电话来评估是否需要做出反捕食者反应。例如，我们可以预期成年人在遭遇捕食者方面比青少年更有经验，因此在发出警报时会引起其他人更强的反捕食者反应。同样，当不止一个人打电话时，我们可以预期对警报呼叫会有更强烈的反应。为了测试这些假设，我们对野生乌鸦进行了回放实验，其中我们操纵了呼叫者的年龄组（成年或青少年）和数量（一个或两个）。我们的结果显示年龄层存在季节性影响，但呼叫者数量没有影响。具体来说，与幼年乌鸦相比，夏季和秋季乌鸦对成年乌鸦的警报信号表现出更强的反捕食行为（警惕姿态），但春季则不然。我们讨论了对这种意想不到的季节性模式的替代解释，并主张对鸟类基于呼叫的交流进行更多研究，以了解什么类型的信息在什么条件下是相关的。