To me it seems counterintuitive that a bird would live in an underground burrow. Even ground-nesting seems, well, dangerous. But plenty of birds do one or the other and thrive, despite the dangers. One of those is the burrowing owl.
The western burrowing owl lives in short grass prairies and deserts across the western US (including Colorado!) and in Central America. These owls live in burrows abandoned by other animals like prairie dogs, tortoises, and ground squirrels. Though not particularly built for creating burrows themselves (though some pairs do!), the owls will maintain and enlarge burrows by digging with their beak and using their feet to move dirt.
Unlike other owl species, burrowing owls hunt during the day as well as at night. They hunt small mammals, insects, amphibians, and even other birds. Though they do fly when hunting, much of it is done by walking, hopping, or running on the ground and grasping prey with their talons. In addition, they will line the entrance to the burrow with dung. This attracts insects, creating an owl buffet. Genius! During brooding season these burrowers will stash food in burrow chambers to ensure they have enough food.
These birds are usually monogamous and live in colonies. A breeding pair works together to raise a clutch of up to a dozen. During incubation and in the first weeks after the eggs hatch, the female stays with the eggs or young, and the males brings food and defend the territory.
I’ve lived in Colorado for 30 years, near prime burrowing owl habitat, and I’ve never seen one. Now I’m on the lookout!