SURVIVAL AND GROWTH OF NESTLING VESPER SPARROWS EXPOSED TO EXPERIMENTAL FOOD REDUCTIONS

We examined the effects of experimental food reductions on the reprodu ctive biology of nestling Vesper Sparrows (Pooecetes gramineus) during 1991 in the Little Missouri National Grasslands, North Dakota. Grassh opper densities on territories around 24 individual nests were experim entally reduced while 31 nests served as controls. There were no signi ficant differences between treatment and control nests in number of yo ung fledged per nest, 2-8 day nestling growth rate, or final 8 day nes tling mass. Nest failure was due to predation (83.3% of nests that fai led), and abandonment or parental death (16.7% of nests that failed). Grasshoppers were the principal food, comprising 67.7% of all identifi ed food items brought to all nests when nestlings were 7-8 days old. B reeding adults on treated territories foraged significantly further fr om the nest than control birds suggesting that birds on treated sites compensated for the reduction in food by altering foraging behavior.