Patting birds on the head is a habit that Linus van Pelt adopted in the early 1960s to cheer himself up. The habit infuriates his sister, Lucy, who fears being known as, "the one whose brother pats birds on the head."
Linus first patted birds on the head in the strip from August 26, 1962. In a number of comic strips over the next several decades, Linus is shown returning to the habit, which is reported to Lucy by Charlie Brown and Violet Gray In spite of giving up patting birds on the head in the strip from June 1, 1967, Linus does it again in the strips from March 11, 1973 and May 10, 1992.
According to Charlie Brown and Lucy, the reason why the habit should be discouraged is that "nobody pats birds on the head."