Why are there silhouettes of Native Americans on the Book Room walls?

The represent some of the 27 Chiefs from the Missouri and the Mississippi that visited DC in 1805.

The 1803-06 Lewis and Clark expedition was not only a journey of discovery but also a diplomatic mission from the United States government to the Native Americans who inhabited the newly acquired Louisiana Territory. Jefferson instructed Lewis and Clark to treat the Native Americans in the most friendly and conciliatory manner. If a few of their influential chiefs, within a practicable distance, wish to visit us, arrange such a visit with them, and furnish them with authority to call on our officers, on their entering the U.S. to have them conveyed to this place at the public expense. The first delegation of Native Americans came from the Osage nation, and arrived in Washington in the summer of 1804. The following fall, a second delegation of about twenty-seven Chiefs from the Missouri and Mississippi departed from St. Louis for the capital. They arrived there two months later, on December 22, 1805.