Winter Food Habits of Lagopus lagopus (Willow Ptarmigan) as a Mechanism to Explain Winter Sexual Segregation
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 IRock 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 (53°07′N, 67°56′W), elevation = 550 m; central Labrador samples were obtained within 25 km of the Lobsitck hydro control structure (53°54′N, 65°09′W), elevation = 550 m; and eastern Labrador samples were obtained within 30 km of Mokami Hill (53°58′N, 6o°08′W), elevation
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., buds, twigs, leaves, and fruit). Segregated crop contents were oven dried at 80 °C for [asymptotically =] 24 hours, and the dry mass by species-structure were recorded for each sample.