Pop Airplay (also called Mainstream Top 40, Pop Songs, and Top 40/CHR) is a 40-song music chart published weekly by Billboard Magazine that ranks the most popular songs of pop music being played on a panel of Top 40 radio stations in the United States. The rankings are based on radio airplay detections as measured by Nielsen Broadcast Data Systems (Nielsen BDS), a subsidiary of the U.S.' leading marketing research company. Consumer researchers, Nielsen Audio (formerly Arbitron), refers to the format as contemporary hit radio (CHR). The current number-one song on the chart is "Birds of a Feather" by Billie Eilish.[1]