WINTER FORAGING AND DIET COMPOSITION OF NORTHERN SHRIKES IN IDAHO

We observed color-banded and/or radio-tagged Northern Shrikes (Lanius excubitor) wintering in southwest Idaho and determined that foraging s uccess of these shrikes was over 69%. Foraging success was dependent u pon the type of prey attacked. Predation success upon arthropods was g reater than 90%, whereas predation upon vertebrates (small mammals and passerines) was substantially lower (56% and 19%, respectively). We c ollected 237 pellets from 12 shrikes and identified 671 individual pre y items contained in these pellets. Arthropods and small mammals were the most important prey items as measured by number (63.9% and 29.8%, respectively) and Index of Relative Importance (38.9% and 59.6%, respe ctively), whereas small mammals were the most important components of shrike diet by biomass contributing 83.1% of the total prey biomass. P asserines were of lesser importance in the winter diet of shrikes acco unting for 11.8% of the biomass but only 1.7% of the Index of Relative Importance.