To study the winter ecology of northern shrikes in southwest Idaho, I observed the activities of six color-banded and six radio-tagged shrikes over the winters of 1988-1989 and 1989-1990. These shrikes occupied winter territories that averaged 216 ha in size (minimum convex polygon method). Over one-half of the activity of each shrike was confined to a core area of approximately 50 ha. Five of nine individual northern shrikes perched in or hunted within mesic areas significantly more than expected and four of the nine utilized grasslands significantly less than expected. Linear habitats such as riparian corridors and rimrock outcroppings appeared to be important to wintering shrikes since the former provided night roost habitat whereas the latter provided warm and productive areas for prey populations.
I located 10 night roosts of northern shrikes. All roosts were found in deciduous shrubs with many small stems. Roosts potentially provide escape from predation and reduce energetic costs through the reduction of wind velocity within the roost.
I collected 237 pellets from 12 shrikes and identified 671 individual prey items contained in these pellets. Arthropods and small mammals were the most important prey items as measured by Index of Relative Importance. Foraging success of northern shrikes was over 69% and was dependent upon the type of prey attacked. Predation success upon arthropods was greater than 90%, whereas predation upon vertebrates (small mammals and passerines) was substantially lower (56% and 19%, respectively).
To assess niche overlap between morphologically and ecologically similar northern shrikes and American kestrels I compared the position of each species upon the following resource axes: (1) macrohabitat, (2) vegetation surrounding hunting perches, (3) height of perches, (4) prey diversity, and (5) prey biomass. Shrikes and kestrels showed considerable overlap (> 0.80) on all axes except prey diversity. Northern shrikes preyed upon a more diverse prey assemblage whereas American kestrels appeared to be a microtine specialist. Multidimensional niche overlap, however, remained high (0.82) between these species which may partially account for five instances of interspecific aggression observed during the study. Northern shrikes and American kestrels attempt to hold mutually exclusive territories through agonistic behaviors and visual displays.