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THE WILSON JOURNAL OF ORNITHOLOGY • Vol 124. No. 4. December 2012 
1974). The software generates upper and lower P- 
values based on the number of observed niche 
overlaps greater or less than the mean niche overlap 
generated by random simulations. Model RA2 
relaxes niche breadth from observed niche overlap 
by assigning a random number for use, but retains 
the resource stales where use was zero. Model RA3 
retains the exact use in the original data, but 
reshuffles the distribution of zeros. We also 
analyzed foraging data using model RA4. which 
retains both the use and zero distributions as in the 
original data. Model RA4 reshuffles only the 
distribution of each use within cells. This model 
has the most stringent assumptions to satisfy and 
may cause Type II error (Gotelli and Entsminger 
2001). We reanalyzed evidence for niche overlap by 
combining all horizontal zones using EeoSitn to 
simulate niche overlap and eliminate arbitrary 
distinctions imposed by the zoning system. 
RESULTS 
We observed chickadees on 6 days between 24 
January and 13 March, totaling 60 hrs of field 
observation time. This amounted to 1,875 and 
1,074 seconds of timing foraging of Boreal and 
Black-capped chickadee zone use, respectively. 
Foraging zone data were recorded for 24 Boreal and 
37 Black-capped chickadees that were in 20 different 
flocks containing Black-capped Chickadees and 15 
flocks containing Boreal Chickadees. This resulted in 
72 (56.2%) Boreal and 56 (43.8%) Black-capped 
chickadee zone observations. Data were drawn from 
eight observations and 178 sec of individual chicka¬ 
dees within Boreal Chickadee flocks, 28 observations 
and 484 sec within Black-capped Chickadee flocks, 
and 92 observations mid 2.293 sec within mixed 
flocks containing both species. 
Microhabitat Use.— Boreal Chickadees foraged 
in only three conifer tree species with 76% of total 
foraging in black spruce. Black-capped Chicka¬ 
dees foraged widely across six conifer and three 
deciduous tree species. Boreal Chickadees spent 
36.5% of the total observation time in the top 
vertical zone (zone I) of trees when foraging, 
while Black-capped Chickadees spent only 4.5% 
°f the observation time in Ihis vertical zone 
(Table I ). There was no significant difference in 
foraging time between species in zone I when 
alpha values were Bonferroni-corrected (P ~ 
0 04 alpha = 0.0045). Neither Black-capped 
( 4 . 1 % occupancy) nor Boreal (1.6% occupancy) 
chickadees spent much time in the zones nearest 
the ground (zones 5-6) (Table I). Boreal and 
Black-capped chickadee foraging time was sim¬ 
ilar in zone 2 (P = 0.19) for 34.36 and 38.05% of 
total observation time, respectively, largely in the 
medial horizontal zone (2 medial). Foraging time 
was similar tor both species in horizontal zones 
with Boreal Chickadees spending less time in the 
basal zone. Differences in foraging time between 
chickadee species were significant only in the 
medial portion of zone I (P - 0.0002). 
Niche Breadth Analysis .—The observed niche 
overlap was larger than expected (simulated) in all 
three models assessed (Table 2). The observed 
mean niche overlap was significantly greater than 
expected by chance and observed niche breadth 
was greater than simulated niche breadth in all 
trials when foraging occupancy was reanalyzed 
with broader zone use. 
DISCUSSION 
Use of black spruce, white spruce, and tamarack 
(in decreasing order) suggests Boreal Chickadees 
may prefer the highest quality boreal habitats of the 
region (Evers 1991). This minimizes competition 
with Black-capped Chickadees which preferentially 
use the more common deciduous and mixed forests 
(Grubb and Bronson 2001. Foote et al. 2010). A 
more open branch structure and dispersed tree¬ 
spacing in deciduous forests appears to offer Boreal 
C hickadees less-desirable habitat. Few data exist on 
interspecific interactions between Boreal and Black- 
capped chickadees, but Black-capped Chickadees 
may be socially dominant to Boreal Chickadees in 
congeneric winter flocks and limit Boreal Chickadee 
foraging outside of high-density conifer regions. 
Minimal differences in foraging microhabitaifl of 
21 zones) arc shown in the medial region of dense 
foliage within the top () to 3 m of conifer crowns. The 
upper regions of spruce trees often contain the 
densest foliage, cone crop, and branch structure on 
(he entire tree, which Black-capped Chickadees may 
be less able to exploit (Ficken et al. 1996). This small 
spatial area within conifer crowns may be the region 
v\heie Boreal Chickadees have a competitive 
advantage. Boreal Chickadees in Alaska and New- 
loundland. where their habitat is far more common, 
apparently forage in a wider range of tree heights, 
suggesting ecological release (Haftom 1972. Vas¬ 
sal lo and Rice 1982). 
Flic null model analysis of Boreal and Black- 
eapped chickadee zone use indicates high niche 
ovci lap (0.676), which is greater than the overlap 
predicted to occur by chance alone. Pianka (1974) 
and Glasser and Price (1988) prov ide explanations 
