We experimentally reduced the food base of nesting Brewer's Sparrows (Spizella breweri) and Sage Thrashers (Oreoscoptes montanus) in a shrubsteppe region of south central Idaho in 1989 and 1990. Frequency and mass of "birdfood" arthropods in pitfall, sweep net, and stickyboard samples were generally lower on sites treated with a broad-spectrum insecticide (malathion) than on untreated sites though the effect varied among taxa. In 1990 O. montanus switched nestling diets to prey taxa not affected by the treatment...