Cues and decision rules in animal migration

The sheer beauty and impressiveness of animal migrations have long puzzled observers and raised the questions of how these animals find their way, what initiates their migrations and how they manage to schedule their journeys at apparently the right times. There are many challenges that animals face before and during migration, and they can be grouped into two major categories namely ‘where to go?’, dealing with orientation and navigation, and ‘when to go?’, dealing with the timing of activities and migration schedules. As these questions are fundamentally different, we also expect different cues to provide relevant information. Furthermore, animals make decisions in relation to their physiological state and therefore, cues can be further categorized into internal and external signals. In this chapter, we tackle these questions not in terms of why animals migrate (ultimate reasons), but how they make the right decisions before and during migration (proximate factors). Migrating animals rely on external and internal information such that they can tune their behaviour to their (changing) requirements and to the development of their seasonal environments. Here, we show that these cues, defined as ‘signals or prompts for action’ (Oxford English Dictionary), are not well understood, although they are most likely highly relevant both for advancing our fundamental understanding of migration and for increasing our capacity to manage and conserve migratory systems under the threat of environmental change. In the following sections, we first characterize migration from different perspectives, as different migration types may require fundamentally different cues and decision rules. Thereafter, we introduce migration in the major migratory taxa that are readily observable to biologists, i.e. insects, fish, reptiles, birds and mammals, and seek to identify the cues that are used for the different steps during each of their migrations. We finish by highlighting the general lessons that can be drawn from this comparative study of cues and decision rules in migration.