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Winter Strategies of Ruffed Grouse in a Mixed Northern
Forest
Bernd Heinrich*
Abstract - I examined behavioral flexibility with respect to potentially conflicting demands
of Bonasa umbellus L. (Ruffed Grouse) adapted to winter conditions in a mixed forest of
western Maine. At the beginning of winter, during the first snow of 15 cm, the grouse roosted
overnight 2–4 m above ground in dense stands of conifers. After 40 cm of snow depth,
they dove into the snow in flight in open areas of deciduous forest and tunneled, at varying
angles and distances from the entrance approach, to den at nigh t and most of the day. With
new snow on top of packed snow near the end of winter, they switched tactics again and
rested and spent the overnight hours in snow molds on the ground under cover of conifer
branches or against tree trunks. The 3 behaviors are discussed in the context of winter diet
and anti-predation strategies.
Introduction
Animals exhibit a variety of solutions for coping in winter. Birds in particular
face the interrelated problem of securing enough food in often greatly reduced
foraging time, and maintaining an elevated body temperature for long durations
of the diurnal cycle. Greater rate of heat loss due to cold and small body size
(Calder 1984) may be compensated for by better insulation, and/or increased
shivering which increases heat production (Cooper and Swanson 1994, Dawson
and Carey 1976, Walsberg 1983) abd requires greater food intake. Hypothermia
and seasonal acclimation are alternate tactics (Chaplin 1976, Liknes et al. 2002,
Marsh and Dawson 1989). Behavioral strategies used by birds in winter include
overnighting in existing tree cavities (e.g., Poecile atricapillus (L.) [Black-capped
Chickadee; Smith 1991], Sitta canadensis L. [Red-breasted Nuthatch; B. Heinrich,
pers. observ.], and Sitta carolinensis Latham [White-breasted Nuthatch; B.
Heinrich, pers. observ.]); excavating tree cavities in the fall specifically for overnighting
(Dryobates pubescens L. [Downy Woodpecker] and Picoides villosus (L.)
[Hairy Woodpecker]; B. Heinrich, pers.observ.; Kilham 1992); huddling in groups
(Regulus satrapa (Lichtenstein) [Golden-crowned Kinglet]; Heinrich 2004); or
burrowing in the snow (Acanthis flammea (L.) [Common Redpoll]; Heinrich 2014).
Winter survival strategies of Bonasa umbellus L. (Ruffed Grouse) could potentially
encompass several of the above strategies (Ber gerud and Gratson 1988).
Ruffed Grouse are one of the best-known game birds living year-round in forests
throughout North America (Atwater and Rusch 1991, Bump et al. 1947, Rusch et
al. 2000), where they feed in the winter on tree buds (Hewitt and Messner 2000,
Huepner and Tester 1988, Jakuba and Gullian 1991). Tree buds are potentially
available in large amounts, although they contain high-cellulose, low-caloric bulk
*Box 153, Weld, ME 04285; bheinrich153@gmail.com.
Manuscript Editor: Susan Z. Herrick
Winter Ecology: Insights from Biology and History
2017 Northeastern Naturalist 24(Special Issue 7):B55–B71
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(Casaway et al. 1976). Additionally, given standard avian energy economics (Dawson
and Hudson 1970, Reinertsen and Haftorn 1986), Ruffed Grouse, because of
their large size (near 500–600 g), face a low rate of heat loss relative to other winter
residents such as kinglets, chickadees, and small finches weighing 5–13 g. They
should thus be less constrained by the energy demands imposed by low temperatures;
however, like many small birds that remain in the Northern Hemisphere in the
winter, Ruffed Grouse overnight in dense coniferous foliage where they gain energy
economy (Thompson and Fritzell 1988a, Whitaker and Stauffer 2003).
Unlike Lagapus spp. (ptarmigan), which molt to cryptic white plumage in the
winter, Ruffed Grouse are camouflaged only in the summer and may thus be constrained
in their foraging as well as roosting behavior in the winter. They are choice
prey of many mainly visually oriented predators (Bergerud and Gratson 1988,
Gormley 1996, Gutierez et al. 2003, Small et al. 1991). They would presumably
be especially vulnerable in the winter to their major aerial predators such as Accipiter
gentilis L. (Northern Goshawk; Eng and Gullion 1962, Tomberg and Fritzell
1988b), both on snow-covered ground as well as in the tops of then-bare deciduous
trees where they often feed. Like some other birds, their choice of night roosts appears
to be based on both safety (Marjakangas 1990) and energetics (Swenson and
Olson 1991). I herein examine Ruffed Grouse snow-use and roosting behavior in
the context of seasonal changes at a site in Maine where the birds experience snowcover
varying from absence to shallow to deep, and have a choice of habitat from
open to stands of dense conifers.
The standard term for typical bird behavior of perching at various levels of
elevation and exposure is “roosting”. However, grouse are also well-known for
“snow-roosting” to avoid predation by burrowing to shelter under the snow (Gullion
1984). For differentiating these 2 behaviors, and also that of resting on the
snow surface in molds, and in common usage with respect to function in other animals,
I here refer to the under-snow sheltering as “denning”. However, no account
had been made of the relation of when, where, and how denning relates to roosting
and the use of molds, and to season or to the lengths of time birds may stay in residency
in these places. Given the ~15 hours of local darkness in winter, and several
hours of den-residence time added either before night or after, snow-den residences
could have been as long as 18 hours. I here examined these behavioral changes in
relation to seasonal and presumably environmental conditions.
Methods
Site description
My observations were conducted in ~50-ha of woodland (44°41'18.89883"N,
70°22'20.37437"W) on York Hill, in western Maine, at an elevation of 350–400 m.
The area contains a patchwork of a wide range of habitat choice and food. Portions
of the forest had been logged at various times over the past 5–50 years, creating
variable-aged stands of Picea–Abies (spruce–fir) and Pinus strobus L. (White
pine) as well as mixed-species deciduous patches of early successional spruce–fir
adjacent to a maturing hardwood forest on a sloping hillside (Fig. 1A) adjacent
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to recently logged areas containing dense regrowth of 4–12 cm diameter Abies
balsamea (L.) Mill. (Balsam Fir) (Fig.1B). The primary winter food of grouse,
buds of primarily Betula spp. (birch), Populus spp. (poplar), and Ostrya virginiana
(Mill.) Koch (Eastern Hop Hornbeam), were distributed throughou t the area.
Figure 1. Ruffed Grouse winter habitat at York Hill, western Maine: (A) deciduous forest
and (B) adjacent Balsam Fir thickets.
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I have informally observed Ruffed Grouse at this site for decades, and as expected
from studies elsewhere (Bergerud and Gratson 1988), they routinely roost
overnight at several meters height in the branches of dense spruce–fir thickets in
spring and fall. In winter, Ruffed Grouse, often seen in groups of 2–4 individuals,
feed at dusk and dawn on buds in the tops of tall birch and poplars. Their overnighting
winter roosts in conifers are easily recognized by numerous scat scattered on
the snow. Similarly, I had routinely encountered over the years in this area grouse
snow dens and molds, and have determined grouse occupancy in a den both by scat
and/or flushing into flight when approached.
Environmental conditions
Since I do not want to assume the environmental variables affecting behavior,
I here separate the observations into the 3 main seasonal periods observed. In the
first period, “February 2015” (2 February to 3 March 2015), the snow was powdery
and near 40 cm in depth on top of additional packed snow below. Both day- and
night-time temperatures were, as is typical in other years, not above 0 °C in the
daytime, and usually near or below -20 °C at night. Almost all of the snow denning
was observed specifically in this period.
In the second period, “March 2015” ( 4 March to 3 April 2015), it had warmed
briefly to daytime temperatures of 8 °C, but temperatures at night continued to remain
below freezing. The snow softened and then crusted, but there were 4–5 cm
of fresh fluffy snow on that crust when the observations were made.
The third period, “January 2016”, (31 December 2015 to 26 January 2016) was
immediately after that year’s first significant storm that deposited ~14–17 cm of
fluffy snow that remained throughout the period. Night-time temperatures ranged
from -25 to -7 °C.
Grouse activity
Ruffed Grouse activity and approximate timing of snow-den construction and
roosting site use were determined by twice-daily surveys for roosting, snow dens,
and resting molds as they occurred anew along an arbitrary 5-km route, chosen
to traverse different habitat. Visibility of Ruffed Grouse activity marks on/in the
snow was ~5 m to either side of the trail. Frequent light snow dustings and snow
disturbance by wind, and varying times of day at which I conducted the surveying,
were also used as aides for estimating den making and occupancy times. The survey
route led through nearly mature hardwood forest of primarily Acer rubrum L. (Red
Maple), A. sacharum Marsh (Sugar Maple), Fraxinus americana L. (White Ash),
Fagus grandifolia Ehrh. (American Beech), Betula papirifera Marsh (Paper Birch),
Betula alleghaniensis Britt. (Yellow Birch), and Populus balsamifera L. (Balsam
Poplar) (Fig. 1A), to secondary growth of the same tree mixture with Balsam Fir
and Picea rubens Sarg. (Red Spruce), and also through young growth of the latter
(Fig. 1B). I located the snow dens by the entry marks left in the snow. I made scat
counts after scraping away the snow surrounding a den and separating the scatclump.
At over-nighting roosts in trees, the scat were clearly visible scattered over
the snow surface.
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I assayed scat to make inferences about den- and mold-occupancy durations.
Ruffed Grouse produce 2 kinds of scat. The first is derived from coarse material
that is passed directly into the large intestine, whereas the other is from fine pasty
liquid material that is retained much longer in the cecum (Casaway et al. 1976).
The intestinal scat of Ruffed Grouse was ~2.5 cm long and relatively solid sausageshaped
(weighing on average 1.8 g each). The cecal scat was amorphous brown
semi-liquid that had usually frozen solid by the time it was assayed in this study in
the field. Given the high fiber diet of tree buds in the winter, Ruffed Grouse produce
the intestinal scat in bulk. An absence of den sites along the survey route in late
afternoon versus their presence the next morning, as well as the number and kind
of scat left in them in the morning, were used as evidence for overnighting dens.
The cecal scat was found only at overnight roosts and dens, and it was deposited
shortly before or directly during the birds’ den departure. At night tree roosts, the
cecal scat, when not seen on the ground, was located instead stuck onto the tree
branches on or under where the bird had roosted.
Ruffed Grouse routinely made snow dens within a meter or two of the snowshoe
tracks of the survey course, and they were therefore apparently not disturbed by
my tracks. To test if the sometimes-observed clumping of den-site locations (within
about 10 m of one another) resulted from grouse using the den-entrance marks on
the snow as a signal for their own denning there, or if alternately they were derived
from birds who had stayed together after their just previous feeding, I made marks
on/in the snow that mimicked grouse snow-den entry marks. One hundred of the
dummy den sites were made available on one day of fresh snow at approximately
equi-distant intervals of at least 30 m over the 5-km den survey trail. These pseudoden
entrances that closely mimicked grouse snow-den entry marks were created by
tossing a bantam rooster carcass on a string into snow, and immediately yanking it
back out. They were accessed twice daily on the regular route survey for 4 successive
days.
Predators
As determined by tracks and sightings, potential predators in the area included
Canis latrans Say (Eastern Coyote), Lynx rufus (Schreber) (Bobcat), Mustela
frenata (Lichtenstein) (Long-tailed Weasel), Neovison vison (Schreber) (Mink),
and Martes pennanti (Erxleben) (Fisher), all of which left tracks in the area. An
Accipiter gentillis L. (Northern Goshawk) and an Accipiter cooperii (Bonaparte)
(Cooper’s Hawk) were both seen once near the area during the current study as well
as before it. No avian predators were currently nesting nearby, although a pair of
Bubo virginianus (Gmelin) (Great-horned Owl) and Buteo jamaicensis (Gmelin)
(Red-tailed Hawk) had nested there earlier.
Results
February 2015
The only Ruffed Grouse I encountered during ~150 hr in the woods in the daytime
were those I flushed from their snow dens on approach, although on the 2
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occasions that I was out for an hour at dusk I saw a pair feeding on buds near the
tops of Yellow Birch. During that month, no grouse tracks were found on the snow
except those occasionally associated within ~1–2 m of a den. In the 111 dens I
examined during this time, 16 were day-dens (the grouse were in and then flushed
from them). Seventy-two were used overnight, as determined from their absence
along the survey route in the evening and presence in the morning. In 23 dens, the
difference between day versus night residency was undetermined.
All grouse dens noted during this time of low temperatures, no wind, and deep
fluffy snow were in relatively open hardwood forest characterized by areas of
cleared snow surface with access by flight. In 109 of the 111 denning occasions,
there were no tracks leading into the den site. Den exit holes had imprints of 1 or 2
wing-beats and usually 1 or 2 foot-steps, although occasionally grouse foottracks
extended for up to 2 m from the exit holes. The den cavity, which contained grouse
intestinal fecal pellets, were usually located within 0.5 m of the entrance hole, but
sometimes the tunnel to the den extended 1 m or more, with one situated 2.7 m in
from the entrance (Fig. 2). The tops of the tunnels were 2–5 cm beneath the snow
surface, and the longer tunnels commonly had what appeared to be a deliberately
made small peek-hole (Fig. 3). As opposed to passive cave-ins, these holes to the
snow surface had snow extruded from them and were vertical and round.
Fig. 2. Diagrammatic configurations of a sample of Ruffed Grouse dens during 40-cm snow
conditions, showing den locations versus entry and exit points into the snow, tunneling, and
(at top left) magnification of a den and (at top right) side view showing locations of the cecal
and intestinal scat arrangement of an over -nighting den.
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Figure 3. Surface observations at sites of Ruffed Grouse snow-dens, February 2015. E
= entry to den, P = “peep hole”, X = exit, W = wing marks on leaving, F = furrow. (A) 3
February—bird tunneled left on entered snow, overnighted by peep hole, and then exited
the next day, leaving wing marks as it took flight. Scale: backpack length = 48 cm. (B) 25
February—birds travelled under snow to right, spending night by peep hole. (C) 27 February—
after spending night by peep hole, bird exited and walked across snow surface leaving
furrow, before taking flight.
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Overnighting dens contained 45–85 (mean of 61) intestinal scat. Scat counts
of the 16 birds flushed from their dens in the daytime varied from 2 to 36, with
those dens from which I flushed the grouse early in the day containing the fewest,
whereas those with over 30 intestinal scat were flushed out near noon. Other,
presumably new, dens near the end of the day also had low scat counts. Day-dens
lacked cecal scat. Dens that were made in the evening and occupied until shortly
after dawn contained 2–5 cecal scat along with the pile of inte stinal scat.
In night-dens, the cecal scat was visible at a glance on the snow surface in the
den, and sometimes at the den exit. But the intestinal scat was piled up, and often
partially buried within the den in a tight multi-layered clump of ~5 cm diameter,
suggesting the bird had apparently not relocated from the same spot for hours. The
usual thin snow layer between the cecal scat on top and intestinal scat under it, and
a lack of disruption of shape of the almost fluid paste-like cecal scat on issuing
and its placement, indicates that the latter was not voided by the grouse until upon,
or just before, leaving the den. I estimated den residence time using an inferred scat
clock proceeding at an intestinal scat production rate of about 4.7 /hr.; this rate was
derived by dividing the mean of 61 scat found in night-dens by the 13 hrs of nighttime
residency.
Ruffed Grouse (n = 16) were flushed from their dens at various times from morning
to late afternoon, on days of sunshine and no snowstorm activity nor unusual
temperature lows. Either they made/entered new snow dens in the morning after
their dawn feeding and then remained denned for various lengths of time, or they
had not left their night dens by morning.
The scat number left in those snow dens of grouse I had flushed in the daytime
increased approximately linearly until at least 13:00 hr, when it reached 36 (Fig. 4).
Applying my calculated scat production rate described above, the grouse should
have produced 37.6 scat if they denned after feeding near dawn from 05:00 hr until
13:00 hr., and ~28 scat if they had denned from 07:00 hr to 13:00 hr. This estimate
suggests that the grouse could have snow-denned for up to at least 6 hours of the
day until I had flushed them. Low scat numbers late in the day presumably indicate
new dennings that I had interrupted.
March 2015
On 4 March, the first day with above-freezing temperatures that year, the Ruffed
Grouses’ behavior changed abruptly. From that day and through the next week,
temperatures rose to +8 °C in the day, although night-time temperatures continued
to be near -7 °C. The top layers of snow crusted at night with occasional dustings
added. I found no more snow dens in the open areas of the hardwood forest on my
circuits at that time. However, there then appeared molds pressed into the softening
snow surface. The 18 snow molds (several cm deep) observed were all under the
cover of live conifer branches or adjacent to thick tree stems. All were associated
with tracks on the snow leading into and out of them. Scat (only the intestinal was
present) numbers per mold ranged from 3 and up to 36 (mean = 7.3).
By 18 March, after 4–6 cm of fresh snow covered a crust, the birds remained in
dense conifer thickets where they left numerous track-ways that, in the length of
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several km of them that I followed, were all under sunlight-excluded cover. Upon
reaching thicket edges, the tracks returned back into the cover. The birds had fed
on seeds shed by several birches that towered over or along the thicket, and on the
evergreen fronds of Dryopterus marginalis (L.) (Marginal Fern) at its edge.
Of the 21 observed individual overnighting locations, none were in either the
deciduous forest nor in mature conifers. All were molds several meters into and
under the cover of the dense young Balsam Firs and spruces growing in the thickets.
January 2016
Evidence of near identical behavior to that observed in March 2015 was found
in the same area after the first snowstorm blanketed the ground with 14–17 cm of
powdery snow. Temperatures at night had dipped to -26 °C, and the snow remained
powdery throughout the period. In my 6 days of approximately equal search in all
habitats, I located 24 overnighting (tree) roosts (Fig. 5A). All were in low (estimated
2–4 m) young Balsam Fir trees. This survey also revealed 23 molds 5–15
cm into the soft snow that had been day residencies under overhanging fir branches
(Fig. 5B). Tracks originating from grouse landing-marks in the snow in open snow
at the thicket edges led to where the birds had made their snow molds in the thickets
(most under less than 1 m below hanging snow-laden boughs, and/or usually next
to a tree trunk). These molds contained 3–14 (mean of 8.8) intestinal, but no cecal,
scat. In contrast, all the 24 overnighting roosts were located (by the scat on the snow
beneath) at the same time in the same area in young fir trees. They were associated
Figure 4. The number of scat left in snow-dens after the Ruffed Grouse had been flushed
from them, indicating that grouse likely denned until mid-day, to then exit and resume denning
shortly afterwards. All scat were of the intestinal variety.
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with 34–72 (mean of 61.6) intestinal and 2–4 (mean of 2.7) cecal scat. I found no
evidence to indicate that any grouse had remained to overnight in the deciduous
woods where they had denned during the deep snow the year befor e.
Approximately 2 weeks later (13 to 26 January 2016), the roosting behavior
of the grouse in the same area changed once more, even though the snow- and
Figure 5. Ruffed Grouse behavior sign in early January 2016, when depth was 16 cm or
less. (A) About 50 intestinal and 3 cecal scat scattered under overnight roosting place of a
grouse at 3 m in live branches of a Balsam Fir tree in a thicket. (B) Three resting day-molds
(M) of 3 grouse near each other at the edge of a barrier (here a brush-pile), each containing
only 8–13 intestinal scat.
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temperature conditions had remained virtually identical. Two night roosts in the
Balsam Fir trees, 10 overnighting molds, and 1 snow den were located. Mean scat
numbers were similar to before, intestinal at 45–85 (mean of 61.2), and cecal at 1–5
Figure 6. Grouse molds in ~16-cm snow conditions in January 2016. (A) A day mold next
to a tree. Note walking tracks in and out of the ~10-cm-deep mold. (B) An over-nighting
mold with cecal pellets (CP) on left.
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(mean of 2.8). Thirty-four day-molds were located with 0–34 (mean of 16.1) intestinal
and no cecal scat. Both the day and night molds (Fig. 6A) were again placed
adjacent to a backing such as brush or a thick tree trunk where an open flight path
remained on the other side(s) of the resting place. The day and night molds (Fig.
6B) were shallow (3–6 cm deep). Three grouse were flushed at 09:30, 13:30, and
14:10 hrs from these molds holding 13, 34, and 2 scat, respecti vely.
Clumping
In the past I had observed Ruffed Grouse in the area in small groups (2 to 5),
while foraging on the snow for birch seeds, perching in trees, and while feeding
on tree buds at dusk. The snow den and mold locations also occasionally indicated
clumping. During the 2015 period, there were 18 apparent groupings within the total
of 111 dens, with 14 pairs of 2 dens within about 6 m of one another, and 4 sets
of 3–5 dens within 10 m. In 2016, as before, some grouse molds were clumped (Fig.
5B); of 48 over-nighting and day-molds, 25 were in groups of 2–4 within 2–10 m
of one another. However, during the 4 days I had made 100 psuedo-den entrances
avialable, none of the 159 natural molds/dens I found were located within such
close near-neighbor distances to the artificial den entrances.
Predators
None of the other 253 dens/molds observed in this study on the transects were
associated with obvious sign indicative of a disturbance by the currently available
potential predators. However, 2 incidences of grouse predation were inferred (from
evidence of tracks, leavings of feathers, and grouse gizzards) in the greater area
surrounding the study sites, during January 2016. A Vulpes vulpes L. (Red Fox) had
killed a grouse at a den containing 25 intestinal scat, and an undetermined predator
had killed another grouse. Snow conditions were not suitable for further inferences.
The gizzards of both kills were packed with birch buds and grit .
Discussion
Birds overwintering in northern forests confront the problem of energy balance
from cold, availability of suitable food and shelter, and reduced time for
foraging (Aschoff 1981, Walsberg 1986). The solution to one problem likely affects
others (Calder 1984). None of the small birds in New England overnight
under snow, although they are potentially more challenged by the cold by their
small size relative to grouse. But because of their winter diet of tree buds and
large body size, Ruffed Grouse face a different mix of problems compared to
most other overwintering woodland birds. For grouse, the energy cost of making
a snow den is presumably negligible, but the benefit of evading predators in snow
that it provides could be high.
Food is a major variable of winter-adaptation, and Ruffed Grouse, in contrast
to the overwintering insectivorous and granivorous birds, switch to a diet of tree
buds after the first snowfall (Doerr et al. 1974). This diet, despite high bulk and
low nutritional content (relative to insects and seeds), has the advantage of great
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abundance and predictability at specific locations. Thus, while most other birds in
the same environment may forage nearly continuously for most of the day, grouse
relying on tree buds need to feed for only brief periods. In Alberta in winter, they
feed daily on average for just 16 min prior to sunrise, and only another 24 min more
to again fill their crops near an hour after sunset (Doerr et al . 1974).
Ruffed Grouse feed on the ground into autumn before the trees set buds. Cryptic
earth-coloration is a defense in summer, fall, and spring. However, normally
through winter when they feed in the tops of bare deciduous trees, these large birds
are visible from several hundred meters. They form small groups presumably not
only because they are drawn together by the productive feeding spots where they
can quickly fill their crops in the evening and in the pre-dawn (Doerr et al. 1974).
Predator avoidance is another possible reason, achieved in groups by taking advantage
of the “many eyes” effect for predator vigilance. Since buds are continuously
available, the crepuscular foraging in tree tops is likely an adaptation reducing
predation. Similarly, during winter, the grouses’ bare-ground–based camouflage on
snow could make them more vulnerable off the trees than on them. In this study,
Ruffed Grouse did feed on the snow surface in late winter, but only under the very
dense cover of young conifers, at locations where birch seed had been spread after
a wind.
Goshawk account for most Ruffed Grouse predation, exceeding that by human
hunters (Bergerud and Gratson 1988, Rusch et al. 2000). Cycles of goshawk abundance
respond to forest grouse populations (Tomberg et al. 2005). Great-horned
Owls prey on grouse as well (Devers et al. 2007) and could hunt them at low light
intensities. Regardless of which feeding time is safer, a short feeding time as such
would reduce exposure, and the grouses’ shift to crepuscular activity and their tendency
to group are both consistent with anti-predator behavior .
On the winter solstice (21 December), at the study site in Maine, there are,
between official sunrise and sunset, 8 hr of day, followed by16 hr of night. With
less than an hour allocated to feeding given the availability of tree buds, Ruffed
Grouse in mid-winter have potentially 23 hours of “free” time available, and being
a large conspicuous bird that is targeted prey may have spurred the evolution of
anti-predator behavior to occupy that time.
In this study, as in others (Bergerud and Gratson 1988, Whitaker and Stauffer
2003), Ruffed Grouse commonly roosted at sites that have been shown by studies
to reduce heat loss by both convection and radiation (Huepener and Tester
1988), and as determined by using grouse taxidermy mounts, the heat loss while
under snow is 14% less than it is while perched in a conifer (a cedar) (Thompson
and Fritzell 1988b). Dense habitat also reduces convection, and grouse generally
prefer dense habitat (Endrulat et al. 2005). As shown in the present study, where
conifers were available in and around the habitat where the grouse lived, the
grouse remained in such habitat even after the first snowfall of ~15 cm had occurred,
and then switched to using open molds in the shallow snow. On the other
hand, in mid-winter when deep snow became available, they then chose open
areas for denning, rather than the immediately adjacent dense stands of conifers,
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even though the snow offered only a relatively slight thermal advantage to the
insulation of conifers (Thompson and Fritzell 1988b). Then, within a day after
that snow became unsuitable for denning because of a crust, they switched back
to overnighting in dense conifers. These behaviors are consistent with respect to
both energy economizing and predator defense.
The snow dens were predominantly in open deciduous woodland, possibly because
of unobstructed access and exit possibilities into and out of the snow. Since
the birds had dived into the snow from flight (whereas they made molds on the snow
by walking around under dense conifers), they needed open space not only to enter,
but also to leave in perhaps variable directions during a forced exit at night. During
escape from a potential predator coming from a random direction near a snow den,
it would be important for the bird to avoid getting entangled in or hitting branches
of brush or a nearby tree.
Snow-denning in the daytime makes energetic sense in storms and at very low
temperatures. But neither storms nor abnormally low temperatures were observed
in this study, yet the grouse snow-denned on sunny days in the open areas and none
were encountered either directly or indirectly by tracks. On the other hand, when
snow denning was not possible, they remained day-active in the dense thickets, as
evidenced by their extensive tracks there.
Although the high visibility of a grouse on snow would change to near invisibility
when it is under it, invisibility in snow is not necessarily invincibility to
predator attacks, if a predator learns to associate den entrances with the presence
of grouse. However, several features should work in favor of the grouses’ safety. In
the absence of frequent snowfalls, the one-time use of dens creates numerous empty
dens and dilutes their utility by a predator looking for grouse. Since dens are made
by a quick dive, given suitable snow it is easy for grouse to make them on almost
any occasion, and a potential predator faces a shell game scenario, especially when
several empty den sites are clumped near a used one.
Nearly half (43%) of the grouse dens assayed were within about 10 m of another
den or resting mold, either because the association of 2 or more dens near each other
is coincidental to the birds staying together, or because they seek existing dens to
rest/overnight near them. If the same clumping effect had applied to the 4-day presence
of 100 pseudo-dens in the same area as the 159 molds/dens observed during
that time, then each had on average a 43% chance of being associated with a den.
But none were, suggesting it was very improbable that they were choosing new den
sites using visual cues of the presence of addtional dens. However, the observed
den clumping, regardless of mechanism, could potentially be an adaptive behavior.
Whether it is remains to be explored.
The structure of the dens, which included odd angles from the entry point and
the occasional long tunneling, suggests a design to foil a potential predator's ability
to predict the grouses' location from the den entry marks on the snow. The location
of the bird within the den was never deep relative to the snow surface, as would
be predicted for enhancing retention of body heat. Instead, the always shallow
submergence in the snow would facilitate quick escape, and the apparent peekNortheastern
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2017 Vol. 24, Special Issue 7
holes observed in the tops of the longer tunnels may have been used by the grouse
in orienting themselves for potential escape directions. Finally, nothing could
make the birds more invisible than being under the snow, with respect to anywhere
above it. A molt into white feathers, such as that of ptarmigan in the Arctic, could
serve the same function as hiding under the snow. However, although the duration
of snow is as long or longer in the Arctic, the snow cover is thin there due to low
precipitation and the conditions conducive for tunneling may be less. Furthermore,
there are fewer, or no, forest thickets in the Arctic to hide in when snow conditions
are unsuitable for snow-denning. I conclude that for Ruffed Grouse in a variable
north temperate climate, the winter behavior of snow-denning strongly favors an
anti-predation strategy. However, the observations leave open the extent to which
and even how much these behaviors may contribute to winter survival in grouse
with no camouflaging molt in winter.
Acknowledgments
I thank Charles H. Sewall and Scott R. Smedley for help with graphics, and an anonymous
reviewer for thoughtfully helpful comments and suggestions .
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