Click the pic to see an excerpt of the story from Four Color/Peanuts #1015.
Patting birds on the head is a habit that Linus van Pelt adopted toward the end of the 1950s 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 a story that appeared in Dell comics' Four Color #1015 (cover-titled Peanuts), dated August-October 1959. He first took up the habit in the strip proper on August 26, 1962.
Linus van Pelt pats birds on the head in the strip from March 20, 1966, reprinted on March 17, 2013.
In a number of comic strips over the next several decades, including Sunday, March 20, 1966 and a daily storyline from May 1967, Linus is shown returning to the habit, which is reported to Lucy by Charlie Brown and Violet. In spite of giving up patting birds on the head in the strip from June 1, 1967,
Rerun van Pelt patting birds on the head in the March 14, 1999 strip.
Linus does it again in the strips from March 11, 1973 and May 10, 1992.
In the strip from March 14, 1999, Lucy is further infuriated when her youngest brother Rerun also adopts the habit.
According to Charlie Brown and Lucy, the reason why the habit should be discouraged is that "nobody pats birds on the head."