IBLE, with cc’s to people otherwise unable to receive ~~~~~ ?
I don’t remember responding to Carl’s note below……. maybe yes in my dreams?
Carl provides excellent discussion, and I’ve gotta say that migration in all of its “formats” has long been one of my topmost two or three favorite subjects regarding birds, and Peter Berthold’s “General Survey” (2001, maybe since updated?) (Oxford Ornithology Series) has been one of my favorite references on topic. I can offer only conjectures, e.g., that “irruptive species” utilize irruption as a precondition to range expansion in a manner similar to “vagrant species” that are doing the same thing but at a slower pace. As an example, some warbler species tend to wander out of their expected ranges much more often than others do, and looking back at state-level CBC data I can see that ECDO are expanding northward way faster than WWDO, yada yada et yada. I’ve often thought of out-of-range birds as genetic outliers that are necessary for a given species’ adaptive survival in a universe that is continually changing, whether they be considered as “typical migrants” or not. And I’ve not often considered “irruption” to be a black-and-white phenomenon for this or that species, vs. not for other species where it is much more subtle. Does this make sense? Seems to me there is a “spectrum” of “irruption” with some species being far less obvious at it than others…?
Maybe somebody can provide clarity, or reference to published data re “migration and propensity toward irruption” as it might relate to genetic variability. But I can’t imagine what such “data” might look like.
Thank you, Carl, 10^6 for your discussion below. Love it !! =)
Larry
W Boise / Garden City