Winter diet and intestinal proportions of rock and willow ptarmigan and sharp-tailed grouse in Ontario

Dwarf birch (Betula glandulosa) and willow (Salix spp.) were the principal winter foods of sympatric willow ptarmigan (Lagopus lagopus), rock ptarmigan (Lagopus mutus), and sharp-tailed grouse (Tympanuchus phasianellus) in northern Ontario. Birch catkins and leaf buds made up 90 and 69% of the diet of sharp-tailed grouse and rock ptarmigan, respectively, whereas willow stems and buds formed 68% of the willow ptarmigan diet. Digestive organ weights differed significantly (P < 0.05) among the species of grouse during winter. Within each species, weights of gizzard, caeca, and small intestine were weakly correlated with individual body weight. Total variance in the weight of each digestive organ among species was partitioned into significant effects of species body weight and also species. Seasonal changes to lighter caeca and small intestines were apparent in prebreeding willow ptarmigan and rock ptarmigan coincident with a change in diet to green, growing willow and birch parts. A precise effect of type of diet upon the digestive organ weight of different grouse species was not perceived and was confounded by influences of species body weight.