This Cassin's Vireo Vireo cassinii was at Rosehill Cemetery, Corpus Christi, Nueces county,
Texas, on November 24, 2019. IDing Cassin's out of range is problematic due to a certain amount of overlap in some key features, especially in the fall when all of the Solitary Vireo complex are in fresh, bright plumage. It is also confounded by hybirds (which are probably unassignable as such) from a small contact zone with Blue-headed Vireo in the Canadian Rockies, and based on geography birds from this area are likely to pass through Texas. Vireos at this time of year are usually silent, but luckily this bird was quite vocal, both before and after playback of Cassin's Vireo vocalizations from a birding app.
Here is a spectrograph of this bird's song (on the right) compared to that of a Cassin's Vireo from California (on the left) obtained from Xeno Canto, establishing that this individual is indeed a Cassin's Vireo:
One distinctive aspect of Cassin's Vireo compared to Blue-headed Vireo is foraging behavior: Blue-headed is typically rather stolid, sitting on a branch perhaps moving its head and/or body but not moving to a new perch for long periods, and does not flit around. Cassin's Vireo behaves more like a slightly less-active Kinglet, rarely perching for more than a few seconds before flitting to a new perch nearby, and often hover-gleaning.
Click here to link to a short vireo of the Rosehill bird that includes some of this behavior.
The degree of crispness on the side of the throat is considered a strong ID feature, but some Cassin's Vireos can resemble some Blue-headed Vireos
in this regard .I searched eBird’s photo archive of Cassin's Vireos, limiting my search to birds from May-Aug in CA, OR, and WA (i.e. the core breeding range in the breeding season, thus BHVI highly unlikely), and was able to find birds with similar or greater sharpness on the sides of the throat; here are links to a few (keep in mind that these individuals are worn and possibly faded at this time of year):