Effects of food and ectoparasites on dispersal age

1
Effects of food and ectoparasites on dispersal
age Victoria Garcia Courtney J. Conway, Arizona
Cooperative Fish and Wildlife Research Unit,
School of Renewable Natural Resources, The
University of Arizona, Tucson, AZ 85721
Results
Figure 2 Juveniles that received food
supplements dispersed about 12 days earlier than
juveniles in the control group (P0.027).
Interactions were not significant.
What factors affect the timing of natal dispersal
in Burrowing owls (Athene cunicularia)?
Figure1 Fate of transmittered juvenile owls. Of
the 67 juveniles that were transmittered, 35
dispersed, 17 died before dispersal, we lost the
signal on 13 so that their fate was unknown, and
2 were still at the natal burrow when our field
season ended in October. Juveniles that died
prior to dispersal or that had unknown fates
were distributed evenly among the 4 treatment
groups.
Introduction Dispersal is an important
component of many ecological and evolutionary
processes (1). For example, dispersal affects
population dynamics via the loss of individuals
from the population, and affects population
genetics via gene flow. Dispersal also affects,
and is affected by, the evolution of social
behaviors such as cooperative breeding (2). The
decision of when to initiate natal dispersal may
influence subsequent survival probability and may
impact an individuals ability to find a suitable
breeding territory the following year (3, 4).
Therefore, optimal timing of natal dispersal
influences fitness, and optimal timing may vary
across time, across regions, and among species
because the relative benefits of early and late
dispersal are context-dependent (5-9). The main
purpose of this study is to determine what
factors affect the timing of natal dispersal in
Burrowing owls (Athene cunicularia).
Accordingly, this study has two objectives 1)
To determine the effect of food supplementation
on the timing of natal dispersal in Burrowing
owls, and 2) To determine the effects of
removing ectoparasites from the nest burrow on
the timing of natal dispersal in Burrowing
owls. The Burrowing owl is an endangered
species in Canada and is listed as threatened in
Mexico (10). Burrowing owls are also listed as
endangered in Minnesota and Iowa and are listed
as a Species of Special Concern in the U.S.
Figure 3 Juveniles that received diatomaceous
earth dust dispersed about 11 days later than
juveniles in the control group (P0.065).
Interactions were not significant.
Figure 4 Juveniles at nests supplemented with
food had a higher index of flea infestation than
juveniles at control nests (P0.003).
Methods The site covers 6500-km2 of sagebrush
steppe and agriculture in the Columbia River
Basin of eastern Washington (Grant and Adams
Counties). Dispersal age We determined age of
initiation of natal dispersal in burrowing owls
by placing a 4.6g radio transmitter on one
juvenile in each of 67 broods. We relocated
transmittered juveniles every 2 days. We assumed
a juvenile had initiated dispersal when it
roosted gt300m away from its nest on gt2
consecutive days (11, 12). Food abundance We
randomly assigned 33 nests to a treatment
(food-supplemented) group and 34 nests to a
control (non-supplemented) group. We
supplemented nests in the treatment group with
food (laboratory mice). We then compared the age
at which transmittered juveniles dispersed from
treatment and control nests using a two-way ANOVA
for a completely-randomized design with factorial
treatment. Ectoparasites We randomly assigned
34 nests to a treatment (dusted for fleas) group
and 33 nests to a control (non-dusted) group. We
treated nests in the dust group using
diatomaceous earth powder. We compared dispersal
age of transmittered juveniles in treatment and
control nests using a two-way ANOVA for a
completely-randomized design with factorial
treatment. To determine whether dusting
reduced flea infestation, we quantified the
density of fleas on juveniles using an index of
flea infestation (0-5). We averaged the flea
index per nest, excluding any juveniles captured
before treatments were initiated or after
fledging age (44 days). We compared flea
infestation at treatment and control nests using
a t-test.
Figure 5 Index of flea infestation did not
differ between juveniles at nests that were
treated with diatomaceous earth and juveniles at
control nests (P0.88).
Figure 6 Dispersal age was not associated with
flea index.

• Discussion and conclusions
• Increased food caused juveniles to disperse
  younger, whereas diatomaceous earth treatments
  caused juveniles to disperse when older.
• However, diatomaceous earth had no effect on flea
  load, but food supplements increased flea load.
• Flea load had no effect on dispersal age. 
• In the future, we will focus on understanding how
  diatomaceous earth affects ectoparasites other
  than fleas, and how ectoparasites affect
  dispersal age.

Acknowledgments We would like to thank the
following for their support and assistance in the
field BLM (Spokane), Columbia NWR, Washington
Dept of Fish and Wildlife, Lisa Ellis, Joey
Jarrell, Rick Keck, Aimee Mitchell, Chris Nadeau,
and Emily Sullivan.  References 1. Gandon, S.,
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