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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 Sexual Segregation 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. Introduction 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 - 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). Methods 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. Results 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). Discussion 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, and age. 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. Acknowledgments 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. Literature Cited Alerstam, T., A. Hedenström, and S. Åkesson. 2003. Long-distance migration: Evolution and determinants. Oikos 103:247–260. Andreev, A.V. 1991. Winter adaptations in the Willow Ptarmigan. Arctic 44:106–114. Bergerud, A.T., and M.W. Gratson 1988. Survival and breeding strategies of grouse. Pp. 473–577, In A.T. Bergerud and M.W. Gratson (Eds.). Adaptive Strategies and Population Ecology of Northern Grouse. University of Minnesota Press, Minneapolis, MN. 809 pp. Bergerud, A.T., S.S. Peters, and R. McGrath. 1963. Determining sex and age of Willow Ptarmigan in Newfoundland. Journal of Wildlife Management 27:700–711. Bryant, J.P., and P.J. Kuropat. 1980. Selection of winter forage by subarctic browsing vertebrates: The role of plant chemistry. Annual Review of Ecology and Systematics 11:261–285. Canada Committee on Ecological Land Classification. 1989. Ecoclimate regions of Canada: First approximation. Environment Canada, Canadian Wildlife Service, Hull, QC, Canada. Ecological Land Classification Series No. 23. 118 pp. Consortium Gauthier and Guillemette – G.R.E.B.E. 1991. Complexe Grande- Baleine. Avant-projet Phase II. Utilisation des arbustaies sur le complexe La Grande et régime alimentaire des lagopèdes; rapport final présenté à Hydro- Québec, vice-présidence Environment. Le Consortium, Saint-Romauld, QC, Canada. Juillet 1991. 78 pp. Fryxell, J.M., and A.R.E. Sinclair. 1988. Causes and consequences of migration by large herbivores. Trends in Ecology and Evolution 3:237–241. Gardarsson, A. 1988. Cyclic populations and some related events in Rock Ptarmigan in Iceland. Pp. 300–329, In A.T. Bergerud and M.W. Gratson (Eds.). Adaptive Strategies and Population Ecology of Northern Grouse. University of Minnesota Press, Minneapolis, MN. 809 pp. 98 Northeastern Naturalist Vol. 14, No. 1 Gardarsson, A., and R. Moss. 1970. Selection of food by Icelandic ptarmigan in relation to its food availability and nutritive value. Pp. 47–71, In A. Watson (Ed.). Animal Populations in Relation to Their Food Resources. Blackwell Scientific Publications, Oxford, UK. 477 pp. Gasaway, W.C. 1976. Volatile fatty acids and metabolizable energy derived from cecal fermentation in the Willow Ptarmigan. Comparative Biochemistry and Physiology 53:115–121. Gill, F.B. 1995. Ornithology, 2nd Edition. Freeman and Company, New York, NY. 766 pp. Gruys, R.C. 1993. Autumn and winter movements and sexual segregation of Willow Ptarmigan. Arctic 46:228–239. Hannon, S.J., P.K. Eason, and K. 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