Deutschlander el al. • AGE-DEPENDENT GEOMAGNETIC POSITIONING 
469 
TABLE I. Values (nT = nanoTesIa) for total magnetic 
intensity and magnetic inclination during holding and 
iesting of Australian Silvcreyes. The birds in the SmiN 
and SimS groups were held in different rooms and the 
ambient values of the geomagnetic field vary slightly. The 
ambient field values of the testing room also varied slightly 
from each holding room. 
SimN group 
SiinS group 
Control (ambient)—holding 
54.000 nT 
55.5(H) nT 
field 
60.25 
60.8(1 
Control (ambient)—testing 
52,500 nT 
52.5(H) nT 
field 
59.25 
59.25 
Average control field value 
53,250 nT 
54.000 ill 
59.75 J 
60 
Experimental—holding field 
42.9(H) nT 
68.800 nT 
51.4(1 
67.50 
Experimental—-testing field 
43,150 nT 
69.135 nT 
50.50 
67.00 
Average experimental field 
43.025 nT 
68.968 nT 
value 
50.95 
67.25 
moves northward along the southeast coast, although 
some birds move westward overwintering in south¬ 
eastern South Australia (Funnel! 2007). Eighteen 
birds were juveniles (or hatch-year) based on skull 
pneumatization, which had not migrated, and 22 
were adults (or alter hatch-year), which presumably 
had previously migrated to and from wintering areas. 
The birds were transported by airplane to 
Sydney, New' South Wales (NSW) and then to 
Armidale, NSW (30 30' S. 151 40' E), where all 
experiments occurred from April through early 
June. Silvcreyes were randomly divided into two 
groups, each containing similar numbers of adults 
and juveniles, and held in two adjacent rooms al 
ihe University of New England’s animal care 
facility. Birds were housed in pairs in non¬ 
magnetic. stainless steel cages (700 X 360 X 
360 mm: Mascot Wire Works. NSW Australia) 
placed in the center of each room within a 
Merritt’s cube-surface coil for magnetic field 
manipulations (Kirschvink 1992). Solid food (a 
high-protein diet consisting of hard-boiled egg, 
buttermilk curd. Madeira cake, fish food, meal¬ 
worms, and apple), fresh water, and artificial 
nectar (38.5% w/v honey with a vitamin supple¬ 
ment) were provided ad libitum. The photoperiod 
in the holding rooms was changed weekly to 
match the natural photoperiod in Armidale. NSW. 
All Silvcreyes were released into the wild 
following our experiments. 
Orientation Tests .—These tests were conducted 
in a separate building, — 100 m from the animal 
care facility. Tests began 30 min before sunset 
and lasted for 90 min, Birds were transported to 
and from the test room in a light-tight box. Each 
evening, 10 Silvcreyes were tested individually in 
funnel-shaped, aluminum cages (Emlen and 
Hmlen 1966) covered with translucent ‘white’ 
Perspex tops. A single incandescent bulb centered 
above the test room provided diffuse illumination. 
Each funnel was lined with typewriter correc¬ 
tion paper (formerly I ipp-Ex. B1C Deutschland 
GmbH. Eschbom, Germany) to record bird 
activity. Birds hop on the Tipp-Ex paper within 
Emlen funnels leaving scratch marks as a record 
of where and how often they hop on the paper 
(Emlen and Emlen 1966). The number of 
scratches on the Tipp-Ex paper was recorded for 
each of 24 15 -sectors, starting at 0-15 . 16-30°. 
to 346-360 . The total number of scratches on the 
Tipp-Ex paper was used as an estimate of nightly 
activity, or migratory restlessness. A bird that left 
<35 scratches was classified as inactive and no 
orientation was recorded for that night. Nightly 
orientation (mean angular direction, a; and 
vector length, r) was calculated from the distri¬ 
bution of scratches in the 24 sectors. Vector 
length provided an indication of the nightly 
concentration of orientation between 0 and 1; a 
larger r value indicated greater clustering in the 
distribution of scratches. 
Control Tests and Experimental Simulations .— 
The orientation of each bird was tested on five 
nights prior to exposure to the experimental 
magnetic field in the ambient magnetic field of 
Armidale (‘control period’; Table 1). Median 
activity, median concentration, and mean vector 
were recorded for each individual for the control 
period from five nightly bearings (Table 2). The 
design of our experiments required active, goal- 
oriented birds w'ilh clear Orientation towards their 
wintering areas; thus, individuals that did not 
exhibit seasonally appropriate orientation, defined 
a priori as a mean vector between 240 clockwise 
to 120 . during control tests were eliminated from 
further experimentation. Tasmanian-breeding Sil- 
vereyes are partial migrants; some individuals 
may not migrate and would not show northward 
autumn movements. Four birds with mean vectors 
during the control period of 223 and 159° (2 
adults) and 158 and 148 (2 juveniles) were 
omitted from experimental tests. 
Birds were exposed to an altered magnetic Held 
using a vertically aligned cube-surface coil 
(Kirschvink 1992) around their holding cages 
