Apparent survival of tree voles
We estimated annual apparent-survival for tree voles using a
Cormack-Jolly-Seber model implemented in R package RMark (Laake &
Rexstad, 2008; White & Burnham, 1999). During annual nest platform
checks we attempted to capture any tree voles that were present and
marked tree voles by clipping the tips of their dorsal fur, making a
high contrast mark readily identifiable using remote camera photographs.
We then resighted those individuals and analyzed 1-day bin-width
encounter histories of marked tree voles that began on the date of
capture. Capture and handling methods were approved by the U.S. Forest
Service Institutional Animal Care and Use permit #2016-009 and Oregon
Department of Fish and Wildlife Scientific Taking Permit 041-18.
Because tree voles molt, growing their fur at unknown intervals and we
had only one capture occasion per year, we right-censored encounter
histories to a survey period such that we minimized the uncertainty that
animals were likely to leave the sample due to their mark fading while
also retaining most of the data. To estimate the longevity of marks we
reviewed sequential photographs of marked female tree voles (n =
40) for which we observed their unique marking fading to became
indistinguishable from an unmarked vole. We estimated that 20% of marks
became indistinguishable at 63-days or fewer although some lasted longer
as we estimated marks faded at a median of 88 days and a mean of 83.4 ±
28.7 days (n = 20). We used 63-days to estimate apparent survival
to minimize underestimates of survival due to mark-loss. To assess
sensitivity of our survival models to encounter history length, we
present survival estimates using encounter histories of 35-days to
84-days.
As tree voles were only marked in the summer months (June – August),
our inferences were limited to June – October. To provide an estimate
of annual survival comparable to previous studies (Swingle et al.,
2010), however, we assumed that if survival and predation risk was
constant year-round, extrapolating our 1-day survival rate to a 1-year
period (365 days) would provide a valid comparison. To assess the
assumption that predation was consistent year-round in our study, we
summarized our qualitative assessment of weasel predation for each month
(Fig. S1a). We pooled data across years but provided separate estimates
for males and females. To estimate the variance of average annual
survival, we temporally rescaled our data using the delta method
(Powell, 2007).
Apparent survival can underestimate actual survival compared to
known-fate estimates as it is impossible to distinguish emigration from
mortality within the model. We presented data on observations of known
fates of marked tree voles, including observations of mortality due to
predators and emigration when a marked tree vole was observed at a
different nest platform than the capture location. In 2017 we applied
passive integrated transmitters to tree voles and recaptured tree voles
in 2018 to provide further information on apparent survival based on the
rate of recapture of these transmitters in live tree voles.