The northern mockingbird is known for its intelligence. A 2009 study showed that the bird was able to recognize individual humans, particularly noting those who had previously been intruders or threats. Also birds recognize their breeding spots and return to areas in which they had greatest success in previous years. Urban birds are more likely to demonstrate this behavior. Finally, the mockingbird is influential in United States culture, being the state bird of five states, appearing in book titles, songs and lullabies, and making other appearances in popular culture.