RE - Ceruleans. I did a lot of atlas blocks (04-08) in Sproul State Forest. I was surprised to find Ceruleans not at all uncommon on the upper parts of high steep slopes. My surprise probably came from a Lower-Trail influenced assumption, as you say, that they were on hillsides along rivers.
Re - Vireos. I wrote the atlas account for YTVI. I wondered why REVI so-outnumbered YTVI., two species so seemingly similar (tho of course with requirements we can't fully grasp). The best hypothesis (a hypothesis in need of factual backing) I could find was that REVI "out-competed" YTVI, pushing them from favored forest interiors to less favorable, more open margins of forests. Which sort of fits with where we tend to find YTVI.
RE - Ceruleans. I did a lot of atlas blocks (04-08) in Sproul State Forest. I was surprised to find Ceruleans not at all uncommon on the upper parts of high steep slopes. My surprise probably came from a Lower-Trail influenced assumption, as you say, that they were on hillsides along rivers.
Re - Vireos. I wrote the atlas account for YTVI. I wondered why REVI so-outnumbered YTVI., two species so seemingly similar (tho of course with requirements we can't fully grasp). The best hypothesis (a hypothesis in need of factual backing) I could find was that REVI "out-competed" YTVI, pushing them from favored forest interiors to less favorable, more open margins of forests. Which sort of fits with where we tend to find YTVI.