Timing is crucial for consequences of migratory connectivity

Migratory animals typically visit several places throughout their journeys. It is widely acknowledged that ‘migratory connectivity’ - the geographic linking of individuals and populations between one life cycle stage and another - has consequences for migrant fitness and the dynamics of their populations, for gene flow and genetic mixing as well as for the communities visited.  

In our paper, we argue that in addition to the spatial dimension of migratory connectivity, i.e. where animals are going, we need to explicitly consider its temporal dimension, i.e. the timing of migration, to fully understand the consequences of migratory connectivity. We suggest that timing of migration can be characterised by three aspects - synchrony, phenology and consistency. Synchrony describes how wide-spread over time individuals of a population migrate. At the extremes, they can either be synchronous – all individuals migrate at the same time –or asynchronous – all individuals migrate at different times.  Migration phenology describes the when migrants move relative to other time-varying factors or processes such as the availability of key-resources or presence of predators. At the two extremes, the migrants’ presence on a particular site fully overlaps with, e.g. resource peaks (‘matched’) or is completely disjoint (‘mismatched’) with them. Finally, consistency describes how repeatable migration phenology and synchrony are over several migrations bouts.

Using examples from throughout the animal kingdom, we illustrate how consequences of migratory connectivity can differ with variations in any of these aspects of timing. For a specific consequence – the transmission of pathogens and parasites – we developed a dynamic network model to quantitatively demonstrate how changes in phenology and synchrony can affect the dynamics of an infectious disease.

The timing of migration – here exemplarily from a non-breeding site via an intermittent staging to a breeding site – can be characterised by synchrony (left panel) and phenology (right panel). Individuals migrants may depart, stay or arrive at the same time, i.e. synchronously (upper-left), or at different times, i.e. asynchronously.

Similarly, the degree of coincidence between migrant visitation and, e.g., resource availability (upper-right panel) has fitness-consequences that may range from positive under complete overlap (‘matched’) to negative when migrant visitation and resource availability are fully disjoint (‘mismatched’).