Where to spend the winter? The role of intraspecific competition and climate in determining the selection of wintering areas by migratory caribou

Depicted as predictable movements, migrations can, however, show important
interannual variations, making the conservation of migratory species
particularly challenging. Plasticity in migratory behaviour allows
individuals to adjust their migratory tactics to maximize their fitness.
Destination of migration, and therefore migration patterns, may vary
according to climatic and environmental conditions encountered during
migration or at the arrival site but also according to competition. In
Northern-Québec and Labrador, Canada, fall migration patterns of caribou
from the Rivière-George (RGH) and the Rivière-aux-Feuilles (RFH) herds
have varied greatly during the last decades. Meanwhile, both herds have
shown large fluctuations in abundance. We assessed the influence of
environmental factors and changes in population size on wintering area
selection. Based on 649 fall migrations of 284 females equipped with ARGOS
collars, we used a machine-learning algorithm, the random forests, to
assess how climate, resources and population size affected the selection
of four different wintering areas. Individuals followed over several years
switched to a different wintering area 45% of the time between
consecutive years, and this probability increased at high population size.
The main determinant of wintering area selection was the population size
for both herds, suggesting intra- and inter-herd competition for wintering
areas. The long migrations of RGH toward the western wintering areas, also
used by RFH, were favoured when the herd was abundant and when the
availability of resources was low at the departure. The migrations of RFH
toward the south-western area increased as RGH declined, possibly because
the past presence of RGH in this area reduced access for caribou from RFH.
These results highlight the flexibility in the migratory behaviour of
caribou in response to variation in competition. Our study is the first to
suggest that wintering area selection can be determined by competition
between populations of the same ungulate species.