Deutschlander et al • AC.E-DEPENDENT GEOMAGNETIC POSITIONING 
475 
compass and are presumed to have an innate 
knowledge of the species-specific migratory direc¬ 
tion. The SimN (and SimS) magnetic fields used did 
not affect orientation of juveniles; this finding rules 
out an effect of the experimental magnetic field on 
the Silvereve's magnetic compass sense or on some 
pre-programmed, innate navigational mechanism. 
The change in orientation of SimN, but not SimS, 
adults is consistent with evidence for a geographic 
position sense perhaps mediated by iron-mineral 
deposits associated with the trigeminal nerve 
(Semm and Beason 1990, Beason and Semin 
1996, Munro et al. 1997, Wiltschko et al. 2006. 
Falkenberg ct al. 2010), 
Fischer et al. (2003), using smaller changes in 
magnetic intensity and inclination, found adult 
Silvereyes exposed to a SimN magnetic field 
failed to show consistent orientation. Reanalysis 
of these same data revealed an initial easterly 
orientation in adults, similar to some of our SimN 
adults, followed by a shift to a more southerly, 
albeit statistically insignificant, direction (Freake 
et al. 2006). Stringent tests of a magnetic map 
hypothesis, or true navigation, require exposing 
animals to geomagnetic values outside their 
normal migratory range to examine if they are 
able to orient towards their goal without using 
route-based cues or familiar landmarks (Yeagley 
1947, 1951; Gould 1980; Moore 1980; Walcott 
1980; Wall raff 1991; Phillips 1996). The mag¬ 
netic field values used by Fischer et al. (2003), at 
least for inclination, were arguably within the 
normal wintering range for Zo.sierops I. lateralis. 
We applied a change in the magnetic field 
approximately double in magnitude (—24% 
change in vertical intensity rather than 12%) used 
by Fischer et al. (2003). The values of inclination 
and intensity for our SimN magnetic field were 
clearly beyond the northern-most latitudes to 
which Silvereyes migrate (Fig. I, and Lane and 
Battam 1971 >. However. SimN adults in our study 
did not show' a reversal from their normal NNE 
orientation as might be expected to compensate 
for 'over shooting’ their normal overwintering 
sites. 
The lack of a goal-oriented response in SimN 
adult Silvereyes is puzzling, hut may be due to an 
inadequate understanding of how birds use 
geographic variation in the magnetic field for 
positioning. There are several possible explana¬ 
tions for these findings. (1) Adults possess a 
magnetic map, but are confused by disparate 
locations specified by inclination and intensity 
(Fig. I). This explanation seems unlikely because 
SimS birds were also exposed to values of 
inclination and intensity that did not specify a 
single geographic location or latitude. (2) Adults 
possess a magnetic map, but their ‘goal’ (i.e., 
wintering site) is not sufficiently specific to result 
in redirection of orientation. The magnetic field 
characteristics experienced by the SimN birds 
represent values beyond the northernmost latitude 
for wintering Australian Silvereyes, particularly 
for those that breed in Tasmania (Lane and 
Battam 1971). but the birds should only reorient 
southwards if they exhibit high fidelity for a 
particular winter territory or range of latitudes. 
Unfortunately there are no published data on 
wintering site fidelity for Australian Silvereyes. 
(3) Both inclination and intensity vary along the 
same NNF.-SSW axis in eastern Australia and 
adults may use magnetic intensity and/or inclina¬ 
tion to derive only one geomagnetic coordinate 
for latitude. Displaced birds may need celestial 
information to identify their longitude from 
magnetic declination (Akesson et al. 2005, 
Thorup and Rabol 2007) or they may search 
longitudinally for familiar geographical land¬ 
marks to locate their east-west position (Mour- 
ilscn 2003). (4) Adults may Use geomagnetic 
spatial variation as a 'sign post’ rather than a map. 
Northern values of magnetic inclination or 
intensity for Silvereyes may be a navigational 
marker to cease oriented migration or to disperse 
randomly or longitudinally. Only adult Silvereyes 
responded to the SimN field, and use of ‘sign 
post’ geomagnetic cues is clearly age- or experi¬ 
ence-dependent if this explanation is correct. (5) 
Finally it is possible that birds exposed to abrupt 
magnetic field changes, which result in novel 
magnetic position ‘signatures’, may disperse in an 
attempt to perceive the gradient in the magnetic 
field; gradual magnetic field changes may 
allow birds to better compensate (Henshaw et al. 
2010 ). 
Our results are consistent with an age- or 
experience-dependent, geomagnetic geographic 
position sense. The reorientation of SimN adults, 
perpendicular to their seasonally appropriate 
migratory direction, does not allow us to distin¬ 
guish between map-based navigation or ‘sign 
post' navigation by adults. SitnN adults did not 
reorient towards the south, and we cannot 
conclusively state whether they have a geomag¬ 
netic-based 'map' for true goal-oriented naviga¬ 
tion. The birds' behavioral response indicates 
