Winter Food Habits of Lagopus lagopus (Willow Ptarmigan) as a Mechanism to Explain Winter Sexual Segregation
Leanne T. Elson, Francis E. Schwab, and Neal P.P. Simon
Northeastern Naturalist, Volume 14, Issue 1 (2007): 89–98
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2007 NORTHEASTERN NATURALIST 14(1):89–98
Winter Food Habits of Lagopus lagopus
(Willow Ptarmigan) as a Mechanism to Explain Winter
Leanne T. Elson1,*, Francis E. Schwab2, and Neal P.P. Simon3,4
Abstract - To determine if the quality of winter diet is related to winter-habitat
sexual segregation, Lagopus lagopus L. (Willow Ptarmigan) were collected in three
wintering areas of Labrador between December and April 1998–2000 (n = 310).
Crop contents were used to evaluate diet differences according to sex, age, and
area. The most prominent item in ptarmigan crops was Salix spp. (willow; range
45–89%). The crops of adult females contained approximately 60% more willow,
the most nutritious food in their diet, and 45% more calories than those of adult
males. All birds collected from western Labrador contained more willow twigs and
buds and more calories than those collected in central or eastern Labrador. The
Gardarsson hypothesis suggests that males winter adjacent to breeding areas to
improve competition for territories, and females choose areas with greater abundance
and quality of food to enhance reproductive success. We could not reject this
hypothesis since the wintering site with the most female-biased sex ratio was where
diets had the greatest mass of willow and total calories.
Most boreal and subarctic bird species migrate to avoid harsh winter
conditions (Gill 1995). Lagopus lagopus L. (Willow Ptarmigan, hereafter
ptarmigan) also migrate between summer and winter areas, which can vary
from 20 km to 160 km apart (Gruys 1993, Hannon et al. 1998). In winter,
males and females move to lower elevations and/or latitudes than their
breeding areas (Gruys 1993, Hannon et al. 1998, Schwab et al. 2005,
Weeden 1964). Adult females, and often juveniles, tend to leave earlier and
winter farther from breeding areas than adult males (Gruys 1993, Weeden
1964). Gruys (1993) suggested that winter sexual segregation results from
sex-specific reproductive strategies (the reproductive-strategy hypothesis):
males winter closer to breeding areas, enabling early selection and defense
of breeding territories, and females winter where cover reduces predation. A
potential result of habitat segregation is that the quantity and quality of
winter food available to males and females is different. Females may select
winter habitat to include a greater abundance and quality of food within a
general strategy of acquiring and conserving energy. Gardarsson (1988) and
Gardarsson and Moss (1970) (working with Lagopus mutus Gmelin [Rock
1Western Newfoundland Model Forest, PO Box 68, Corner Brook, NL, Canada,
A2H 6C3. 2College of the North Atlantic, Labrador West Campus, Labrador City,
NL, Canada A2V 2Y1. 3Newfoundland and Labrador Department of Natural Resources,
Happy Valley-Goose Bay, NL, Canada, A0P 1EO. 4Deceased. *Corresponding
author - firstname.lastname@example.org.
90 Northeastern Naturalist Vol. 14, No. 1
Ptarmigan]) suggested that females feed on higher nutritive quality foods
than males in winter and prior to egg laying, to offset greater reproductive
energy demands (the Gardarsson hypothesis). The reproductive-strategy
hypothesis makes no prediction about food quality and female habitat
choice, whereas the Gardarsson hypothesis specifically predicts that females
select wintering areas with higher food quality.
The difference in quality between willow and other food items allows
for the evaluation of the Gardarsson hypothesis. In Alaska, up to 94% of
ptarmigan winter food consists of buds and twigs of Salix spp. (willow)
(West and Meng 1966). They consume lesser amounts of Betula
glandulosa Michx. (dwarf birch), Populus tremuloides Michx. (trembling
aspen) (Gasaway 1976, Moss 1973), Empetrum nigrum L. (crowberry), and
Vaccinium vitis-idaea L. (partridgeberry) (Weeden 1969). Dwarf birch and
trembling aspen have higher lipid content and, therefore, more calories
than willow (West and Meng 1966). However, willow is more nutritious
because of its lower toxin and lignin content and higher proportion of
cellulose, phosphorous, and protein (Bryant and Kuropat 1980, Gardarsson
and Moss 1970, Gasaway 1976, Moss 1983). Willow buds are more nutritious
than twigs because they contain more crude protein and less lignin,
but repeated browsing reduces the amount of buds in the diet, increasing
the proportion of twigs (Gasaway 1976).
If Gardarsson’s hypothesis is the mechanism explaining winter-habitat
sexual segregation, then ptarmigan crops should contain more total calories
and more nutritious food (i.e., willow) in areas with the higher proportion
of adult females. We subject this prediction to crop-content data from
ptarmigan in Labrador, an area displaying winter sexual segregation
(Schwab et al. 2005).
Ptarmigan were obtained from three areas in Labrador: Western Labrador
samples were obtained within 20 km of Javelin Mountain (53º07'N,
67º56'W), elevation 550 m; central Labrador samples were obtained within
25 km of the Lobsitck hydro control structure (53º54'N, 65º09'W), elevation
550 m; and eastern Labrador samples were obtained within 30 km of
Mokami Hill (53º58'N, 60º08'W), elevation < 75 m (Fig. 1). Both central and
western climates are classified Low Subarctic with dominant vegetation
consisting of Picea mariana Mill. (black spruce) and minor amounts of
Abies balsamea (L.) Mill. (balsam fir), Picea glauca (Moench) Voss (white
spruce), and Larix laricina (DuRoi) K. Koch (eastern larch). The eastern
climate is classified Perhumid High Boreal with dominant vegetation consisting
of black spruce and balsam fir on shallow upland soils and white
spruce, Betula papyrifera Marsh. (white birch), and trembling aspen on
well-drained soils (Canada Committee on Ecological Land Classification
1989, Lopoukhine et al. 1977). The ecological classifications available to
the authors do not mention willow. However, in western Labrador, willow is
2007 L.T. Elson, F.E. Schwab, and N.P.P. Simon 91
rare as compared with dwarf birch, which dominates mesic and drier soils,
and alder,which dominates wetter soils. Willow tends to be abundant in the
shrub layer on moist soils associated with nearby streams, lakes, and receiving
slope areas (Ryan 1995). Lake and stream edges are more abundant in
western than in eastern Labrador. In eastern Labrador, ptarmigan were taken
from the edge of a 1980s lakeside burn, dominated by white birch and aspen.
Hunters collected ptarmigan between late December and early April (see
Schwab et al. 2005). Of 310 samples, 178 were collected during the winters
of 1998–99 and 1999–2000 in western Labrador, 97 during the winter of
1999–2000 in central Labrador, and 35 during the winter of 1998–99 in
eastern Labrador. Our samples came from areas where hunters had snowmobile
or vehicle access, potentially biasing the data to vegetation in such
areas. Because there are no field marks that would allow discrimination
between ages and sexes in the field, the ptarmigan samples likely represent
sex and age ratios in these areas. Sex was determined by necropsy, and age
(adult or juvenile) was determined by comparing feather pigmentation on the
eighth and ninth primary feathers (Bergerud et al. 1963). Crop contents of
each ptarmigan were separated according to plant species and structure (i.e.,
Figure 1. Map of study area.
92 Northeastern Naturalist Vol. 14, No. 1
buds, twigs, leaves, and fruit). Segregated crop contents were oven dried at
80 °C for 24 hours, and the dry mass by species-structure were recorded
for each sample.
The total calories of the major crop items and the dry mass of the two
predominant plant species within crops (willow and birch) among sexes,
ages, and areas were compared using mean and standard error. The caloric
values of the major crop items (willow buds and twigs, birch buds, twigs,
and catkins; partridgeberry leaves, and trembling aspen buds) from West and
Meng (1966) were the only values available, but accounted for 96% of all
crop-content masses. We used the average calories/gram values of Salix
richardsoni Hook. (Richardson's willow) and Salix glauca var. villosa (D.
Don ex Hook.) Anderss. (grayleaf willow), which are common in Labrador
(Ryan 1995), from West and Meng (1966) for all willow species. The
calories/gram values of dwarf birch were used for all birch species.
Three general categories of crop contents accounted for 99% of the crop
mass: five plant species, fine gravel, and lead shot (Table 1). Willow was the
most prevalent food item averaging 4.85 g ± 0.38 (SE) per ptarmigan crop,
or 83% ± 1.50 (SE) of total dry food items per ptarmigan crop (Table 1).
Birch varied among age, sex, and area with most being present in the crops
from eastern Labrador. Five of the birds sampled had empty crops: three
immature females, one immature male, and one adult male.
Ptarmigan crops from western Labrador contained more calories (mean ±
SE = 3.65 x 104 ± 3.38 x 103) than those from central (mean ± SE = 1.91 x 104
± 1.57 x 103) and eastern Labrador (mean ± SE =1.00 x 104 ± 1.74 x 103).
Within western Labrador, adult males had, on average, approximately 60%
more calories than other sexes and ages, but the large standard error indicates
an imprecise estimate for males (Fig. 2). In central and eastern Labrador,
calories among sexes and ages were similar. Ptarmigan crops from
western Labrador contained more total willow (mean ± SE = 3.16 ± 0.28)
than those from central (mean ± SE =1.30 ± 0.24) and eastern Labrador
(mean ± SE = 6.47 ± 0.62). The pattern of average willow crop contents
among ages and sexes within areas was virtually identical to those of
average calories (Fig. 3).
In western Labrador, all ptarmigan (regardless of sex and age) contained
more calories and willow twigs and buds than ptarmigan from other areas.
The adult-female-per-adult-male ratio in western Labrador was 4.7 to 1,
whereas central and eastern Labrador had 0.4 and 1 adult female per adult
male, respectively (Schwab et al. 2005). While the crops of western Labrador
adult males contained greater forage quality than those of adult females,
the overwhelming proportion of adult females wintering there suggests they
2007 L.T. Elson, F.E. Schwab, and N.P.P. Simon 93
Table 1. Average dry mass (g) of major winter crop items in Labrador Willow Ptarmigan by sex, age, and area.
Central (n = 97) East (n = 35) West (n = 178)
Adult Immature Adult Immature Adult Immature
Crop items Female Male Female Male Female Male Female Male Female Male Female Male Average
Betula glandulosa bud 0.01 0.02 0.02 0.03 0.01 0.00 0.00 0.01 0.01 0.00 0.03 0.02 0.02
Betula glandulosa catkin 0.00 0.00 0.00 0.01 0.45 0.11 0.09 0.53 0.28 0.17 0.21 0.15 0.15
Betula glandulosa twig 0.07 0.06 0.30 0.08 0.02 0.02 0.00 0.02 0.03 0.00 0.05 0.09 0.06
Betula papyrifera bud 0.07 0.02 0.05 0.02 0.01 0.01 0.02 0.00 0.04 0.09 0.01 0.06 0.03
Betula papyrifera catkin 0.02 0.06 0.00 0.09 0.45 0.13 0.09 0.02 0.30 0.10 0.19 0.23 0.16
Betula papyrifera twig 0.17 0.21 0.35 0.05 0.49 0.00 0.03 0.12 0.05 0.03 0.02 0.04 0.09
Betula spp. total 0.31 0.34 0.71 0.27 1.39 0.25 0.22 0.69 0.70 0.37 0.48 0.56 0.50
Rocks/grit 0.14 0.28 0.00 0.28 0.11 0.00 0.12 0.23 0.04 0.07 0.06 0.02 0.11
Salix spp. bark 0.44 0.64 0.70 0.83 0.11 0.12 0.05 0.05 0.41 0.80 0.35 0.49 0.42
Salix spp. bud 0.37 0.27 0.58 0.33 0.02 0.11 0.06 0.10 1.62 3.14 1.02 1.16 0.83
Salix spp. twig 3.17 2.26 3.60 3.22 0.88 1.73 1.01 1.84 5.83 10.04 4.61 5.33 4.02
Salix spp. total 3.55 2.54 4.18 3.55 0.89 1.84 71.08 1.93 7.46 13.18 5.63 6.49 4.85
Total dry 3.94 3.23 5.02 4.10 2.43 2.14 1.61 3.09 8.21 13.57 6.15 7.07 5.46
94 Northeastern Naturalist Vol. 14, No. 1
Figure 2. Mean and standard error of total calories in wintering Labrador Willow
Ptarmigan crops by area (with adult female:adult male ratios), sex, and age.
2007 L.T. Elson, F.E. Schwab, and N.P.P. Simon 95
Figure 3. Mean and standard error of dry mass of Salix buds and twigs in wintering
Labrador Willow Ptarmigan crops by area (with adult female:adult male ratios), sex,
96 Northeastern Naturalist Vol. 14, No. 1
winter in areas with the most nutritious foods. These results are consistent
with predictions from the Gardarsson (1988) hypothesis, which states that
females select wintering areas with greater food quality to offset their
reproductive energy demands. However, since we had no control over the
time of day from early December to late April when ptarmigan were taken,
sex, age, and location patterns may be obscured. We have no reason to
suspect this biased our data, but likely added noise.
Our observations contrast with the conclusion of Consortium Gautheir
and Guillemett – G.R.E.B.E. (1991), who found sexual segregation in wintering
ptarmigan, but stated no statistical difference in diets between sexes.
Despite the lack of a statistical significant difference, we feel there is a
biological difference as their wintering females contained 2.5 times the mass
of willow buds as compared with their wintering males. Since willow buds
are the more nutritious food (Gasaway 1976), this is further evidence that
food influences where females winter.
Despite area and sexual differences in food items, the most prominent
winter ptarmigan food was willow buds and twigs as commonly reported
(Gasaway 1976, Moss 1973, Weeden 1969, West and Meng 1966). This is
expected since willow is easier to digest, has greater protein and phosphorous,
and has less lignin than most other items in their diet (Bryant and
Kuropat 1980, Gardarsson and Moss 1970, Gasaway 1976, Moss 1983).
However, we found more birch in our ptarmigan crops than others: Birch
ranged from 13–34% within eastern Labrador crops, 6–17% in central Labrador,
and 6–9% in western Labrador, while Gasaway (1976) and Moss
(1973), both in Alaska, found only 5–6% birch. In eastern Labrador, white
birch is more abundant than in central and western Labrador (Canada Committee
on Ecological Land Classification 1989, Wilton 1965), potentially
explaining the greater amount of birch in the eastern crops. We found few
berries in the ptarmigan crops, but these are not common winter foods. It is
likely that wind-reduced snow cover presented the opportunity for ptarmigan
to forage on berries (Weeden 1969).
Since the areas with the greater food frequently have greater visual
cover, females may have selected for greater cover (Bergerud and Gratson
1988, Gruys 1993, Hannon et al. 2003), supporting the reproductive-strategy
hypothesis. That is, the greater quality of food in female crops may
have been coincident with selection for cover rather than a causal factor.
However, ptarmigan have several non-vegetative cover predator-avoidance
strategies: White plumage against snow, the tendency of ptarmigan to
freeze in the presence of a predatory threat, and their habit of burrowing
into snow for cover (Andreev 1991, Hannon et al. 1998). Where the reproductive-
strategy hypothesis emphasizes female survival from predators,
most literature, for a variety of taxa, state that obtaining greater energy
resources is a primary reason for migration (Alerstam et al. 2003, Fryxell
and Sinclair 1988, Gill 1995, Wilmshurst et al. 1999). Further, Savory
(1975) found the total weight of Lagopus lagopus scoticus Lath. (Red
2007 L.T. Elson, F.E. Schwab, and N.P.P. Simon 97
Grouse) eggs laid increased with daily food intake prior to, but not during,
laying. While it is possible that winter sexual segregation is due, in part, to
females selecting habitats with greater cover, we note the following: migration
is generally perceived as a strategy to obtain additional energy;
female ptarmigan enhance reproduction by obtaining energy prior to laying;
and ptarmigan possess structural and behavioral anti-predator strategies
that preclude the need for visual cover. We suggest that the greater
energy resources obtained during winter by females is an important consideration
in ptarmigan winter sexual-habitat segregation.
R. Holmberg, B. Stutchbury, K Wiebe, and an anonymous referee improved
the manuscript with critical comments. Assistance with collection of the Willow
Ptarmigan was provided by B. Griffin, W. Lyall, and G. O’Brien. Ptarmigan age,
sex, and weights were determined by F. Phillips, while S. Nash assisted with
dissection. Funding was provided by Human Resources Development Canada, the
College of the North Atlantic, and the Newfoundland and Labrador Department of
Natural Resources. Neal Simon died tragically at 32 years of age, during the
preparation of this manuscript. He did more research on and understood better the
forest ecosystems of Labrador than any other person. Yet, he died with only his
potential demonstrated. We mourn the loss of our colleague and friend.
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