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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124. No. 3. September 2012 
requires an animal to ascertain its position along a 
gradient by comparing the local value of a map 
component to a 'home' or familiar value, such as 
a breeding site. Bi-coordinate positioning (i.e.. 
distinguishing both latitude and longitude) re¬ 
quires perception of two non-parallel gradients 
(e.g., Wiltschko and Wiltschko 1995. Phillips 
1996). Geomagnetic intensity and inclination vary 
mainly along a north-south axis in many geo¬ 
graphic regions, and geomagnetic information 
alone may provide birds w ith only a unicoordinate 
map limited to latitudinal information (Mouritsen 
2003). Moreover, migratory birds would likely 
have to learn the pattern of magnetic gradients 
within their 'home' or migratory range to 
accommodate regional and temporal variation in 
the geomagnetic field (Phillips and Deutschlander 
1997, Frcake et al. 2006). Thus, consistent with 
geographic displacement experiments (e.g., Per- 
deck 1958). juvenile migrants on their first 
migration would not be expected to use a 
magnetic map. 
Direct tests of the involvement of geomagnetic 
cues in map-based navigation are few and. in most 
cases, involve homing behavior in non-avian 
species (reviewed in Freake et al. 2006). Geo¬ 
graphic displacements need to be simulated by 
altering only the geomagnetic field to test for 
the role of geomagnetic cues in positioning and 
orientation. Only tw'o studies (Fischer et al. 2003, 
Henshaw et al. 2010) have directly examined the 
effect(s) of geomagnetic displacements on migra¬ 
tory passerines, specifically Australian Silvereyes 
(Zosterops 1. lateralis) and Lesser Whitethroats 
(Sylvia curruca ). Adults that had made at least 
one migration prior to the experiments in both 
studies were exposed to magnetically-simulated 
geographic displacements while en route to a goal 
(i.e., wintering areas for Australian Silvereyes and 
breeding areas for Lesser Whitethroats). Both 
species exhibited remarkably similar orientation 
responses. Birds exposed to a 'magnetic displace¬ 
ment southward towards the origin of their 
migratory flight continued to orient towards the 
north, their seasonally appropriate direction. Birds 
exposed to geomagnetic values found beyond 
their goal became relatively disoriented. These 
results are consistent with migratory birds using 
spatial variation in the geomagnetic field to perceive 
position, but the lack of redirected orientation 
owards their goal leaves many unanswered ques- 
tmns about the function of the magnetic field in 
geographic-positioning. Moreover, juveniles were 
not tested in either experiment to be sure their 
magnetic compass sense was unaffected by the 
experimental magnetic fields and to ensure that 
other 'innate' navigation processes (such as 'sign 
post* navigation) were unaffected. 
We exposed both juvenile and adult Australian 
Silvereyes to magnetic field values simulating 
different geographic positions during autumn 
migration. A simulated northern displacement 
(SimN) w'as produced by decreasing both mag¬ 
netic inclination and total intensity to values 
found to the north of the normal wintering range 
for the Tasmanian population of Silvereyes. A 
simulated southern displacement (SimS). pro¬ 
duced by increasing magnetic inclination and 
total intensity, provided magnetic field values 
near, or to the south, of the origin of their autumn 
migratory journey in Tasmania. Our goal was to 
learn if the effect of the experimental magnetic 
displacements is both age- and displacement- 
specific. We used larger changes in the magnetic 
field than used previously (Fischer et al. 2003) to 
ensure locations that were specified were well 
outside the range of this subspecies. Juveniles 
should be unaffected by the simulated displace¬ 
ment treatments and should continue to orient 
north-northeast (NNE). their typical autumn 
direction, if geomagnetic values do not affect 
their magnetic compass or serve as innate 'sign 
posts’. The orientation of adults, unlike juveniles, 
should depend on the simulated displacement. 
Adults in the SimS group should perceive their 
location to be south of their breeding area and 
should continue to orient in the seasonally 
appropriate migratory direction to the NNE. SimN 
adults should perceive their geographic position 
as being to the north of their normal winter range 
and should exhibit a change in behavior that 
consists of reorientation towards specific over¬ 
wintering sites, dispersal to search for new over¬ 
wintering sites, or cessation of migratory behavior 
consistent with termination of migration. 
METHODS 
Forty Australian Silvereyes were captured in 
breeding areas prior to autumn migration near 
Hobart. Tasmania (42 54' S. 147 18' E). Silvereyes 
of this subspecies migrate in autumn, mainly during 
dawn and dusk (Funnell and Munro 2007) to 
wintering sites on the Australian continent ranging 
from southeastern South Australia to southern 
Queensland (Lane and Battam 1971. Griffioen 
and Clarke 2002); the majority of the population 
