2009 NORTHEASTERN NATURALIST 16(4):491–500
Canada Warbler Habitat Use of Northern Hardwoods in
Vermont
Jameson F. Chace1,2,4,*, Steven D. Faccio3,5, and Abraham Chacko1
Abstract - We examined habitat use by Wilsonia canadensis (Canada Warbler),
a migratory songbird population in a 40-year decline. We used a long-term forest
bird monitoring program in Vermont to compare the structural components of sites
of warbler presence and absence. Habitats occupied included lowland Picea-Abies
(spruce-fir), northern hardwood, and Quercus-Carya (oak-hickory) forests, and Acer
rubrum-Thuja (Red Maple-cedar) and cedar-fir swamps. Northern hardwood forest
detections were explored in greater detail due to the greater extent of coverage in
Vermont, the higher number of survey points (n = 80), and high percentage of Canada
Warbler detections at those points (29%). Within the northern hardwood forests,
warblers occurred in patches with a lower canopy height and higher percent ground
cover of shrubs and ferns than patches where warblers were not detected. These three
parameters were also the strongest set of competing Akaike’s information criterion
model scores based on the patch attributes. In the northern hardwoods of the northeast,
the conditions of reduced average canopy height and increased ground cover
are created naturally by wind throw, ice storms, and insect damage, as well as under
some forms of timber management. Canada Warblers appear to prefer these forest
structural conditions because they provide abundant foraging strata, conceal nesting
sites, and expose song perches.
Introduction
Wilsonia canadensis L. (Canada Warbler) is a Nearctic-Neotropical migratory
bird that winters in South America and breeds in the northeastern
United States, southeastern Canada, and through the transitional hardwoodboreal
forests extending west into Alberta. This species inhabits a variety
of upland and lowland habitats in the Atlantic Northern Forests of New
England (Conway 1999, Dettmers 2003). Patchily distributed across the
breeding range, the Canada Warbler has highest densities in swamps where
excessive wet conditions and poor soils limit canopy closure and favor shrub
growth and in areas having forest floors with pronounced relief. In upland
forests, Canada Warblers appear to be disturbance specialists, moving into
regenerating forest patches following wind-throw (Hagan and Grove 1999),
ice damage (Faccio 2003), or timber removal (King and DeGraaf 2000).
The Canada Warbler is one among several migratory birds breeding in
North America that have exhibited long-term population declines, raising
1Department of Biology, Villanova University, Villanova, PA 19085. 2Center for
Northern Forest Research, PO Box 192, Island Pond, VT 05846. 3Conservation Biology
Department, Vermont Institute of Natural Science, Woodstock Road, Quechee,
VT, 05059. 4Current address - Department of Biology and Biomedical Science, Salve
Regina University, 100 Ochre Point Avenue, Newport, RI 02840. 5Current address -
Vermont Center for Ecostudies, PO Box 420, Norwich, VT 05055. *Corresponding
author - jameson.chace@salve.edu.
492 Northeastern Naturalist Vol. 16, No. 4
concerns about habitat loss and degradation in breeding and nonbreeding
areas (Martin and Finch 1995, Robbins et al. 1989). Results from the North
American Breeding Bird Survey indicate nearly four decades of Canada
Warbler decline throughout the northeastern portion of its range. Estimates
of annual population change since 1980 range between -3.8% and -7.3%
(Sauer et al. 2005). Reasons for the declines are unknown; however, loss of
breeding habitat may be a contributing factor (Conway 1999). The Northeast
Endangered Species and Wildlife Diversity Technical Committee, Partners
in Flight, and the North American Bird Conservation Initiative (NABCI)
have identified the Canada Warbler as one of the region’s highest priorities
for conservation and research (Dettmers 2003, Rich et al. 2004, Therres
1999). Our objective was to measure patch occurrence by Canada Warblers
across a diversity of forested habitat types within large contiguous blocks
of mature forest in Vermont and determine specific structural requirements
within the upland northern hardwood forests.
Methods
Study area
We surveyed for all bird species at 27 study sites distributed throughout
large contiguous patches of forest in Vermont. Nineteen sites (70%) were
in two of the state’s biophysical regions (Northern Green Mountains and
Champlain Valley), and from 1 to 3 sites were located in 5 of the 6 other
biophysical regions of the state (Fig. 1). Each site was in mature, contiguous,
interior forest, and consisted of at least 4 point-count stations (124 total point
counts) spaced at least 200 m apart. Elevations ranged from 310 to 810 m.
Sites were located in forest stands representing eight cover types:
northern hardwood (n = 18), lowland Picea-Abies (spruce-fir; n = 2), Quercus-
Carya (oak-hickory; n = 2), Thuja-Abies (cedar-fir) swamp (n = 1), dry
oak (n = 1), lakeside floodplain (n = 1), Tsuga-Pinus (hemlock-pine; n = 1),
and Acer rubrum L.-Thuja (Red Maple-cedar) swamp (n = 1). Northern
hardwood forests are the state’s dominant forest type, consisting primarily of
Fagus grandifolia Ehrh. (American Beech), Acer saccharum Marsh (Sugar
Maple), and Betula lutea Michaux (Yellow Birch), with lesser amounts of
B. papyrifera Marsh (Paper Birch) and Picea rubens Sargent (Red Spruce)
at higher elevations. Lowland spruce-fir forests are restricted to the colder
regions of the state, primarily the Northeast Highlands (Fig. 1), and were
dominated by Abies balsamea Miller (Balsam Fir ) and Red Spruce, with
lesser amounts of Pinus strobus L. (White Pine), Red Maple, Larix laricina
K. Koch (Tamarack), and Betula spp. (birches). Both lowland spruce-fir
sites in our study were forested wetlands. Oak-hickory forests, which occur
in the warmer valleys, are composed primarily of Quercus rubra L. (Red
Oak), Carya ovata Miller (Shagbark Hickory), and Ostrya virginiana Miller
(Hophornbeam). Cedar-fir swamps are dominated by Thuja occidentalis L.
(Northern White Cedar) and Balsam Fir, with lesser amounts of Red Spruce,
Tamarack, and Red Maple. Dry oak forests, found on rocky ridge tops, are
composed of Red Oak and Q. alba L. (White Oak), occasionally with lesser
2009 J.F. Chace, S.D. Faccio, and A. Chacko 493
amounts of Q. prinus L. (Chestnut Oak). Lakeside floodplain forests occur
primarily along the shores of Lake Champlain, and are dominated by A.
saccharinum L. (Silver Maple) and Fraxinus pennsylvanica Marsh (Green
Ash), with lesser amounts of Ulmus americana L. (American Elm) and Salix
spp. (willows). Hemlock-pine forests are dominated by Tsuga canadensis L.
(Eastern Hemlock) and White Pine, often with a mix of Red or Sugar Maple
and American Beech. Red Maple-cedar swamps occur primarily in the
Champlain Valley, and are dominated by Red Maple, Northern White Cedar,
and F. nigra Marsh (Black Ash).
Figure 1. Distribution of study sites (black dots) in Vermont by biophysical region.
494 Northeastern Naturalist Vol. 16, No. 4
Bird surveys
Point counts (10 min) were conducted twice annually during June as
part of the Vermont Forest Bird Monitoring Program (FBMP). The FBMP
uses point-count surveys at mature, interior forest sites throughout the state
to collect habitat-specific baseline data on species composition and relative
abundance (Faccio et al. 1998). Surveys began by 0600 hrs and concluded
by 1000 hrs (EDT), and were not conducted on days with rain or moderate
to high winds. Experts in visual and aural bird identification recorded all
individual birds within two distinct distance classes from the center point:
within and beyond the 50-m radius. Only Canada Warblers detected within
50 m were included in this analysis. Each point was scored for Canada Warbler
“presence” (≥1 warbler detected) or “absence” (no detections) on either
of the two survey days per year in either 2001 or 2002.
Vegetation measurement
Vegetation was measured at each point-count location in June and July
2001 or 2002 and generally followed the BBIRD protocol (Martin et al.
1997). Four 11.3-m radius plots were established at each point-count location,
one centered on the census point, and three located 35 m distant at
angles of 0, 120, and 240 degrees from the center point. Litter depth, ground
cover estimates, and shrub/sapling density were measured within 5 m of
the center of each vegetation plot; canopy metrics were obtained within the
11.3-m radius. Average organic-litter depth (cm) was calculated from 12
evenly spaced locations along two 10-m perpendicular lines oriented to the
cardinal directions and crossing at the plot center. Ground-cover metrics
consisted of visual estimates of the percent cover between 0 and 50 cm in
height, and within 5 m of the plot center, for the following 10 variables:
total green vegetation (Total); woody perennial plants (Shrub); ferns (Fern);
grasses and sedges (Grass); broad-leaf, non-woody plants (Forb); mosses
(Moss); ground covered with leaf litter (Leaf Litter); downed logs (>12 cm
dbh, >2.5 m long; Logs); open ground and rocks (Bare); and standing water
(Water). Each ground-cover type was measured independent of all other
categories, such that the sum would typically be greater than the “Total.”
All shrubs and saplings (>50 cm in height, <8 cm dbh) were identified to
species and counted within 5 m of the plot center. Shrubs and saplings were
classified by species (or “dead”) as either small (<2.5 cm diameter at 10-
cm height) or large (≥2.5 cm). Trees were classified by size class (Class 1:
8–23 cm dbh; Class 2: 23–38 cm dbh; Class 3: >38 cm dbh) for each species
(including dead standing snags) and counted within the 11.3-m radius plot.
Canopy height of an average tree in the plot was measured with a clinometer.
Percent canopy cover was estimated from the average of four readings of a
convex, spherical crown forest densiometer held at waist height, measured at
plot center oriented to the cardinal directions; canopy cover was partitioned
as the total canopy cover and upper canopy cover (>5 m from the ground).
Statistical analysis
All tests were nonparametric because data were not normally distributed,
even after log-transformation. All statistics were conducted using
2009 J.F. Chace, S.D. Faccio, and A. Chacko 495
SAS JMP 5.0 (SAS Institute Inc., Cary, NC). Descriptive statistics include
means and standard error unless otherwise noted; univariate statistics used
a Bonferroni-adjusted alpha. The Wilcoxon-Mann-Whitney two-sample
test was used to compare occupied and unoccupied point-count locations in
univariate analyses (Bonferroni-adjusted alpha). Using an information-theoretic
approach, we evaluated sets of competing models investigating Canada
Warbler detections in northern hardwood forests of Vermont by comparing
Akaike’s Information Criterion scores (AIC; Burnham and Anderson 2002).
The models were based on the most likely factors derived from univariate
tests (P < 0.05) after highly correlated variables (Pearson product-moment
correlation) were removed. Through an iterative process using ≤5 variables,
the most robust model was selected.
Results
Regional scale
Canada Warblers were detected within 50 m of 31 of the 124 census
points. The 31 points occurred at 16 of the 27 study sites (Table 1). Canada
Warblers were detected at study sites in all biophysical regions sampled
except Vermont Valley (one site); the Southern Vermont Piedmont was
not sampled. The majority of detections (71%) occurred at sites within the
Northern Green Mountains (Fig. 1), the biophysical region with the greatest
coverage of study sites and predominately covered by northern hardwood
forest. Overall, habitats occupied by Canada Warblers included northern
hardwood forest, lowland spruce-fir forest, oak-hickory forest, cedar-fir
swamp, and Red Maple-cedar swamp (Table 1).
Within habitat type: Northern hardwoods
We limited quantitative analysis to northern hardwood forests because
the majority of study sites occupied by Canada Warblers were northern
hardwood forests (Table 1), while other habitat types were relatively undersampled.
Northern hardwoods comprised 80 of the 124 of the survey points
(65%), of which Canada Warblers occupied 23 points (29%). In northern
hardwood forests, Canada Warblers were present at points that had signifi-
cantly greater total ground cover and lower canopy height (Table 2).
Table 1. Canada Warbler presence (n = 31) and absence (n = 93) in major forest habitat types
in Vermont.
Habitat type Survey points % occupied
Northern hardwood 80 29
Lowland spruce-fir 10 30
Oak-hickory 10 20
Cedar-fir swamp 5 40
Dry oak 5 0
Lakeside floodplain 5 0
Hemlock-pine 5 0
Red Maple-cedar swamp 4 25
496 Northeastern Naturalist Vol. 16, No. 4
We evaluated sets of competing AIC models investigating Canada Warbler
detections in northern hardwood forests of Vermont based on the most
significant variables (P < 0.2), as determined in univariate tests (Table 2),
to explain Canada Warbler presence or absence. No more than five variables
were used to model Canada Warbler presence in the northern hardwood
points (n = 80). Total ground cover was removed from models because of
a high correlation with low (<50 cm) shrub cover (r = 0.69), fern cover
(0.51), and forb cover (0.56). The best model for predicting Canada Warbler
detections at northern hardwood points was based on vegetative structure,
including shrub and fern cover, and canopy height (Table 3). Canopy height
was highly correlated with total ground cover (r = -0.38; F1, 82 = 13.447,
P = 0.0004), shrub cover (r = -0.39; F1, 82 = 15.1733, P = 0.0002), fern cover
(r = -0.21; F1, 82 = 3.9348, P = 0.051) and number of small alive stems (r =
-0.34 ; F1, 82 = 10.452, P = 0.0018). Additional variables included in the
model made only marginal changes in log-likelihood with less predictive
power than the model presented (Table 3).
Table 2. Habitat characteristics associated with Canada Warbler presence and absence in northern
hardwood forest (n = 80); means (SE) of four 11.3-m radius (0.04-ha) plots at each point.
Ground-cover estimates and number of saplings were measured within a 5 m radius of census
point (0.0078 ha). * indicates statistical significance (Bonferroni-adjusted alpha, P = 0.0021).
Absent Present
Character (n = 57) (n = 23) Wilcoxon Z P
Average leaf litter depth (cm) 3.7 (0.2) 4.1 (0.4) 0.6084 0.5500
Ground cover:
% total ground cover 53.6 (2.5) 72.6 (3.9) 3.7838 0.0002*
% shrub cover 15.2 (1.5) 28.4 (4.4) 2.6911 0.0071
% fern cover 12.1 (1.4) 20.5 (3.1) 2.9779 0.0029
% grass cover 1.9 (0.5) 4.4 (1.8) 1.7986 0.0700
% forb cover 15.9 (1.6) 23.2 (3.1) 2.1023 0.0360
% moss cover 6.7 (0.5) 10.1 (1.3) 2.4388 0.0150
% leaf cover 85.1 (1.1) 85.6 (1.5) 0.0643 0.9000
% downed logs 6.3 (0.7) 6.3 (0.7) 0.8212 0.4100
% bare ground 3.0 (0.4) 3.6 (1.0) 0.2426 0.8100
% water cover 0.6 (0.3) 0.05 (0.05) 2.0380 0.0420
Canopy:
Average canopy height (m) 20.34 (0.4) 16.77 (0.97) 3.4919 0.0005*
% canopy cover 91.7 (0.4) 92.1 (1.2) 1.0932 0.2700
% canopy cover >5 m 88.7 (0.6) 85.1 (1.6) 1.7856 0.0700
Sapling/shrubs:
No. dead small stems 1.4 (0.2) 1.6 (0.4) 0.1291 0.9000
No. dead large stems 1.1 (0.1) 2.0 (0.5) 1.0258 0.3100
No. living small stems 38.3 (2.9) 47.3 (5.9) 1.1871 0.2300
No. living large stems 12.5 (0.8) 16.3 (1.8) 1.6969 0.0890
Trees:
No. trees 8–22.9 cm dbh 14.6 (0.7) 18.7 (1.8) 2.0241 0.0430
No. trees 23.0–37.9 cm dbh 3.7 (0.2) 3.7 (2.1) 0.2476 0.8000
No. trees >38.0 cm dbh 1.3 (0.1) 1.2 (0.1) 0.2493 0.8000
No. dead trees 8–22.9 cm dbh 1.8 (0.2) 2.8 (0.4) 1.9140 0.0600
No. dead trees 23.0–37.9 cm dbh 0.6 (0.1) 0.7 (0.1) 0.5727 0.0600
No. dead trees >38.0 cm dbh 0.2 (0.03) 0.2 (0.05) 0.7127 0.4700
2009 J.F. Chace, S.D. Faccio, and A. Chacko 497
Discussion
In Vermont, Canada Warblers occupy breeding sites in a variety of forest
types, including northern hardwoods, Red Maple-cedar swamps, lowland
spruce-fir forests, cedar-fir swamps, and oak-hickory forests (Table 1).
Within northern hardwood forests, warbler detections are related to the
structure of the canopy and understory. Canada Warblers use sites that have
lower average canopy height and higher coverage of low (<50 cm) ground
cover (Table 2), primarily ferns and shrubs (Table 3).
The structural attributes described here may be important to Canada Warblers
for several reasons. First, Canada Warblers sing from exposed, elevated
perches (J.F. Chace and S.D. Faccio, unpubl. data; Kendeigh 1945), and lower
average canopy height may be related to canopy openings, snapped tops, and
other structural features associated with canopy disturbance events (e.g.,
wind-throw, ice storms, insect damage). Second, high densities of shrubs
and saplings, such as those found in disturbed forests or forested wetlands,
provide cover and favorable foraging structure. Two studies of warbler foraging
behavior, conducted in upland forests of New Hampshire and Wisconsin,
found that Canada Warblers concentrate their feeding effort in shrubs and low
tree branches at heights of 3–5 m (Sabo and Holmes 1983, Sodhi and Paszkowski
1995). This range corresponds with the leafy stratum often formed by
wetland shrubs (Miller 1999) and by regenerating forests 6–20 years after harvest
(Hagan et al. 1997). Lastly, Canada Warblers nest on the ground, usually
on raised hummocks under dense ground cover (Conway 1999; J.F. Chace,
pers. observ.). Structurally complex forest floors, with hummocks, rootballs,
downed woody debris, and dense patches of vegetation may provide concealment
for nests and young. The use of sites with higher ground cover of ferns
and shrubs may provide cover for Canada Warbler nests.
Several previous studies have shown that Canada Warblers occupy a
variety of forest types with structural conditions similar to those found in
this study. In this study, we found warblers at points with lower than average
canopy height, perhaps indicative of past wind-throw and ice storm damage.
Canopy disturbance resulting from wind-throw, insect outbreaks, ice storms,
Table 3. Aikiake’s information criterion (AICc) for structural habitat components associated
with Canada Warbler presence in northern hardwood forests sites (n = 85) in Vermont; best ten
models of 3 to 5 variables are presented. K = number of model parameters.
Model Log-likelihood K AICc Δ AICc
SHRUB, CANHT, FERNS 38.158615 5 13.9227 0
SHRUB, CANHT, FERNS + SHRUBS ALIVE large 36.314240 6 16.1971 -2.2744
SHRUB, CANHT, FERNS + WATER 36.469787 6 16.2008 -2.2781
SHRUB, CANHT, FERNS + MOSS 36.480940 6 16.2011 -2.2784
SHRUB, CANHT, FERNS + FORBS 36.956112 6 16.2123 -2.2896
SHRUB, CANHT, FERNS + TREES (Class 1) 37.249450 6 16.2192 -2.2965
SHRUB, CANHT, FERNS + HIGH CANOPY 37.728050 6 16.2302 -2.3076
SHRUB, CANHT, FERNS + GRASS 38.059350 6 16.2378 -2.3152
SHRUB, CANHT, FERNS + DEAD TREES (Class 1) 38.158603 6 16.2401 -2.3175
SHRUB, CANHT, FERNS, MOSS + WATER 34.128043 7 18.5208 -4.5981
498 Northeastern Naturalist Vol. 16, No. 4
and timber harvest often create these conditions through increased sunlight
penetration that results in vigorous shrub and sapling growth, and through
an increase to forest floor complexity due to downed woody material. Hagan
and Grove (1999) frequently encountered Canada Warblers in small treefall
gaps within large blocks of mature forest in Maine, and Hagan and Meehan
(2002) reported a positive correlation with dead-tree basal area and understory
stem density. In another Vermont study, Faccio (2003) found Canada
Warblers in small (0.1–0.2 ha) canopy gaps within an extensively forested
landscape during three years following a damaging ice storm in 1998. However,
these gaps did not increase Canada Warbler abundance within the forest
as a whole (Faccio 2003).
While all points surveyed in this study were not managed for timber
harvesting, timber removal may create suitable structural conditions for the
Canada Warbler. In the industrial forests of Maine, Hagan et al. (1997) found
Canada Warblers most abundant in young, scrubby re-growth 6–20 years following
both partial-cuts and clearcuts, particularly when some unharvested
trees remained. This finding mirrored results from a previous study of birdhabitat
relations in Maine timberlands, in which Canada Warblers were found
only in regenerating stands dominated by stems <10 cm dbh and >2 m in
height; highest counts occurred in areas where saplings exceeded 4.5 m and
where loggers had left some trees in the overstory (Titterington et al. 1979). In
a New Hampshire study, King and DeGraaf (2000) found Canada Warblers to
be significantly more abundant in 3- to 5-year-old clearcuts and shelterwood
cuts, than in mature, closed canopy hardwood stands. These other studies are
consistent with our finding of Canada Warblers detected in northern hardwood
patches with high ground cover and high density of large stems.
Canada Warbler populations appear to be decreasing across their range
(Sauer et al. 2005), precipitating a need for understanding the habitat
conditions that promote occupancy. Canada Warblers in the mature northern
hardwood forests of Vermont may benefit from factors that create small canopy
gaps and promote understory growth. Perhaps small-group-selection harvesting
is a candidate management tool for this songbird with a high regional
conservation emphasis. We caution that Canada Warbler detections at a single
point does not necessarily equate with successful forest-patch occupancy nor
is a proxy for successful pairing or reproductive success. The reproductive
response of these warblers to different harvesting regimes remains untested.
Bock and Jones (2004), in a review of the relationship between avian density
and reproduction, found that 30% of the studies had a negative correlation.
Clearly, studies that measure the reproductive success of Canada Warblers
across different forest types and harvesting regimes is needed.
The decline of the Canada Warbler population is perplexing. Long-term
declines on the breeding grounds of the United States and Canada occur in
regions where mature forests are abundant and habitat conditions seem to be
suitable. Certainly, impacts from wind throw, ice storms, and insect damage
in these forests today create small canopy gaps that these warblers appear to
respond to (Faccio 2003, Hagan and Grove 1999). Mature forests are extensive
in Canada, where 80% of the global Canada Warbler population breeds.
2009 J.F. Chace, S.D. Faccio, and A. Chacko 499
Yet this portion of the population declined by 4.5% between 1968 and 2007
(Savignac 2008). Habitat loss and degradation on the breeding grounds have
impacted the population; however, population limitation most likely stems
from habitat loss on the species’ winter range in South America (Lambert
and Faccio 2005). The primary over-wintering grounds of the Canada Warbler
includes the mid-elevation subtropical forests of the northern Andes,
especially Columbia (Conway 1999, Robinson et al. 1995). These are among
the most threatened habitats in the world, where 90% of forest has been
cleared by activities relating to agriculture, fuel wood harvesting, cultivation
of illegal drugs, and herbicide treatments to eradicate drug crops (Davis et al.
1997). Clearly, there is a need for research on habitat use and survivorship
of nonbreeding populations of the Canada Warbler.
Acknowledgments
We thank James Gillis, Rob Gregory, Cristin Chiha, Jenn McLaughlin, and
Jennifer Schomp for measuring vegetation at the Forest Bird Monitoring Project
point-count stations. We are especially grateful to the following volunteers and
field staff who helped conduct bird surveys: Carl Anderson, Jayson Benoit, Ernie
Buford, Chip Darmstadt, Brett Engstrom, Ted Gaine, Mary Gaudette, Jim Graves,
Warren King, Mark LaBarr, Everett Marshall, Sean Macfaden, Bryan Pfeiffer, Alan
Quackenbush, Charlie Rabatin, Chris Rimmer, Sue Staats, Ruth Stewart, and Ned
Swanberg. Clait Braun and four anonymous reviewers provided very valuable comments
on earlier drafts. Financial support for this project was generously provided by
the William P. Wharton Foundation, Sweet Water Trust, and Villanova University.
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