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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123. No. 2. June 2011 
sphere and are among the most studied bird species 
(Birkhead 1991. Trost 1999). Males and females 
typically work together to construct a large, domed 
structure made of sticks in a tree or shrub (Erpino 
1968. Buitron 1988, Trost 1999). This dome is 
anchored by a grass-lined mud cup. The dome of 
sticks over the nest cup presumably decreases 
predation risk of adults, young, and eggs, espe¬ 
cially by owls and larger corvids (Erpino 1968, 
Baeyens 1981). The dome usually has one, narrow, 
often difficult to identify entrance. 
Our observations were in an area of mixed farm¬ 
land —8 km southwest of the town of Olds, Alberta. 
Canada (51 45' N. 114‘ 14' W; 1,036 m asl). Mag¬ 
pies are common in this area and typically nest in 
willow (SalLx babbim) trees, which grow through¬ 
out the local area in drainages and other untillable 
locations. KRS frequently observed magpies in late 
April 2008 near a wooden nest box constructed for 
Wood Ducks (Aix sponsa). This box had been 
erected in spring 2006 but was unused in 2006 and 
2007. The box was mounted 4.1 m above ground 
on a live poplar (Populus spp.) tree, 25 m from 
open water at the edge of a slough. Magpies had 
built nests in several locations within 300 m of this 
tree in previous years although the nests were 
typically <3 m from the ground in willows rather 
than in other available poplar trees. The box was 
60 cm tall and 26.7 cm deep and wide (floor area = 
711 cm 2 ). A rectangular entrance hole 11.4 cm 
wide and 10 cm tall was centered 43.8 cm above 
the bottom of the box. The box contained a large 
undomed nest of twigs topped by a grass-lined mud 
cup when checked in September 2008. The cup was 
sufficiently high within the box that an incubating 
or brooding adult could see out the box entrance 
hole. The carcass of a fully feathered magpie 
nestling was in the cup. The cup was soiled with 
feces suggesting that at least several young had 
survived late into the nestling stage and probably 
fledged. All nesting material was removed from the 
box at this time. 
Magpies again used the box in 2009. It seems 
likely the same pair was involved although this was 
not confirmed as the adults were unmarked. The 
nest contained four, possibly five, hatchlings when 
first checked on 13 May. One young had Hedged to 
the tree near the box and at least three young were 
visible in the nest entrance on 10 June (http://www. 
youtube,com/watch?v=153T1 xtyqpM). All nest¬ 
ing material was again removed from the box on 
15 August. The box was not used bv any species in 
2010 . 
DISCUSSION 
This is the first report to our knowledge of a 
magpie using an enclosed, tree-like cavity with a 
relatively smull entrance hole. Holyoak (1967) 
noted the presence of a nest of a Eurasian Magpie 
(P. pica) in a “hole” in a cliff on the Calf of Man 
in southern England, but provided no details on 
the nature of this hole, i.e., whether it was a tme 
cavity or simply a crevice. 
Whether a novel trait increases in frequency 
within a population depends in part on how that 
trait affects individual fitness. Use of a cavity for 
nesting has potential advantages for magpies 
including greater shelter from precipitation, solar 
radiation, and wind. The nest would also be 
better concealed from predators and probably 
would be more resistant than a typical nest to 
entry by certain predators, e.g.. crows (Conus 
spp.) and common raccoons (Procxon lotor). 
However, cavity nesting would also have several 
potential costs. Given their size, magpies would 
require relatively large cavities with large 
entrance holes, which will be scarcer than other 
types of cavities. Magpies rarely reuse nests 
(typically <25% of the time; Trost 1999, sec also 
Antonov and Atanasova 2003), which exacer¬ 
bates this problem. Magpies potentially could use 
many of the cavities created by nesting Northern 
Flickers (Colaptes aura(us). Wiebe (2001) re¬ 
ported the mean ± SD diameter of entrance holes 
to flicker nests in western Canada is 6.42 ± 
0.91 cm (n = 143). Measurement of museum 
specimens suggests that most adult magpies 
would fit through a hole of this diameter. The 
mean ± SD maximum diameter of six anatom¬ 
ically complete adults preserved in alcohol was 
5.7 ± 0.5 cm (range: 5.1-6.3 cm) whereas that 
for 9 individuals prepared as stuffed study skins 
was 6.4 ± 0.3 cm (range: 5.9-6.7 cm). However, 
after entering a flicker cavity, magpies would be 
more confined than in a traditional nest. Mean : 
SD floor area of 139 flicker cavities examined by 
Wiebe (2001) was 166 ± 77 enr whereas data :n 
Silloway (1900 ) suggests the area of the mud nest 
cup in a magpie nest is typically about a third 
larger than the floor area of flicker nests. 
Magpies using flicker cavities would also be in 
direct competition for nest sites not only with 
flickers (which reuse nest cavities) but also other 
species that use flicker holes, most notably 
American Kestrels ( Falco span erius). a cavity - 
nesting raptor. Kestrels would likely dominate 
