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why not found west of the cascades ?
Very interesting question, for which I don't have a good answer. I suspect that part of it is a niche thing. It occurs to me that they tend to be in places where American Crows are not resident. It is little known to most folks that American Crows vacate most of eastern Oregon during the winter months and in some places east of the Cascades they are absent even during the warmer months. Perhaps magpies are the default corvids in places where crows are absent or non-resident, but cannot compete with crows where they are resident. I will do some research when I have the time and see if I can come up with a better and less speculative answer.
Just did a quick online search and found some references that seem to confirm my earlier speculation. I found one reference to work in Sweden that talked about competition between European Black-billed Magpies and Carrion Crows (very similar to Am. Crow). It refers to "interference competition" between these species, which share similar diets and foraging habits. The paper notes that crows have been known to kill adult magpies and that they regularly raid magpie nests, predate the young and eggs and/or cause magpies to abandon nests, that in some cases are then taken over by crows.