The Digestion and Intake of Winter Foods by Wild Ptarmigan in Alaska

Moss, Robert

An indirect technique for measuring the digestibility of foods of ptarmigan is described, using the magnesium which occurs naturally in the foods as a digestibility marker. The method was tested in the laboratory and then applied to three species of ptarmigan in the field. It was essential to use the crop contents for estimating the magnesium content of the diet. Hand-picked samples of foods could not be used because they sometimes contained less magnesium than the same food in the crops. Daily food intakes and energy budgets were calculated by weighing heaps of woody droppings left by birds roosting for known periods. The technique for measuring digestibility was simple and precise when applied to two diets consisting mainly of one food. A third diet included two major foods; the magnesium content of each food was fairly similar and so a fairly large error in estimating the proportions of the two foods in the diet could be tolerated with little effect on the result. In a fourth case, the magnesium content of two of the four major foods was markedly different from the other two, and in this case a relatively small error in estimating the species composition of the diet would have led to appreciable error in its calculated digestibility. Factors for calculating the species composition of diets from counts of epidermal fragments of plants in the droppings were derived. This was done by comparing the crop contents with counts of plant epidermal fragments in samples from the large intestine. These factors had to be used carefully because they differed on different occasions. This was when the nature of materials classified under one heading differed, e.g., 'birch'might be twigs on one occasion and catkins on another. The winter energy budgets of free-living ptarmigan were compared with earlier work on captive Willow Ptarmigan. The 'cost of free-living'was about 20% of the daily energy budget of Willow Ptarmigan but negligible for Rock and White-tailed Ptarm

The Digestion and Intake of Winter Foods by Wild Ptarmigan in Alaska

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