Elon Musk has made one of the most visible changes to Twitter since he took control of the social media company last fall: replacing its widely recognized bird logo.

In a tweet early Sunday morning Eastern time, Mr. Musk said that “soon we shall bid adieu to the twitter brand and, gradually, all the birds.” About 24 hours later, a stylized, black-and-white X appeared on the company’s website in place of the blue bird logo.

Twitter’s corporate accounts also adopted the new branding, which was projected onto the side of the company’s headquarters in San Francisco overnight.

“X” is a term for what Mr. Musk has described as an “everything app” that could combine social media, instant messaging and payment services, akin to the popular Chinese app WeChat.