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THE WILSON JOURNAL OF ORNITHOLOGY . Vol 123. No. 3. September 2011 
TABLE 3. Candidate model set accounting for variation 
Thrush at Fort Simpson, NT (20 nests, 37 time periods). 
in provisioning rate during the night period by Swainson's 
Model 
LL 
K 
AlCr 
AAIC r 
Umodellx)' 
AIC. 
All parameters 
Day length 
Temperature 
Edge distance 
Canopy tree density 
Nestling age + nestling number 
-45.06 
-52.75 
-59.20 
-60.95 
-61.90 
-62.02 
8 
3 
3 
3 
3 
4 
111.27 
112.23 
125.13 
128.63 
130.53 
133.28 
0.00 
0.97 
13.86 
17.36 
19.26 
22.01 
1.00 
0.62 
0.00 
0.00 
0.00 
0.00 
0.62 
0.38 
0.00 
0.00 
0.00 
0.00 
1 Model likelihood given data r - exp( - 
i 4 AtC, ). 
(19.08 hrs) when civil twilight extends through 
the night (i.e., nautical twilight does not occur) 
suggesting some ambient light is required for 
nocturnal foraging. A similar threshold does not 
exist at Chinchaga where 3.67 hrs were darker 
than civil twilight at the solstice. No birds were 
recorded feeding during the night period at 
Chinchaga because all observations were darker 
than civil twilight. We did not find support for 
forest structure covariales (edge proximity and 
canopy density) affecting ambient light availabil¬ 
ity and nocturnal provisioning rate: non-forest 
songbirds may require even less light and may be 
able to extend their provisioning period further in 
open habitats 
Day length (hrs) 
.. nG ' 3 - . Provisi °ning rale by Swainson's Thrush durinc 
mp » g ‘nt ‘"W 
finding that night-provisioned nests are not more 
successful or more productive than nests not 
provisioned during the night period, it is unlikely 
that night provisioners are having greater difficulty 
meeting their brood's energy needs during the day 
given that days are longer. A more plausible 
explanation for compensation is that night provi¬ 
sioning parents are distributing an equal and 
sufficient amount of chick-rearing effort over more 
active hours, resulting in a lower hourly rate of 
provisioning but similar total provisioning over the 
entire day. It is not clear why parents do not take 
Night provisioners are compensating for a lower 
rate of energy delivery during the day compared to 
non-night provisioners rather than delivering 
additional ‘bonus' energy. This agrees with our 
