Foods Eaten by Tree Sparrows in Relation to Availability During Summer in Northern Manitoba

The amount of animal matter in the stomach contents of tree sparrows ( Spizella arborea ) collected at Churchill, Manitoba, increased from June until the middle of July but quickly dropped off in August. Chief food items were seeds of 6 species of Carex ; seeds, flowers and fruits of Empetrum and Vaccinium ; adult Diptera and Araneida; and larvae of Coleoptera and Lepidoptera. From available foods tree sparrows selected several species of Carex and ignored others. Snails though abundant were not eaten, and several larval insect forms were selected out of proportion to their occurrence in availability samples. The gross caloric value of the average summer diet was 5211 cal./g. dry weight and the kcal./day or the equivalent of 5.06 and 6.33 g. dry wt. of diet items. On the basis of food supply it is doubtful that gross energy of available foods limits the breeding density of tree sparrows at Churchill.