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Toxicity and digestive inhibition are commonly thought of as the primary postingestive consequences by which plant secondary metabolites (PSM's) limit herbivore food selection. However, food selection may also be modified by nutritional costs imposed by detoxication processes and nutrient dilution. Few studies have determined the magnitude of these costs for vertebrate herbivores and their ecological significance. Research clarifying the mechanisms by which PSM's repel animals may give new insights into the development of repellents for nuisance wildlife and improve our ability to predict ecological interactions involving herbivores. Using captive ruffed grouse (Bonasa umbellus), we tested whether PSM ingestion interferes with energy and nitrogen retention and whether food selection was related to costs associated with detoxication and nutrient dilution. Two feeding experiments were conducted. In one experiment, grouse (Group 1) were fed quaking aspen (Populus tremuloides) flower buds with different levels of conifeql benzoate (CB), the primary PSM in the buds. In the other experiment, birds (Group 2) were given a formulated diet treated with CB (6.5 % dry wt.). We measured energy and nitrogen utilization efficiencies and the output of detoxication products. Intake of CB was associated with decreased food utilization efficiencies in both experiments. The low energy utilization efficiency of Group 1 birds consuming high levels of CB, was mainly attributed to nutrient dilution by CB. These birds retained 24% less metabolizable energy than when feeding on buds with low CB levels (P = 0.010). Group 1 birds excreted 10 to 14% of their daily metabolizable energy intake as glucuronic acid and ornithine. Excretion of detoxication conjugates and ammonium increased with CB intake in both experiments. Nitrogen excreted in the form of ornithine and ammonium accounted for approximately 30% of the daily nitrogen intake in both experiments. During the high CB trials, the excretion of ornithine conjugates alone increased Group 1 birds' minimum daily nitrogen requirement by 90.0% over that required when consuming a diet containing no PSM's. Relative to Group 1 birds, Group 2 birds excreted a higher proportion of their daily nitrogen intake as ornithine, despite having one-forth the nitrogen intake as Group 1 birds and despite being in negative nitrogen balance. For the development of repellents to control nuisance wildlife, additional research should be conducted to determine the extent to which repellency can be increased by raising nutrient dilution costs or by manipulating detoxication pathways.