Diet switching and food delivery by shrubsteppe passerines in response to an experimental reduction in food

We experimentally reduced the food base of nesting Brewer's Sparrows (Spize lla breweri) and Sage Thrashers (Oreoscoptes montanus) in a shrubsteppe reg ion of south central Idaho in 1989 and 1900. Frequency and mass of "birdfoo d" 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. Time b etween nestling food deliveries was greater for S. breweri on the treated t han untreated site in 1989. In 1990 there were no between-site differences, but there was an increase in delivery time on the treated plot after treat ment; this difference was within the range of delivery times recorded on th e untreated plot. Malathion applications did reduce the food base, but plas ticity in passerine behavior and emergence characteristics of some prey tax a ameliorated indirect effects of food reduction to birds.