Species: V. atricapillus
The Black-capped Vireo (Vireo atricapillu) is a small endangered songbird found in the southern US (chiefly Texas) and Mexico.
The Vireo is about 4.5 inches long. As one might expect, the males have black caps (the females are more of a slate grey). Olive green on top, and white (males) or greenish-yellow (females) on the chest. Black beaks and white eye circles around auburn eyes complete the ensemble.
I've heard their song as angry-sounding, though the recording I heard didn't seem that way. They're very territorial birds, using their song to keep other Vireo away.
As of this writing, they're hanging out on the south coast of Mexico for the winter (wise birds), but around mid-March they will return to north-central Mexico, Texas, and three counties in Oklahoma (it used to be more, in addition to Kansas, but no longer) for mating season, which will extend to the end of August.
The Vireo builds its nest within about a meter of the ground under low-hanging shrub canopy that helps protect it from predators. Typically, the desired area will be 1/3 - 2/3 shrubs and 1/3 - 2/3 grassland, usually heavy on the shrubbery.
This sort of land is early successional, meaning it's usually emerging three to five years after a transformational event (traditionally wildfire).
As mentioned above, the Vireo is endangered (though IUNC lists it as merely vulnerable and the US may follow suit soon if things go well). Habitat loss has been the largest problem for the Vireo. Development by humans is a major issue. In addition to using land, we also like to suppress wildfire (especially in lands bordering our newly built Wal-Marts), so ashe juniper (which we all call cedar for some reason) takes over, replacing the shrubby grasslands, and shin oaks grow into relatively large trees instead of shinneries. To fix this, we can do prescribed burns (problematic near developed land--people are paranoid about wildfire) or take a chainsaw to the land, ploughing down the ashe juniper and 'shinning' the shin oak. (That's what I've been doing most of this week in the Travis County Balcones Canyonlands Preserve. Fire with chainsaws!)
Humans have also killed off a lot of the natural predators of deer, who have consequently exploded, population-wise, and eat up the low-hanging shrub canopy, letting snakes and other predators into the Vireo nests. And we seem to attract invasive red imported fire ants, which can destroy a Vireo nest pretty darn quickly.
Fourthly, the Brown-headed cowbird is doing well. The cowbirds are nest parasites, who lay their eggs in Vireo nests and let the Vireo then care for their young. (Since the baby cowbird is larger than the baby Vireo, it gets most of the food and attention, often leading to the untimely death of the Vireo young.)
Mostly notes from visiting the three BCPs. Also Wikipedia and http://www.tpwd.state.tx.us/huntwild/wild/species/endang/animals/birds/bcv.phtml