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THE WILSON JOURNAL OF ORNITHOLOGY • VoL 123, No. 3. September 2011 
FIG. 3. First nest-box departures of female Eastern Screech-Owls 
Falmouth, Massachusetts. 
compared to local sunset. 1999-2010, 
when with very few exceptions, only the female 
resided in the box (Fig. 3). There was no 
significant difference across all years between 
departure delays of males and females, relative to 
sunset (Table I). The same is true for individual 
years outside the nestling period. For example, 
mean (± SD) male departure delay in 2010 was 
21.72 ± 10.02 min, and mean female delay was 
21.73 ± 5.72 min (Table 1). However, when both 
birds were in the box. mainly during the early 
stages of the nestling period, the male was the first 
to leave (by 22.9 ± 16.7 min, n = 13). Males 
spent the days hidden outside the box during most 
of the nestling period, but within sight of it. The 
female's departures prior to the nestling period on 
clear nights in 2010 were significantly (P = 0.05) 
later than on cloudy nights, whereas there was no 
significant difference for the male (Table 1). 
Females had a consistent trend towards earlier 
and earlier departures as the nestling season 
progressed, averaging as much as 2.6 min/day 
over a 25-day period (Fig. 4, all P < 0.01). 
DISCUSSION 
There was a tight correlation (r = 0.98) 
between times of sunset and first departure from 
the nest box throughout the amplitude of the 
annual daylight cycle, suggesting the owls time 
Iheir departures quite consistently to a certain 
ambient light level. Significant deviations oc¬ 
curred exclusively during the nestling period. The 
distribution of the departure times relative to 
sunset was near normal (Fig. 2) with the left-hand 
tail (early departures) representing the latter part 
ol the nestling period, and the right-hand tail (late 
departures) mostly representing the early part of 
the nestling period, during which the female 
broods the nestlings. Late departures may also be 
due to an abundance of food in the box (F. R. 
Gehlbach. pers. comm.). I did not inspect the nest 
box while birds were in residence and judged the 
beginning of the nestling period by the first 
observation ol food deliveries by both adults, and 
its end by the departure of the last nestling. The 
duration of the nestling period was 31 days in 
TABLE I Nest-box departure delays (minutes i "1 
Easicrn Screech-Owls relative to local sunset. 1999-2010 
Falmouth, Massachusetts. 
Category 
Mean ± SD 
» 
Males (all years) 
20.56 ± 9.99 
262 
Females (all years) 
21.65 ± 14.46 
220 
Male (2010. all nights)” 
21.72 ± 10.02 
60 
Female (2010. all nights)“ 
21.73 ± 5.72 
18 
Female (2010, clear) 3 
25.40 - 3.96 
9 
Female 12010. overcast)” 
18.07 ± 4.87 
9 
Male (2010, clear)” 
25.48 ± 8.56 
25 
Male (2010, overcast)" 
17.64 ± 9.01 
35 
Pre-nestling period. 
