SHORT COMMUNICATIONS 
821 
avoid competition with closely related species. 
Similar conclusions were noted by Alutalo et al. 
1 1987) in Finland, based on elimination experiments 
in winter Hocks. 
Niche partitioning between Boreal and Black- 
capped chickadees has not been studied during 
winter, a period in which both species are in 
greatest contact. Descriptive (Stewart and Aldrich 
1952) or summer accounts (Vassallo and Rice 
1982) suggest Black-capped Chickadees more 
frequently occupy deciduous and less dense 
forests, usually at lower heights within trees, 
while Boreal Chickadees exploit regions of dense 
conifer foliage most commonly in spruce crowns. 
We investigated microhabitat partitioning in 
mixed-species chickadee Hocks within boreal 
forests at a McCormick Wilderness Area study 
site in western Marquette County. Michigan, USA 
in January-March 2011. We predicted the more 
abundant Black-capped Chickadee might restrict 
rarer Boreal Chickadees to confined regions ot 
microhabitats. 
METHODS 
Study Area— Five primary study locations near 
the McCormick Wilderness Area (centered at 4b 
38' 39.04" N. 88 02' 40.87" W) were identified as 
quality Boreal Chickadee habitat based on habitat 
descriptions from Evers (1991) and lliekman 
12011 ). and secondarily through analysis of aerial 
photographs. Black-capped Chickadees also occur 
in this urea and regularly use these habitats lor 
nesting and foraging. The habitat consisted ot 
mature boreal forest patches ranging from 1.7 knr 
in size to smaller, narrow belts of 0.35 knr. and 
isolated boreal islands that were only 0.1 km in 
size within a maple-yellow birch [Acer spp .-Helnla 
alleghaniensis) matrix. All boreal lores! patches 
were close to the Peshekec River or to Baraga 
Creek, often forming narrow bands of boreal 
habitat along streams. Tree species in the boreal 
patches in decreasing order of estimated domi¬ 
nance were: black spruce, white spruce (Picea 
Klauca), tamarack (Liirix laricina). balsam Mr 
{Abies huhamea), white birch {Be tala papyrifera). 
white pine (Piiuts strohus ), red maple (Acer 
mbrum ), quaking aspen (Populus tremuloides ), 
white cedar (Thuja occidentalis). and jack pine 
(Finns banksiuna). 
Data Collection. —Observers systematically 
searched the study sites for chickadee flocks on 
6 days between 24 January and 13 March 2011. 
Flock size, species composition, general flock 
location, and time of observation were recorded 
once a Hock with chickadees was located. 
Individual observers focused on one individual 
chickadee of either species for as long as possible 
(but <10 min) for each Hock. The lice species in 
which a bird was foraging and estimated height of 
tree were recorded. Observers visually divided 
each tree used for foraging into zones by estimating 
3-m vertical areas (c.g.. 21-m tree = 7 zones) that 
each contained three horizontal /ones (basal, 
medial, distal) per 3-m vertical zone. Observers 
recorded the number of seconds using stop-watches 
that a focal chickadee foraged in different zones of 
the tree. Shifts to new zone positions and trees were 
recorded as discrete observations. 1 iming stopped 
when the focal chickadee stopped foraging. The 
number of foraging observations within zones was 
recorded per individual chickadee followed to 
ascertain each bird's contribution to the data set. 
Zones used to segregate chickadee foraging to a 
specific location within trees were numbered from 
tree crowns to the tree base following MacArthur 
(1958). Trees were numbered from the top so those 
of varying height could be compared on a zone-by¬ 
zone basis while retaining as much similarity in 
vegetation structure. Chickadees loraging in tiees 
<10 m in height were not used in the data analysis. 
Statistical Analysis.— We analyzed differences 
between Boreal and Black-cuppcd chickadee 
foraging time in nine tree species where chicka¬ 
dees were observed foraging. Chickadee Imaging 
time in each tree species was scaled to the total 
number of seconds chickadees were observed 
foraging by species throughout the study, and the 
total number of individuals observed. The numbei 
of individual chickadees observed per species was 
estimated from detailed records ot I lock locations. 
Totals were based on the maximum number of 
each species recorded each day plus addition ol 
individuals recorded on subsequent days that were 
>8 km from previous observations. 
Differences in foraging zone occupancy between 
Boreal and Black-capped chickadees within trees 
in each of 21 zones were evaluated (unpaired t- 
tests) with Bonferroni correction of the alplta-v alue 
to account for repeated tests (Cabin and Mitchell 
20(X)). We calculated niche overlap between Boreal 
and Black-capped chickadees based on foraging 
zone data using EcoSim 7.0 (Gotelli and Entsminger 
2001 ) to evaluate the likelihood of niche overlap. 
Each null model calculated niche overlap with 
different assumptions about the specialization ol the 
species compared based on Pianka s index (Pianka 
