Territory acquisition mediates the influence of predators and climate on juvenile red squirrel survival

1) Juvenile survival to first breeding is a key life history stage for all
taxa. Survival through this period can be particularly challenging when it
can coincide with harsh environmental conditions such as a winter climate
or food scarcity, leading to highly variable cohort survival. However, the
small size and dispersive nature of juveniles generally makes studying
their survival more difficult. 2) In territorial species, a key life
history event is the acquisition of a territory. A territory is expected
to enhance survival, but how it does so is not often identified. We tested
how the timing of territory acquisition influenced the winter survival of
juvenile North American red squirrels Tamiasciurus hudsonicus, hereafter
red squirrels, and how the timing of this event mediated sources of
mortality. We hypothesized that securing a territory prior to when food
resources become available would reduce juvenile susceptibility to
predation and climatic factors over winter. 3) Using 27 years of data on
the survival of individually-marked juvenile red squirrels, we tested how
the timing of territory acquisition influenced survival, whether the
population density of red squirrel predators and mean temperature over
winter were related to individual survival probability, and if territory
ownership mediated these effects. 4) Juvenile red squirrel survival was
lower in years of high predator abundance and in colder winters. Autumn
territory owners were less susceptible to lynx Lynx canadensis, and
possibly mustelid Mustela and Martes spp., predation. Autumn territory
owners had lower survival in colder winters, but surprisingly non-owners
had higher survival in cold winters. 5) Our results show how the timing of
a life history event like territory acquisition can directly affect
survival and also mediate the effects of biotic and abiotic factors later
in life. This engenders a better understanding of the fitness consequences
of the timing of key life history events.