Abstracf. We examined the effects of experimental food reductions on the reproductive biology of nestling Vesper Sparrows (Pooecetesgramineus) during 199 1 in the Little Missouri National Grasslands, North Dakota. Grasshopper densities on territories around 24 individual nests were experimentally reduced while 3 1 nests served as controls. There were no significant differences between treatment and control nests in number of young fledged per nest, 2-8 day nestling growth rate, or final 8 day

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... stling mass. Nest failure was due to predation (83.3% of nests that failed), and abandonment or parental death (16.7% of nests that failed). Grasshoppers were the principal food, comprising 67.7% of all identified food items brought to all nests when nestlings were 7-8 days old. Breeding adults on treated territories foraged significantly further from the nest than control birds suggesting that birds on treated sites compensated for the reduction in food by altering foraging behavior.