Unravelling processes between phenotypic plasticity and population dynamics in migratory birds

Populations can rapidly respond to environmental change via adaptive
phenotypic plasticity, which can also modify interactions between
individuals and their environment, affecting population dynamics. Bird
migration is a highly plastic resource-tracking tactic in seasonal
environments. However, the link between the population dynamics of
migratory birds and migration tactic plasticity is not well understood.
The quality of staging habitats affects individuals’ migration timing and
energy budgets in the course of migration, and can consequently affect
individuals’ breeding and overwintering performance, and impact population
dynamics. Given staging habitats being lost in many parts of the world,
our goal is to investigate responses of individual migration tactics and
population dynamics in the face of loss of staging habitat, and to
identify the key processes connecting them. We started by constructing and
analysing a general full-annual-cycle individual-based model with a
stylized migratory population to generate hypotheses on how changes in the
size of staging habitat might drive changes in individual stopover
duration and population dynamics. Next, through the interrogation of
survey data, we tested these hypotheses by analysing population trends and
stopover duration of migratory waterbirds experiencing loss of staging
habitat. Our modelling exercise led to us posing the following hypotheses:
the loss of staging habitat generates plasticity in migration tactics,
with individuals remaining on the staging habitat for longer to obtain
food due to a reduction in per capita food availability. The subsequent
increasing population density on the staging habitat has knock on effects
on population dynamics in the breeding and overwintering stage. Our
empirical results were consistent with the modelling predictions. Our
results demonstrate how environmental change that impacts one
energetically costly life history stage in migratory birds can have
population dynamics impacts across the entire annual cycle via phenotypic
plasticity.