Short distance migration: The role of stopovers and transitional ranges
Authors
Merrell, Jerrod Lee
Issue Date
2022
Type
Thesis
Language
Keywords
Brownian Bridge Movement Model , Migration , Mule Deer , Short-distance Migration , Stopover , Transitional range
Alternative Title
Abstract
Animal migration is seasonal movements between distinct ranges not used in other times of the year. Migration is a way to avoid harsh seasonal environments, obtain higher
quality forage, and mitigate predation risk. Associated with migration is the use of
stopovers. Stopovers are used by migratory species as locations along the migration route
used to rest and replenish somatic reserves. Stopovers are well documented for long
distance migrations with a focus on spring migrations, and how migrators track the
phenology of spring green-up. We analyzed data from both autumn and spring migrations
to see if stopover use was an important part of migratory behavior, and to identify any
differences in use of stopovers between autumn and spring migrations. We captured
female mule deer in summer range and fitted them with GPS iridium collars. Collars
collected location data at a rate of 2 GPS location per day while a deer was in seasonal
range. Once migration was initiated, collars collected 1 GPS location per hour. This
method gave us fine scale movement data for periods of migration. GPS data were
collected for autumn and spring migrations from autumn 2015 until spring of 2018. We
developed a frequency distribution of these stops and found that a 2-day stop during
migration was the most frequent duration of a stop. We determined that stopover use was
an important part of migration behavior by this population. We identified that stops
during this short distance migration ranged from 1 to 145 days. We used snow water
equivalent to determine the onset of winter. The first day with a snow water equivalent of
10 mm or more was assigned as the onset of winter. The frequency distribution had a
decline at stops with a duration of 10 days. Based on the frequency distribution of stops
by this population of mule deer, we conservatively labeled a stop with a duration of 1-10
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days a stopover. A stop with a duration of 10 days or longer we considered to be
transitional range. Stopovers and transitional ranges were used during both autumn and
spring migrations. We found that stopovers were used in similar rates in all autumn and
spring migrations with the exception of the spring migration of 2018 where use was
significantly higher. Use of transitional range was similar for autumn and spring
migrations from autumn 2015 to spring of 2016. There was significantly higher use of
transitional range for the autumn migration of 2017 and spring migration of 2018. Years
with onset of winter occurring later in the year positively influence the number of days
spent in transitional range. Conversely, less time was spent in transitional range when the
onset of winter occurred earlier in the year. Timing of snowfall did not affect the start of
migration which was consistent across time of the study. When looking at the proportion
of days stopped while on migration (combining stopovers and transitional range), was a
significantly higher proportion during autumn migrations when compared to spring. The
short distance migration used by this population of mule deer allows for flexibility in
migrational behavior. We observed variation among individuals in use of stopovers and
transitional ranges. We also found that the population varies its use of stopovers and
transitional ranges based on the onset of winter and the first snowfall of the winter
season. This information will elucidate individual variation in migratory behavior as well
as, at a population level, how migratory behavior can be influenced by weather events.