Though today it holds a visitor’s center, exhibit space, and offices, the Smithsonian Institution Building, or "Castle," once also contained residential spaces. The Castle was home to the Institution’s first Secretary, Joseph Henry, and his family from 1855 to 1878. During the building’s early years it also included apartments for Smithsonian employees and visiting scholars.

But some other inhabitants of the Castle have been less conventional. In the late 19th century, barn owls took roost in the building's lofty towers. These uninvited occupants were nevertheless welcomed and were the object of study by researchers. The Smithsonian collections contain two owl eggs gathered from the roof of the Castle in 1861 and 1865 (specimens USNM B 9986 and 9693), the latter conserved by ornithologist, Spencer F. Baird, who would later become second Secretary of the Smithsonian. Ornithologists collected samples of the owls' pellets, droppings containing undigested remains of the rodents they ate, to study hunting habits. Another later Secretary and ornithologist, Alexander Wetmore, gathered over two thousand pellets from the Castle towers while working for the Biological Survey in 1913.

Though the owls were useful study subjects, they were at some times pests. Residents of the Castle noted that the owls often crashed into their windows, startling them from their work. When the owls would dive to catch prey at night they would nearly collide with superstitious guards who patrolled the National Mall and took the swooping birds to be curses of bad luck. The guards complained to sixth Secretary Wetmore during his tenure from 1945–1952 and asked that he remove the owls, but he wittily replied that "our guards must remain dauntless to any and all attacks." However, by the 1950s the owls had outstayed their welcome. A large quantity of droppings that had accumulated had generated an unpleasant smell and caused the floor of one tower to collapse. The owls were put out and the windows were barred.

But in 1971, eighth Secretary S. Dillon Ripley, yet another ornithologist interested in the tower owls as much as his predecessors, decided to reinstate the winged residents. He believed that the owls could hunt the rats attracted by the newly placed garbage cans on the National Mall. Ripley argued that the owls would be beneficial "for reasons of biological harmony as well as tradition." He wrote to former-Secretary Wetmore inquiring about the history of the tower owls, and set about equipping replacements. Barn owls to be put in the towers were trained at the National Zoological Park to breed in captivity and hunt live prey. In 1974, a male and female were placed in the northwest tower of the Castle. Alex (named for Wetmore) and his mate Athena were fed dead lab rodents and soon hatched seven young owlets.

Once the observers felt that the owls were comfortable in their new home, they unbarred the windows. But by December 1975 the last of the owl family had, quite literally, flown the coop. Not to be deterred, Ripley commissioned a second pair of trained owls from NZP, this time named "Increase" and "Diffusion" for language found in the Institution’s bequest (under a clause in his will, the Institution’s founding donor, James Smithson, left his fortune to the United States to found under the name of the Smithsonian Institution, "an establishment for the increase and diffusion of knowledge among men."). The aptly-named owl duo was placed in the tower in January 1977, and hatched three young owlets that spring. However, when given the chance to fly to freedom, the second family also took to the air never to return to the Castle tower.

Now the towers are uninhabited. And though Ripley was saddened by the lack of traditional residents, those who had to tend to the owls probably were not. Writing in 1993, one such caretaker remembers climbing the ladder to place a bag full of dead rodents in the coop, dressed in a protective suit and helmet to guard against (as Ripley put it) "more than a gentle tap" on the head. Ripley himself once received an aerial attack when, while poking his head in to take a look at the birds, an owlish deposit fell squarely in his eye.