Fitness consequences of anthropogenic subsidies for a partially migratory wading bird

Human activities are forcing wildlife to confront selective pressures
different from those under which they evolved. In seasonal environments,
migration evolved as an adaptation to fluctuating resource availability.
Anthropogenic subsidies modify resource dynamics by providing a steady
food source that is not subject to seasonality. Globally, many migratory
populations are becoming increasingly resident in response to food
supplementation. While these population-level shifts are assumed to arise
from changing fitness consequences of individual behavior in response to
resource dynamics, these mechanisms are often difficult to quantify and
disentangle. Here, we quantified fitness consequences of responses to
anthropogenic subsidies in partially migratory wood storks (Mycteria
americana) in the heavily urbanized southeastern U.S. First, we tested
whether individual migratory behavior is linked to different responses to
anthropogenic subsidies. Second, we quantified fitness consequences of
these behavioral responses. We found that, in our system, migration and
residency are alternative behavioral tactics associated with different
responses to food supplementation. In turn, the use of anthropogenic
resources alters a fitness component by enhancing nest survival. These
results provide a mechanistic examination of how animals may respond to
human-modified resource dynamics and how fitness consequences of
individual tactics may translate into behavioral shifts at the population
level.