The company detailed the change in a blog post where it explains the rationale for switching up the icon to something that resembles a person. However, there's some interesting rationale at work here.
"Regardless, people have come to associate the circle head with masculinity, and because of this association, we felt that it was important to explore alternate head shapes," the post explains. "We reviewed many variations of our figure, altering both the head and shoulders to feel more inclusive to all genders." Twitter says it settled on a more neutral gray color scheme to give less prominence to account with a default photo. They're less like to stand out in your timeline than those colorful eggs. Of course, if you really wanted to use the old profile pics, there's not anything anyone can do about it -- except mute or block you.
Yesterday the service announced it was removing @replies from the 140-character limit -- a change users are still getting used to. Earlier this month, Twitter started allowing users to mute any accounts with the default avatar so you can still keep those folks out of your feed should the need arise. For now, we're off to sort out our new profile pics.