Hi Ted,
I think the Teller Farm Eastern Meadowlark is a fine old Sturnella magna magna ("Eastern" Eastern). The song is high pitched (averaging around 4 kHz vs. 3-3.5 on Lillian's), the thing has midnight black head stripes and kerchief, and the view of the
tail feathers that we get in your video taken from behind the bird (dorsal surface of tail) shows dark webs on the two inner tail feathers that should be all white on Lillian's.
A checklist with some tracks of what I IDed as a Lillian's on Gunbarrel Hill, Boulder County, in 2018. I think the song stands out as being quite distinct from the "king of the earth" song of nominate Easterns:
https://ebird.org/checklist/S46522659
I'll be adding the videos mentioned in the comments to the checklist here shortly, but it might take them a while to process. The skinny is that it is a
S. magna magna that sings a high-pitched Eastern song, but had no problem switching to a bubbly Western Meadowlark song when it wanted. The clincher from the ID standpoint was that it called like an Eastern. As Nathan Pieplow might tell you, meadowlarks
can learn each others' songs, but the calls are innate.
One more anecdote that I have observed on the Teller Farm Eastern. It's in an irrigated hay field. It's the kind of place one might expect to observe a
S. magna magna back east. It's far from the dry grasslands inhabited by Lillian's. The Lillian's on Gunbarrel Hill was in a weedy patch of pasture grasses and thistles amongst a prairie dog colony. Good for Westerns and Lillian's. You may note a distinct
grass in the 2013 Eastern videos- that's New Mexican Feather Grass. In good years it produces a distinct tall structure that can be seen from miles away. I think that's what drew it into that area among the matrix of short-stature mixed-grass prairie in the
surrounding landscape.
Christian Nunes
Boulder, CO