At the individual level seasonal interactions are defined as non-lethal effects that operate via the body condition of migratory birds. This may function in many ways, migratory birds can accumulate body reserves in one season and use those reserves in a following season, or they can occupy habitats of differing quality that affect their body condition to such an extent that it will affect their migratory schedule and eventually their reproductive output.
It is important to understand to what extent different stages of the annual cycle of migratory birds are linked to propose effective conservation and management measures. To understand seasonal interactions it is also vitally important to know how different geographic areas are connected by the journeys of individual birds.
A population level seasonal interaction occurs as a result of density dependent effects. If a mortality event in winter reduces the number of birds in a given population, then fewer birds will return to breeding grounds the following spring. As a consequence of the lower density, a higher proportion of individuals can occupy high quality habitat and achieve high reproductive output. So as a consequence of the mortality event in winter there is an elevated per capita reproductive output the next spring.
