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THE WILSON JOURNAL OF ORNITHOLOGY • Vol 123. No. 4. December 2011 
TABLE I. Factor loadings on first three principal component axes and interpretations for weather variables 
corresponding to the day that migrating birds were discovered absent. Variables with high positive or negative factor 
loadings are in bold. 
Variables 
48 hrs before 
Temperature change 
Wind speed change 
Air pressure change 
24 hrs before 
Temperature change 
Wind speed change 
Air pressure change 
24 hrs after 
Temperature change 
Wind speed change 
Air pressure change 
48 hrs after 
Temperature change 
Wind speed change 
Air pressure change 
Proportion variance 
Cumulative variance 
Overall 
PC A 1 
0.96 
0.97 
-0.77 
0.77 
0.98 
-0.85 
0.18 
0.15 
0.61 
0.65 
0.63 
0.39 
0.51 
0.51 
Wanner, windier, and lower 
pressure before 
PCA II 
0.17 
-0.07 
-0.40 
-0.31 
- 0.11 
-0.25 
0.90 
0.89 
-0.37 
-0.53 
-0.30 
0.48 
0.22 
0.74 
Warmer and windier 
soon after 
PCAra 
-0.04 
0.05 
0.35 
-0.15 
0.07 
0.02 
-0.08 
-0.08 
0.66 
0.12 
- 0.66 
0.75 
0.13 
0.87 
Pressure increasing and winds 
calming well after 
lower pressure conditions during the days preced¬ 
ing departure. The second principal coinponem 
corresponded to increased temperature and wind 
24 hrs after departure. The third PCA axis was 
characterized by increasing pressure and calmino 
winds 48 hrs after departure. 
DISCUSSION 
Fall range sizes during or prior to migration of 
radio-marked birds in our study were substantially 
larger than territory sizes of breedinc Sage 
Sparrows (2 ha: Wiens et al. 1985. Misenhelter 
and Rotenberry 2000. Vander Haegan et al. 2000) 
implying that resource use or availability or 
exploratory movements differ across seasons The 
increasing amount of area used by birds as the 
season progressed (Puehlo Valley [Sep| vs. Silver 
State Valley [Oct] and Carson Sink (late Oct and 
Novi) may reflect pre-migratory movements or 
vanabd.ty m habitat quality among sites or within 
seasons. We were unable to follow any Carson 
Sink birds beyond 15 days due to study logistics- 
arge distances moved by non-migratory birds at 
th s sue may represent pre-migratory movements 
for departures after our study concluded. 
3fi e Sparrows did not exhibit gradual move¬ 
ments south as we had anticipated. Rather, birds 
departed and were not subsequently located by 
conventional telemetry techniques. We made a 
key assumption that birds not relocated within 
35 km had truly departed. Given that one bird 
made a daily movement >15 km. other birds also 
may have made similar movements that went 
undetected, although we continued to listen for 
signals from these birds at the study site and 
surrounding region. We were unable to distin¬ 
guish radio-marked birds as residents that had 
bred or were reared in the immediate vicinity, or 
as migrants from farther north using sites as 
stopover habitats. 
The PCA axes indicated passage of cold from 
weather systems during the day or evening prior 
to assumed departure day. The transition (mm a 
warm, windy, low pressure period to a cooler, 
calmer, higher pressure period triggered depar¬ 
tures of marked birds from our study sites. This 
pattern is well-described for fall migrants "i 
central and eastern North America ('Hass)er et "T 
1963, Able 1972. Allen et al. 1996. Calvert ei al. 
2009), but has not previously been reported tor 
shrubsteppe passerines of the intermouniam 
region. Most birds were not under observation 
